Merge pull request #354 from mitchmindtree/no-eventloop-rebased

Removing the `EventLoop` - rebased

.gitignore

@@ -3,3 +3,4 @@
 .cargo/
 .DS_Store
 recorded.wav
+rls*.log

Cargo.toml

@@ -24,6 +24,7 @@ ringbuf = "0.1.6"
 [target.'cfg(target_os = "windows")'.dependencies]
 winapi = { version = "0.3", features = ["audiosessiontypes", "audioclient", "coml2api", "combaseapi", "debug", "devpkey", "handleapi", "ksmedia", "mmdeviceapi", "objbase", "std", "synchapi", "winbase", "winuser"] }
 asio-sys = { version = "0.1", path = "asio-sys", optional = true }
+parking_lot = "0.9"
 
 [target.'cfg(any(target_os = "linux", target_os = "dragonfly", target_os = "freebsd", target_os = "openbsd"))'.dependencies]
 alsa-sys = { version = "0.1", path = "alsa-sys" }

asio-sys/src/bindings/mod.rs

@@ -7,7 +7,7 @@ use self::errors::{AsioError, AsioErrorWrapper, LoadDriverError};
 use std::ffi::CStr;
 use std::ffi::CString;
 use std::os::raw::{c_char, c_double, c_long, c_void};
-use std::sync::{Arc, Mutex, Weak};
+use std::sync::{Arc, Mutex, MutexGuard, Weak};
 
 // Bindings import
 use self::asio_import as ai;
@@ -85,7 +85,7 @@ pub struct SampleRate {
 }
 
 /// Holds the pointer to the callbacks that come from cpal
-struct BufferCallback(Box<FnMut(i32) + Send>);
+struct BufferCallback(Box<dyn FnMut(i32) + Send>);
 
 /// Input and Output streams.
 ///
@@ -235,6 +235,9 @@ struct BufferSizes {
     grans: c_long,
 }
 
+#[derive(Clone, Copy, Debug, PartialEq, Eq)]
+pub struct CallbackId(usize);
+
 lazy_static! {
     /// A global way to access all the callbacks.
     ///
@@ -244,7 +247,7 @@ lazy_static! {
     /// Options are used so that when a callback is removed we don't change the Vec indices.
     ///
     /// The indices are how we match a callback with a stream.
-    static ref BUFFER_CALLBACK: Mutex<Vec<Option<BufferCallback>>> = Mutex::new(Vec::new());
+    static ref BUFFER_CALLBACK: Mutex<Vec<(CallbackId, BufferCallback)>> = Mutex::new(Vec::new());
 }
 
 impl Asio {
@@ -419,6 +422,8 @@ impl Driver {
         // To pass as ai::ASIOCallbacks
         let mut callbacks = create_asio_callbacks();
 
+        let mut state = self.inner.lock_state();
+
         // Retrieve the available buffer sizes.
         let buffer_sizes = asio_get_buffer_sizes()?;
         if buffer_sizes.pref <= 0 {
@@ -429,13 +434,12 @@ impl Driver {
         }
 
         // Ensure the driver is in the `Initialized` state.
-        if let DriverState::Running = self.inner.state() {
-            self.stop()?;
+        if let DriverState::Running = *state {
+            state.stop()?;
         }
-        if let DriverState::Prepared = self.inner.state() {
-            self.dispose_buffers()?;
+        if let DriverState::Prepared = *state {
+            state.dispose_buffers()?;
         }
-
         unsafe {
             asio_result!(ai::ASIOCreateBuffers(
                 buffer_infos.as_mut_ptr() as *mut _,
@@ -444,8 +448,8 @@ impl Driver {
                 &mut callbacks as *mut _ as *mut _,
             ))?;
         }
+        *state = DriverState::Prepared;
 
-        self.inner.set_state(DriverState::Prepared);
         Ok(buffer_sizes.pref)
     }
 
@@ -566,13 +570,14 @@ impl Driver {
     ///
     /// No-op if already `Running`.
     pub fn start(&self) -> Result<(), AsioError> {
-        if let DriverState::Running = self.inner.state() {
+        let mut state = self.inner.lock_state();
+        if let DriverState::Running = *state {
             return Ok(());
         }
         unsafe {
             asio_result!(ai::ASIOStart())?;
         }
-        self.inner.set_state(DriverState::Running);
+        *state = DriverState::Running;
         Ok(())
     }
 
@@ -589,12 +594,26 @@ impl Driver {
     /// Adds a callback to the list of active callbacks.
     ///
     /// The given function receives the index of the buffer currently ready for processing.
-    pub fn set_callback<F>(&self, callback: F)
+    ///
+    /// Returns an ID uniquely associated with the given callback so that it may be removed later.
+    pub fn add_callback<F>(&self, callback: F) -> CallbackId
     where
         F: 'static + FnMut(i32) + Send,
     {
         let mut bc = BUFFER_CALLBACK.lock().unwrap();
-        bc.push(Some(BufferCallback(Box::new(callback))));
+        let id = bc
+            .last()
+            .map(|&(id, _)| CallbackId(id.0.checked_add(1).expect("stream ID overflowed")))
+            .unwrap_or(CallbackId(0));
+        let cb = BufferCallback(Box::new(callback));
+        bc.push((id, cb));
+        id
+    }
+
+    /// Remove the callback with the given ID.
+    pub fn remove_callback(&self, rem_id: CallbackId) {
+        let mut bc = BUFFER_CALLBACK.lock().unwrap();
+        bc.retain(|&(id, _)| id != rem_id);
     }
 
     /// Consumes and destroys the `Driver`, stopping the streams if they are running and releasing
@@ -618,56 +637,71 @@ impl Driver {
     }
 }
 
-impl DriverInner {
-    fn state(&self) -> DriverState {
-        *self.state.lock().expect("failed to lock `DriverState`")
-    }
-
-    fn set_state(&self, state: DriverState) {
-        *self.state.lock().expect("failed to lock `DriverState`") = state;
-    }
-
-    fn stop_inner(&self) -> Result<(), AsioError> {
-        if let DriverState::Running = self.state() {
+impl DriverState {
+    fn stop(&mut self) -> Result<(), AsioError> {
+        if let DriverState::Running = *self {
             unsafe {
                 asio_result!(ai::ASIOStop())?;
             }
-            self.set_state(DriverState::Prepared);
+            *self = DriverState::Prepared;
         }
         Ok(())
     }
 
-    fn dispose_buffers_inner(&self) -> Result<(), AsioError> {
-        if let DriverState::Initialized = self.state() {
+    fn dispose_buffers(&mut self) -> Result<(), AsioError> {
+        if let DriverState::Initialized = *self {
             return Ok(());
         }
-        if let DriverState::Running = self.state() {
-            self.stop_inner()?;
+        if let DriverState::Running = *self {
+            self.stop()?;
         }
         unsafe {
             asio_result!(ai::ASIODisposeBuffers())?;
         }
-        self.set_state(DriverState::Initialized);
+        *self = DriverState::Initialized;
         Ok(())
     }
 
-    fn destroy_inner(&mut self) -> Result<(), AsioError> {
-        // Drop back through the driver state machine one state at a time.
-        if let DriverState::Running = self.state() {
-            self.stop_inner()?;
+    fn destroy(&mut self) -> Result<(), AsioError> {
+        if let DriverState::Running = *self {
+            self.stop()?;
         }
-        if let DriverState::Prepared = self.state() {
-            self.dispose_buffers_inner()?;
+        if let DriverState::Prepared = *self {
+            self.dispose_buffers()?;
         }
         unsafe {
             asio_result!(ai::ASIOExit())?;
             ai::remove_current_driver();
         }
+        Ok(())
+    }
+}
+
+impl DriverInner {
+    fn lock_state(&self) -> MutexGuard<DriverState> {
+        self.state.lock().expect("failed to lock `DriverState`")
+    }
+
+    fn stop_inner(&self) -> Result<(), AsioError> {
+        let mut state = self.lock_state();
+        state.stop()
+    }
+
+    fn dispose_buffers_inner(&self) -> Result<(), AsioError> {
+        let mut state = self.lock_state();
+        state.dispose_buffers()
+    }
+
+    fn destroy_inner(&mut self) -> Result<(), AsioError> {
+        {
+            let mut state = self.lock_state();
+            state.destroy()?;
 
             // Clear any existing stream callbacks.
             if let Ok(mut bcs) = BUFFER_CALLBACK.lock() {
                 bcs.clear();
             }
+        }
 
         // Signal that the driver has been destroyed.
         self.destroyed = true;
@@ -863,11 +897,9 @@ extern "C" fn buffer_switch_time_info(
 ) -> *mut ai::ASIOTime {
     // This lock is probably unavoidable, but locks in the audio stream are not great.
     let mut bcs = BUFFER_CALLBACK.lock().unwrap();
-    for mut bc in bcs.iter_mut() {
-        if let Some(ref mut bc) = bc {
+    for &mut (_, ref mut bc) in bcs.iter_mut() {
         bc.run(double_buffer_index);
     }
-    }
     time
 }
 

examples/beep.rs

@@ -1,37 +1,26 @@
 extern crate anyhow;
 extern crate cpal;
 
-use cpal::traits::{DeviceTrait, EventLoopTrait, HostTrait};
+use cpal::traits::{DeviceTrait, StreamTrait, HostTrait};
 
 fn main() -> Result<(), anyhow::Error> {
     let host = cpal::default_host();
     let device = host.default_output_device().expect("failed to find a default output device");
     let format = device.default_output_format()?;
-    let event_loop = host.event_loop();
-    let stream_id = event_loop.build_output_stream(&device, &format)?;
-    event_loop.play_stream(stream_id.clone())?;
-
     let sample_rate = format.sample_rate.0 as f32;
+    let channels = format.channels;
     let mut sample_clock = 0f32;
 
     // Produce a sinusoid of maximum amplitude.
-    let mut next_value = || {
+    let mut next_value = move || {
         sample_clock = (sample_clock + 1.0) % sample_rate;
         (sample_clock * 440.0 * 2.0 * 3.141592 / sample_rate).sin()
     };
 
-    event_loop.run(move |id, result| {
-        let data = match result {
-            Ok(data) => data,
-            Err(err) => {
-                eprintln!("an error occurred on stream {:?}: {}", id, err);
-                return;
-            }
-        };
-
+    let stream = device.build_output_stream(&format, move |data| {
         match data {
             cpal::StreamData::Output { buffer: cpal::UnknownTypeOutputBuffer::U16(mut buffer) } => {
-                for sample in buffer.chunks_mut(format.channels as usize) {
+                for sample in buffer.chunks_mut(channels as usize) {
                     let value = ((next_value() * 0.5 + 0.5) * std::u16::MAX as f32) as u16;
                     for out in sample.iter_mut() {
                         *out = value;
@@ -39,7 +28,7 @@ fn main() -> Result<(), anyhow::Error> {
                 }
             },
             cpal::StreamData::Output { buffer: cpal::UnknownTypeOutputBuffer::I16(mut buffer) } => {
-                for sample in buffer.chunks_mut(format.channels as usize) {
+                for sample in buffer.chunks_mut(channels as usize) {
                     let value = (next_value() * std::i16::MAX as f32) as i16;
                     for out in sample.iter_mut() {
                         *out = value;
@@ -47,7 +36,7 @@ fn main() -> Result<(), anyhow::Error> {
                 }
             },
             cpal::StreamData::Output { buffer: cpal::UnknownTypeOutputBuffer::F32(mut buffer) } => {
-                for sample in buffer.chunks_mut(format.channels as usize) {
+                for sample in buffer.chunks_mut(channels as usize) {
                     let value = next_value();
                     for out in sample.iter_mut() {
                         *out = value;
@@ -56,5 +45,12 @@ fn main() -> Result<(), anyhow::Error> {
             },
             _ => (),
         }
-    });
+    }, move |err| {
+        eprintln!("an error occurred on stream: {}", err);
+    })?;
+    stream.play()?;
+
+    std::thread::sleep(std::time::Duration::from_millis(1000));
+
+    Ok(())
 }

examples/feedback.rs

@@ -10,18 +10,21 @@ extern crate anyhow;
 extern crate cpal;
 extern crate ringbuf;
 
-use cpal::traits::{DeviceTrait, EventLoopTrait, HostTrait};
+use cpal::traits::{DeviceTrait, HostTrait, StreamTrait};
 use ringbuf::RingBuffer;
 
 const LATENCY_MS: f32 = 150.0;
 
 fn main() -> Result<(), anyhow::Error> {
     let host = cpal::default_host();
-    let event_loop = host.event_loop();
 
     // Default devices.
-    let input_device = host.default_input_device().expect("failed to get default input device");
-    let output_device = host.default_output_device().expect("failed to get default output device");
+    let input_device = host
+        .default_input_device()
+        .expect("failed to get default input device");
+    let output_device = host
+        .default_output_device()
+        .expect("failed to get default output device");
     println!("Using default input device: \"{}\"", input_device.name()?);
     println!("Using default output device: \"{}\"", output_device.name()?);
 
@@ -29,12 +32,6 @@ fn main() -> Result<(), anyhow::Error> {
     let mut format = input_device.default_input_format()?;
     format.data_type = cpal::SampleFormat::F32;
 
-    // Build streams.
-    println!("Attempting to build both streams with `{:?}`.", format);
-    let input_stream_id = event_loop.build_input_stream(&input_device, &format)?;
-    let output_stream_id = event_loop.build_output_stream(&output_device, &format)?;
-    println!("Successfully built streams.");
-
     // Create a delay in case the input and output devices aren't synced.
     let latency_frames = (LATENCY_MS / 1_000.0) * format.sample_rate.0 as f32;
     let latency_samples = latency_frames as usize * format.channels as usize;
@@ -50,25 +47,13 @@ fn main() -> Result<(), anyhow::Error> {
         producer.push(0.0).unwrap();
     }
 
-    // Play the streams.
-    println!("Starting the input and output streams with `{}` milliseconds of latency.", LATENCY_MS);
-    event_loop.play_stream(input_stream_id.clone())?;
-    event_loop.play_stream(output_stream_id.clone())?;
-
-    // Run the event loop on a separate thread.
-    std::thread::spawn(move || {
-        event_loop.run(move |id, result| {
-            let data = match result {
-                Ok(data) => data,
-                Err(err) => {
-                    eprintln!("an error occurred on stream {:?}: {}", id, err);
-                    return;
-                }
-            };
-
+    // Build streams.
+    println!("Attempting to build both streams with `{:?}`.", format);
+    let input_stream = input_device.build_input_stream(&format, move |data| {
         match data {
-                cpal::StreamData::Input { buffer: cpal::UnknownTypeInputBuffer::F32(buffer) } => {
-                    assert_eq!(id, input_stream_id);
+            cpal::StreamData::Input {
+                buffer: cpal::UnknownTypeInputBuffer::F32(buffer),
+            } => {
                 let mut output_fell_behind = false;
                 for &sample in buffer.iter() {
                     if producer.push(sample).is_err() {
@@ -79,8 +64,16 @@ fn main() -> Result<(), anyhow::Error> {
                     eprintln!("output stream fell behind: try increasing latency");
                 }
             },
-                cpal::StreamData::Output { buffer: cpal::UnknownTypeOutputBuffer::F32(mut buffer) } => {
-                    assert_eq!(id, output_stream_id);
+            _ => panic!("Expected input with f32 data"),
+        }
+    }, move |err| {
+        eprintln!("an error occurred on input stream: {}", err);
+    })?;
+    let output_stream = output_device.build_output_stream(&format, move |data| {
+        match data {
+            cpal::StreamData::Output {
+                buffer: cpal::UnknownTypeOutputBuffer::F32(mut buffer),
+            } => {
                 let mut input_fell_behind = None;
                 for sample in buffer.iter_mut() {
                     *sample = match consumer.pop() {
@@ -91,18 +84,30 @@ fn main() -> Result<(), anyhow::Error> {
                         },
                     };
                 }
-                    if let Some(_) = input_fell_behind {
-                        eprintln!("input stream fell behind: try increasing latency");
+                if let Some(err) = input_fell_behind {
+                    eprintln!("input stream fell behind: {:?}: try increasing latency", err);
                 }
             },
-                _ => panic!("we're expecting f32 data"),
+            _ => panic!("Expected output with f32 data"),
         }
-        });
-    });
+    }, move |err| {
+        eprintln!("an error occurred on output stream: {}", err);
+    })?;
+    println!("Successfully built streams.");
+
+    // Play the streams.
+    println!(
+        "Starting the input and output streams with `{}` milliseconds of latency.",
+        LATENCY_MS
+    );
+    input_stream.play()?;
+    output_stream.play()?;
 
     // Run for 3 seconds before closing.
     println!("Playing for 3 seconds... ");
     std::thread::sleep(std::time::Duration::from_secs(3));
+    drop(input_stream);
+    drop(output_stream);
     println!("Done!");
     Ok(())
 }

examples/record_wav.rs

@@ -6,21 +6,21 @@ extern crate anyhow;
 extern crate cpal;
 extern crate hound;
 
-use cpal::traits::{DeviceTrait, EventLoopTrait, HostTrait};
+use cpal::traits::{DeviceTrait, HostTrait, StreamTrait};
 
 fn main() -> Result<(), anyhow::Error> {
     // Use the default host for working with audio devices.
     let host = cpal::default_host();
 
     // Setup the default input device and stream with the default input format.
-    let device = host.default_input_device().expect("Failed to get default input device");
+    let device = host
+        .default_input_device()
+        .expect("Failed to get default input device");
     println!("Default input device: {}", device.name()?);
-    let format = device.default_input_format().expect("Failed to get default input format");
+    let format = device
+        .default_input_format()
+        .expect("Failed to get default input format");
     println!("Default input format: {:?}", format);
-    let event_loop = host.event_loop();
-    let stream_id = event_loop.build_input_stream(&device, &format)?;
-    event_loop.play_stream(stream_id)?;
-
     // The WAV file we're recording to.
     const PATH: &'static str = concat!(env!("CARGO_MANIFEST_DIR"), "/recorded.wav");
     let spec = wav_spec_from_format(&format);
@@ -29,28 +29,15 @@ fn main() -> Result<(), anyhow::Error> {
 
     // A flag to indicate that recording is in progress.
     println!("Begin recording...");
-    let recording = std::sync::Arc::new(std::sync::atomic::AtomicBool::new(true));
 
     // Run the input stream on a separate thread.
     let writer_2 = writer.clone();
-    let recording_2 = recording.clone();
-    std::thread::spawn(move || {
-        event_loop.run(move |id, event| {
-            let data = match event {
-                Ok(data) => data,
-                Err(err) => {
-                    eprintln!("an error occurred on stream {:?}: {}", id, err);
-                    return;
-                }
-            };
-
-            // If we're done recording, return early.
-            if !recording_2.load(std::sync::atomic::Ordering::Relaxed) {
-                return;
-            }
+    let stream = device.build_input_stream(&format, move |data| {
         // Otherwise write to the wav writer.
         match data {
-                cpal::StreamData::Input { buffer: cpal::UnknownTypeInputBuffer::U16(buffer) } => {
+            cpal::StreamData::Input {
+                buffer: cpal::UnknownTypeInputBuffer::U16(buffer),
+            } => {
                 if let Ok(mut guard) = writer_2.try_lock() {
                     if let Some(writer) = guard.as_mut() {
                         for sample in buffer.iter() {
@@ -60,7 +47,9 @@ fn main() -> Result<(), anyhow::Error> {
                     }
                 }
             },
-                cpal::StreamData::Input { buffer: cpal::UnknownTypeInputBuffer::I16(buffer) } => {
+            cpal::StreamData::Input {
+                buffer: cpal::UnknownTypeInputBuffer::I16(buffer),
+            } => {
                 if let Ok(mut guard) = writer_2.try_lock() {
                     if let Some(writer) = guard.as_mut() {
                         for &sample in buffer.iter() {
@@ -69,7 +58,9 @@ fn main() -> Result<(), anyhow::Error> {
                     }
                 }
             },
-                cpal::StreamData::Input { buffer: cpal::UnknownTypeInputBuffer::F32(buffer) } => {
+            cpal::StreamData::Input {
+                buffer: cpal::UnknownTypeInputBuffer::F32(buffer),
+            } => {
                 if let Ok(mut guard) = writer_2.try_lock() {
                     if let Some(writer) = guard.as_mut() {
                         for &sample in buffer.iter() {
@@ -80,12 +71,14 @@ fn main() -> Result<(), anyhow::Error> {
             },
             _ => (),
         }
-        });
-    });
+    }, move |err| {
+        eprintln!("an error occurred on stream: {}", err);
+    })?;
+    stream.play()?;
 
     // Let recording go for roughly three seconds.
     std::thread::sleep(std::time::Duration::from_secs(3));
-    recording.store(false, std::sync::atomic::Ordering::Relaxed);
+    drop(stream);
     writer.lock().unwrap().take().unwrap().finalize()?;
     println!("Recording {} complete!", PATH);
     Ok(())

src/host/alsa/mod.rs

@@ -1,11 +1,14 @@
 extern crate alsa_sys as alsa;
 extern crate libc;
 
-pub use self::enumerate::{Devices, default_input_device, default_output_device};
+use std::{cmp, ffi, io, mem, ptr};
+use std::sync::Arc;
+use std::thread::{self, JoinHandle};
+use std::vec::IntoIter as VecIntoIter;
 
-use ChannelCount;
 use BackendSpecificError;
 use BuildStreamError;
+use ChannelCount;
 use DefaultFormatError;
 use DeviceNameError;
 use DevicesError;
@@ -14,20 +17,15 @@ use PauseStreamError;
 use PlayStreamError;
 use SampleFormat;
 use SampleRate;
-use SupportedFormatsError;
 use StreamData;
-use StreamDataResult;
 use StreamError;
 use SupportedFormat;
+use SupportedFormatsError;
+use traits::{DeviceTrait, HostTrait, StreamTrait};
 use UnknownTypeInputBuffer;
 use UnknownTypeOutputBuffer;
-use traits::{DeviceTrait, EventLoopTrait, HostTrait, StreamIdTrait};
 
-use std::{cmp, ffi, ptr};
-use std::sync::Mutex;
-use std::sync::mpsc::{channel, Sender, Receiver};
-use std::sync::atomic::{AtomicUsize, Ordering};
-use std::vec::IntoIter as VecIntoIter;
+pub use self::enumerate::{default_input_device, default_output_device, Devices};
 
 pub type SupportedInputFormats = VecIntoIter<SupportedFormat>;
 pub type SupportedOutputFormats = VecIntoIter<SupportedFormat>;
@@ -47,7 +45,6 @@ impl Host {
 impl HostTrait for Host {
     type Devices = Devices;
     type Device = Device;
-    type EventLoop = EventLoop;
 
     fn is_available() -> bool {
         // Assume ALSA is always available on linux/freebsd.
@@ -65,15 +62,12 @@ impl HostTrait for Host {
     fn default_output_device(&self) -> Option<Self::Device> {
         default_output_device()
     }
-
-    fn event_loop(&self) -> Self::EventLoop {
-        EventLoop::new()
-    }
 }
 
 impl DeviceTrait for Device {
     type SupportedInputFormats = SupportedInputFormats;
     type SupportedOutputFormats = SupportedOutputFormats;
+    type Stream = Stream;
 
     fn name(&self) -> Result<String, DeviceNameError> {
         Device::name(self)
@@ -94,95 +88,132 @@ impl DeviceTrait for Device {
     fn default_output_format(&self) -> Result<Format, DefaultFormatError> {
         Device::default_output_format(self)
     }
-}
 
-impl EventLoopTrait for EventLoop {
-    type Device = Device;
-    type StreamId = StreamId;
-
-    fn build_input_stream(
-        &self,
-        device: &Self::Device,
-        format: &Format,
-    ) -> Result<Self::StreamId, BuildStreamError> {
-        EventLoop::build_input_stream(self, device, format)
+    fn build_input_stream<D, E>(&self, format: &Format, data_callback: D, error_callback: E) -> Result<Self::Stream, BuildStreamError> where D: FnMut(StreamData) + Send + 'static, E: FnMut(StreamError) + Send + 'static {
+        Ok(Stream::new(Arc::new(self.build_stream_inner(format, alsa::SND_PCM_STREAM_CAPTURE)?), data_callback, error_callback))
     }
 
-    fn build_output_stream(
-        &self,
-        device: &Self::Device,
-        format: &Format,
-    ) -> Result<Self::StreamId, BuildStreamError> {
-        EventLoop::build_output_stream(self, device, format)
-    }
-
-    fn play_stream(&self, stream: Self::StreamId) -> Result<(), PlayStreamError> {
-        EventLoop::play_stream(self, stream)
-    }
-
-    fn pause_stream(&self, stream: Self::StreamId) -> Result<(), PauseStreamError> {
-        EventLoop::pause_stream(self, stream)
-    }
-
-    fn destroy_stream(&self, stream: Self::StreamId) {
-        EventLoop::destroy_stream(self, stream)
-    }
-
-    fn run<F>(&self, callback: F) -> !
-    where
-        F: FnMut(Self::StreamId, StreamDataResult) + Send,
-    {
-        EventLoop::run(self, callback)
+    fn build_output_stream<D, E>(&self, format: &Format, data_callback: D, error_callback: E) -> Result<Self::Stream, BuildStreamError> where D: FnMut(StreamData) + Send + 'static, E: FnMut(StreamError) + Send + 'static {
+        Ok(Stream::new(Arc::new(self.build_stream_inner(format, alsa::SND_PCM_STREAM_PLAYBACK)?), data_callback, error_callback))
     }
 }
 
-impl StreamIdTrait for StreamId {}
 
-struct Trigger {
-    // [read fd, write fd]
-    fds: [libc::c_int; 2],
-}
+struct TriggerSender(libc::c_int);
 
-impl Trigger {
-    fn new() -> Self {
-        let mut fds = [0, 0];
-        match unsafe { libc::pipe(fds.as_mut_ptr()) } {
-            0 => Trigger { fds: fds },
-            _ => panic!("Could not create pipe"),
-        }
-    }
-    fn read_fd(&self) -> libc::c_int {
-        self.fds[0]
-    }
-    fn write_fd(&self) -> libc::c_int {
-        self.fds[1]
-    }
+struct TriggerReceiver(libc::c_int);
+
+impl TriggerSender {
     fn wakeup(&self) {
         let buf = 1u64;
-        let ret = unsafe { libc::write(self.write_fd(), &buf as *const u64 as *const _, 8) };
+        let ret = unsafe { libc::write(self.0, &buf as *const u64 as *const _, 8) };
         assert!(ret == 8);
     }
+}
+
+impl TriggerReceiver {
     fn clear_pipe(&self) {
         let mut out = 0u64;
-        let ret = unsafe { libc::read(self.read_fd(), &mut out as *mut u64 as *mut _, 8) };
+        let ret = unsafe { libc::read(self.0, &mut out as *mut u64 as *mut _, 8) };
         assert_eq!(ret, 8);
     }
 }
 
-impl Drop for Trigger {
+fn trigger() -> (TriggerSender, TriggerReceiver) {
+    let mut fds = [0, 0];
+    match unsafe { libc::pipe(fds.as_mut_ptr()) } {
+        0 => (TriggerSender(fds[1]), TriggerReceiver(fds[0])),
+        _ => panic!("Could not create pipe"),
+    }
+}
+
+impl Drop for TriggerSender {
     fn drop(&mut self) {
         unsafe {
-            libc::close(self.fds[0]);
-            libc::close(self.fds[1]);
+            libc::close(self.0);
         }
     }
 }
 
+impl Drop for TriggerReceiver {
+    fn drop(&mut self) {
+        unsafe {
+            libc::close(self.0);
+        }
+    }
+}
 
 #[derive(Clone, Debug, PartialEq, Eq)]
 pub struct Device(String);
 
 impl Device {
+    fn build_stream_inner(&self, format: &Format, stream_type: alsa::snd_pcm_stream_t) -> Result<StreamInner, BuildStreamError> {
+        let name = ffi::CString::new(self.0.clone()).expect("unable to clone device");
+
+        let handle = unsafe {
+            let mut handle = ptr::null_mut();
+            match alsa::snd_pcm_open(
+                &mut handle,
+                name.as_ptr(),
+                stream_type,
+                alsa::SND_PCM_NONBLOCK,
+            ) {
+                -16 /* determined empirically */ => return Err(BuildStreamError::DeviceNotAvailable),
+                -22 => return Err(BuildStreamError::InvalidArgument),
+                e => if let Err(description) = check_errors(e) {
+                    let err = BackendSpecificError { description };
+                    return Err(err.into());
+                }
+            }
+            handle
+        };
+        let can_pause = unsafe {
+            let hw_params = HwParams::alloc();
+            set_hw_params_from_format(handle, &hw_params, format)
+                .map_err(|description| BackendSpecificError { description })?;
+
+            alsa::snd_pcm_hw_params_can_pause(hw_params.0) == 1
+        };
+        let (buffer_len, period_len) = unsafe {
+            set_sw_params_from_format(handle, format)
+                .map_err(|description| BackendSpecificError { description })?
+        };
+
+        if let Err(desc) = check_errors(unsafe { alsa::snd_pcm_prepare(handle) }) {
+            let description = format!("could not get handle: {}", desc);
+            let err = BackendSpecificError { description };
+            return Err(err.into());
+        }
+
+        let num_descriptors = {
+            let num_descriptors = unsafe { alsa::snd_pcm_poll_descriptors_count(handle) };
+            if num_descriptors == 0 {
+                let description = "poll descriptor count for stream was 0".to_string();
+                let err = BackendSpecificError { description };
+                return Err(err.into());
+            }
+            num_descriptors as usize
+        };
+
+        let stream_inner = StreamInner {
+            channel: handle,
+            sample_format: format.data_type,
+            num_descriptors,
+            num_channels: format.channels as u16,
+            buffer_len,
+            period_len,
+            can_pause,
+        };
+
+        if let Err(desc) = check_errors(unsafe { alsa::snd_pcm_start(handle) }) {
+            let description = format!("could not start stream: {}", desc);
+            let err = BackendSpecificError { description };
+            return Err(err.into());
+        }
+
+        Ok(stream_inner)
+    }
+
     #[inline]
     fn name(&self) -> Result<String, DeviceNameError> {
         Ok(self.0.clone())
@@ -450,48 +481,7 @@ impl Device {
     }
 }
 
-pub struct EventLoop {
-    // Each newly-created stream gets a new ID from this counter. The counter is then incremented.
-    next_stream_id: AtomicUsize, // TODO: use AtomicU64 when stable?
-
-    // A trigger that uses a `pipe()` as backend. Signalled whenever a new command is ready, so
-    // that `poll()` can wake up and pick the changes.
-    pending_command_trigger: Trigger,
-
-    // This field is locked by the `run()` method.
-    // The mutex also ensures that only one thread at a time has `run()` running.
-    run_context: Mutex<RunContext>,
-
-    // Commands processed by the `run()` method that is currently running.
-    commands: Sender<Command>,
-}
-
-unsafe impl Send for EventLoop {
-}
-
-unsafe impl Sync for EventLoop {
-}
-
-enum Command {
-    NewStream(StreamInner),
-    PlayStream(StreamId),
-    PauseStream(StreamId),
-    DestroyStream(StreamId),
-}
-
-struct RunContext {
-    // Descriptors to wait for. Always contains `pending_command_trigger.read_fd()` as first element.
-    descriptors: Vec<libc::pollfd>,
-    // List of streams that are written in `descriptors`.
-    streams: Vec<StreamInner>,
-
-    commands: Receiver<Command>,
-}
-
 struct StreamInner {
-    // The id of the stream.
-    id: StreamId,
-
     // The ALSA channel.
     channel: *mut alsa::snd_pcm_t,
 
@@ -513,209 +503,177 @@ struct StreamInner {
 
     // Whether or not the hardware supports pausing the stream.
     can_pause: bool,
-
-    // Whether or not the sample stream is currently paused.
-    is_paused: bool,
-
-    // A file descriptor opened with `eventfd`.
-    // It is used to wait for resume signal.
-    resume_trigger: Trigger,
-
-    // Lazily allocated buffer that is reused inside the loop.
-    // Zero-allocate a new buffer (the fastest way to have zeroed memory) at the first time this is
-    // used.
-    buffer: Vec<u8>,
 }
 
-#[derive(Copy, Debug, Clone, PartialEq, Eq, Hash)]
-pub struct StreamId(usize);
+// Assume that the ALSA library is built with thread safe option.
+unsafe impl Send for StreamInner {}
+
+unsafe impl Sync for StreamInner {}
 
 enum StreamType { Input, Output }
 
+pub struct Stream {
+    /// The high-priority audio processing thread calling callbacks.
+    /// Option used for moving out in destructor.
+    thread: Option<JoinHandle<()>>,
 
-impl EventLoop {
-    #[inline]
-    fn new() -> EventLoop {
-        let pending_command_trigger = Trigger::new();
+    /// Handle to the underlying stream for playback controls.
+    inner: Arc<StreamInner>,
 
-        let mut initial_descriptors = vec![];
-        reset_descriptors_with_pending_command_trigger(
-            &mut initial_descriptors,
-            &pending_command_trigger,
-        );
+    /// Used to signal to stop processing.
+    trigger: TriggerSender,
+}
 
-        let (tx, rx) = channel();
-
-        let run_context = Mutex::new(RunContext {
-                                         descriptors: initial_descriptors,
-                                         streams: Vec::new(),
-                                         commands: rx,
+/// The inner body of the audio processing thread. Takes the polymorphic
+/// callback to avoid generating too much generic code.
+fn stream_worker(rx: TriggerReceiver,
+                 stream: &StreamInner,
+                 data_callback: &mut (dyn FnMut(StreamData) + Send + 'static),
+                 error_callback: &mut (dyn FnMut(StreamError) + Send + 'static)) {
+    let mut descriptors = Vec::new();
+    let mut buffer = Vec::new();
+    loop {
+        descriptors.clear();
+        // Add the self-pipe for signaling termination.
+        descriptors.push(libc::pollfd {
+            fd: rx.0,
+            events: libc::POLLIN,
+            revents: 0,
         });
 
-        EventLoop {
-            next_stream_id: AtomicUsize::new(0),
-            pending_command_trigger: pending_command_trigger,
-            run_context,
-            commands: tx,
-        }
-    }
-
-    #[inline]
-    fn run<F>(&self, mut callback: F) -> !
-        where F: FnMut(StreamId, StreamDataResult)
-    {
-        self.run_inner(&mut callback)
-    }
-
-    fn run_inner(&self, callback: &mut dyn FnMut(StreamId, StreamDataResult)) -> ! {
+        // Add ALSA polling fds.
+        descriptors.reserve(stream.num_descriptors);
+        let len = descriptors.len();
+        let filled = unsafe {
+            alsa::snd_pcm_poll_descriptors(
+                stream.channel,
+                descriptors[len..].as_mut_ptr(),
+                stream.num_descriptors as libc::c_uint,
+            )
+        };
+        debug_assert_eq!(filled, stream.num_descriptors as libc::c_int);
         unsafe {
-            let mut run_context = self.run_context.lock().unwrap();
-            let run_context = &mut *run_context;
-
-            'stream_loop: loop {
-                process_commands(run_context);
-
-                reset_descriptors_with_pending_command_trigger(
-                    &mut run_context.descriptors,
-                    &self.pending_command_trigger,
-                );
-                append_stream_poll_descriptors(run_context);
-
-                // At this point, this should include the command `pending_commands_trigger` along
-                // with the poll descriptors for each stream.
-                match poll_all_descriptors(&mut run_context.descriptors) {
-                    Ok(true) => (),
-                    Ok(false) => continue,
-                    Err(err) => {
-                        for stream in run_context.streams.iter() {
-                            let result = Err(err.clone().into());
-                            callback(stream.id, result);
-                        }
-                        run_context.streams.clear();
-                        break 'stream_loop;
-                    }
+            descriptors.set_len(len + stream.num_descriptors);
         }
 
-                // If the `pending_command_trigger` was signaled, we need to process the comands.
-                if run_context.descriptors[0].revents != 0 {
-                    run_context.descriptors[0].revents = 0;
-                    self.pending_command_trigger.clear_pipe();
+        let res = unsafe {
+            // Don't timeout, wait forever.
+            libc::poll(descriptors.as_mut_ptr(), descriptors.len() as libc::nfds_t, -1)
+        };
+        if res < 0 {
+            let description = format!("`libc::poll()` failed: {}", io::Error::last_os_error());
+            error_callback(BackendSpecificError { description }.into());
+            continue;
+        } else if res == 0 {
+            let description = String::from("`libc::poll()` spuriously returned");
+            error_callback(BackendSpecificError { description }.into());
+            continue;
         }
 
-                // The set of streams that error within the following loop and should be removed.
-                let mut streams_to_remove: Vec<(StreamId, StreamError)> = vec![];
+        if descriptors[0].revents != 0 {
+            // The stream has been requested to be destroyed.
+            rx.clear_pipe();
+            return;
+        }
 
-                // Iterate over each individual stream/descriptor.
-                let mut i_stream = 0;
-                let mut i_descriptor = 1;
-                while (i_descriptor as usize) < run_context.descriptors.len() {
-                    let stream = &mut run_context.streams[i_stream];
-                    let stream_descriptor_ptr = run_context.descriptors.as_mut_ptr().offset(i_descriptor);
-
-                    // Only go on if this event was a pollout or pollin event.
-                    let stream_type = match check_for_pollout_or_pollin(stream, stream_descriptor_ptr) {
+        let stream_type = match check_for_pollout_or_pollin(stream, descriptors[1..].as_mut_ptr()) {
             Ok(Some(ty)) => ty,
             Ok(None) => {
-                            i_descriptor += stream.num_descriptors as isize;
-                            i_stream += 1;
+                // Nothing to process, poll again
                 continue;
             },
             Err(err) => {
-                            streams_to_remove.push((stream.id, err.into()));
-                            i_descriptor += stream.num_descriptors as isize;
-                            i_stream += 1;
+                error_callback(err.into());
                 continue;
             }
         };
-
         // Get the number of available samples for reading/writing.
         let available_samples = match get_available_samples(stream) {
             Ok(n) => n,
             Err(err) => {
-                            streams_to_remove.push((stream.id, err.into()));
-                            i_descriptor += stream.num_descriptors as isize;
-                            i_stream += 1;
+                let description = format!("Failed to query the number of available samples: {}", err);
+                error_callback(BackendSpecificError { description }.into());
                 continue;
             }
         };
 
         // Only go on if there is at least `stream.period_len` samples.
         if available_samples < stream.period_len {
-                        i_descriptor += stream.num_descriptors as isize;
-                        i_stream += 1;
             continue;
         }
 
         // Prepare the data buffer.
         let buffer_size = stream.sample_format.sample_size() * available_samples;
-                    stream.buffer.resize(buffer_size, 0u8);
+        buffer.resize(buffer_size, 0u8);
         let available_frames = available_samples / stream.num_channels as usize;
 
         match stream_type {
             StreamType::Input => {
-                            let result = alsa::snd_pcm_readi(
+                let result = unsafe {
+                    alsa::snd_pcm_readi(
                         stream.channel,
-                                stream.buffer.as_mut_ptr() as *mut _,
+                        buffer.as_mut_ptr() as *mut _,
                         available_frames as alsa::snd_pcm_uframes_t,
-                            );
+                    )
+                };
                 if let Err(err) = check_errors(result as _) {
                     let description = format!("`snd_pcm_readi` failed: {}", err);
-                                let err = BackendSpecificError { description };
-                                streams_to_remove.push((stream.id, err.into()));
+                    error_callback(BackendSpecificError { description }.into());
                     continue;
                 }
 
                 let input_buffer = match stream.sample_format {
                     SampleFormat::I16 => UnknownTypeInputBuffer::I16(::InputBuffer {
-                                    buffer: cast_input_buffer(&mut stream.buffer),
+                        buffer: unsafe { cast_input_buffer(&mut buffer) },
                     }),
                     SampleFormat::U16 => UnknownTypeInputBuffer::U16(::InputBuffer {
-                                    buffer: cast_input_buffer(&mut stream.buffer),
+                        buffer: unsafe { cast_input_buffer(&mut buffer) },
                     }),
                     SampleFormat::F32 => UnknownTypeInputBuffer::F32(::InputBuffer {
-                                    buffer: cast_input_buffer(&mut stream.buffer),
+                        buffer: unsafe { cast_input_buffer(&mut buffer) },
                     }),
                 };
                 let stream_data = StreamData::Input {
                     buffer: input_buffer,
                 };
-                            callback(stream.id, Ok(stream_data));
+                data_callback(stream_data);
             },
             StreamType::Output => {
                 {
                     // We're now sure that we're ready to write data.
                     let output_buffer = match stream.sample_format {
                         SampleFormat::I16 => UnknownTypeOutputBuffer::I16(::OutputBuffer {
-                                        buffer: cast_output_buffer(&mut stream.buffer),
+                            buffer: unsafe { cast_output_buffer(&mut buffer) },
                         }),
                         SampleFormat::U16 => UnknownTypeOutputBuffer::U16(::OutputBuffer {
-                                        buffer: cast_output_buffer(&mut stream.buffer),
+                            buffer: unsafe { cast_output_buffer(&mut buffer) },
                         }),
                         SampleFormat::F32 => UnknownTypeOutputBuffer::F32(::OutputBuffer {
-                                        buffer: cast_output_buffer(&mut stream.buffer),
+                            buffer: unsafe { cast_output_buffer(&mut buffer) },
                         }),
                     };
 
                     let stream_data = StreamData::Output {
                         buffer: output_buffer,
                     };
-                                callback(stream.id, Ok(stream_data));
+                    data_callback(stream_data);
                 }
                 loop {
-                                let result = alsa::snd_pcm_writei(
+                    let result = unsafe {
+                        alsa::snd_pcm_writei(
                             stream.channel,
-                                    stream.buffer.as_ptr() as *const _,
+                            buffer.as_ptr() as *const _,
                             available_frames as alsa::snd_pcm_uframes_t,
-                                );
+                        )
+                    };
 
-                                if result as i32 == -libc::EPIPE {
+                    if result == -libc::EPIPE as i64 {
                         // buffer underrun
                         // TODO: Notify the user of this.
-                                    alsa::snd_pcm_recover(stream.channel, result as i32, 0);
+                        unsafe { alsa::snd_pcm_recover(stream.channel, result as i32, 0) };
                     } else if let Err(err) = check_errors(result as _) {
                         let description = format!("`snd_pcm_writei` failed: {}", err);
-                                    let err = BackendSpecificError { description };
-                                    streams_to_remove.push((stream.id, err.into()));
+                        error_callback(BackendSpecificError { description }.into());
                         continue;
                     } else if result as usize != available_frames {
                         let description = format!(
@@ -724,8 +682,7 @@ impl EventLoop {
                             available_frames,
                             result,
                         );
-                                    let err = BackendSpecificError { description };
-                                    streams_to_remove.push((stream.id, err.into()));
+                        error_callback(BackendSpecificError { description }.into());
                         continue;
                     } else {
                         break;
@@ -734,280 +691,46 @@ impl EventLoop {
             },
         }
     }
-
-                // Remove any streams that have errored and notify the user.
-                for (stream_id, err) in streams_to_remove {
-                    run_context.streams.retain(|s| s.id != stream_id);
-                    callback(stream_id, Err(err.into()));
-                }
-            }
-        }
-
-        panic!("`cpal::EventLoop::run` API currently disallows returning");
-    }
-
-    fn build_input_stream(
-        &self,
-        device: &Device,
-        format: &Format,
-    ) -> Result<StreamId, BuildStreamError>
-    {
-        unsafe {
-            let name = ffi::CString::new(device.0.clone()).expect("unable to clone device");
-
-            let mut capture_handle = ptr::null_mut();
-            match alsa::snd_pcm_open(
-                &mut capture_handle,
-                name.as_ptr(),
-                alsa::SND_PCM_STREAM_CAPTURE,
-                alsa::SND_PCM_NONBLOCK,
-            ) {
-                -16 /* determined empirically */ => return Err(BuildStreamError::DeviceNotAvailable),
-                -22 => return Err(BuildStreamError::InvalidArgument),
-                e => if let Err(description) = check_errors(e) {
-                    let err = BackendSpecificError { description };
-                    return Err(err.into());
-                }
-            }
-            let hw_params = HwParams::alloc();
-
-            set_hw_params_from_format(capture_handle, &hw_params, format)
-                .map_err(|description| BackendSpecificError { description })?;
-
-            let can_pause = alsa::snd_pcm_hw_params_can_pause(hw_params.0) == 1;
-
-            let (buffer_len, period_len) = set_sw_params_from_format(capture_handle, format)
-                .map_err(|description| BackendSpecificError { description })?;
-
-            if let Err(desc) = check_errors(alsa::snd_pcm_prepare(capture_handle)) {
-                let description = format!("could not get capture handle: {}", desc);
-                let err = BackendSpecificError { description };
-                return Err(err.into());
-            }
-
-            let num_descriptors = {
-                let num_descriptors = alsa::snd_pcm_poll_descriptors_count(capture_handle);
-                if num_descriptors == 0 {
-                    let description = "poll descriptor count for capture stream was 0".to_string();
-                    let err = BackendSpecificError { description };
-                    return Err(err.into());
-                }
-                num_descriptors as usize
-            };
-
-            let new_stream_id = StreamId(self.next_stream_id.fetch_add(1, Ordering::Relaxed));
-            if new_stream_id.0 == usize::max_value() {
-                panic!("number of streams used has overflowed usize");
-            }
-
-            let stream_inner = StreamInner {
-                id: new_stream_id.clone(),
-                channel: capture_handle,
-                sample_format: format.data_type,
-                num_descriptors: num_descriptors,
-                num_channels: format.channels as u16,
-                buffer_len: buffer_len,
-                period_len: period_len,
-                can_pause: can_pause,
-                is_paused: false,
-                resume_trigger: Trigger::new(),
-                buffer: vec![],
-            };
-
-            if let Err(desc) = check_errors(alsa::snd_pcm_start(capture_handle)) {
-                let description = format!("could not start capture stream: {}", desc);
-                let err = BackendSpecificError { description };
-                return Err(err.into());
-            }
-
-            self.push_command(Command::NewStream(stream_inner));
-            Ok(new_stream_id)
-        }
-    }
-
-    fn build_output_stream(
-        &self,
-        device: &Device,
-        format: &Format,
-    ) -> Result<StreamId, BuildStreamError>
-    {
-        unsafe {
-            let name = ffi::CString::new(device.0.clone()).expect("unable to clone device");
-
-            let mut playback_handle = ptr::null_mut();
-            match alsa::snd_pcm_open(
-                &mut playback_handle,
-                name.as_ptr(),
-                alsa::SND_PCM_STREAM_PLAYBACK,
-                alsa::SND_PCM_NONBLOCK,
-            ) {
-                -16 /* determined empirically */ => return Err(BuildStreamError::DeviceNotAvailable),
-                -22 => return Err(BuildStreamError::InvalidArgument),
-                e => if let Err(description) = check_errors(e) {
-                    let err = BackendSpecificError { description };
-                    return Err(err.into())
-                }
-            }
-            let hw_params = HwParams::alloc();
-
-            set_hw_params_from_format(playback_handle, &hw_params, format)
-                .map_err(|description| BackendSpecificError { description })?;
-
-            let can_pause = alsa::snd_pcm_hw_params_can_pause(hw_params.0) == 1;
-
-            let (buffer_len, period_len) = set_sw_params_from_format(playback_handle, format)
-                .map_err(|description| BackendSpecificError { description })?;
-
-            if let Err(desc) = check_errors(alsa::snd_pcm_prepare(playback_handle)) {
-                let description = format!("could not get playback handle: {}", desc);
-                let err = BackendSpecificError { description };
-                return Err(err.into());
-            }
-
-            let num_descriptors = {
-                let num_descriptors = alsa::snd_pcm_poll_descriptors_count(playback_handle);
-                if num_descriptors == 0 {
-                    let description = "poll descriptor count for playback stream was 0".to_string();
-                    let err = BackendSpecificError { description };
-                    return Err(err.into());
-                }
-                num_descriptors as usize
-            };
-
-            let new_stream_id = StreamId(self.next_stream_id.fetch_add(1, Ordering::Relaxed));
-            if new_stream_id.0 == usize::max_value() {
-                return Err(BuildStreamError::StreamIdOverflow);
-            }
-
-            let stream_inner = StreamInner {
-                id: new_stream_id.clone(),
-                channel: playback_handle,
-                sample_format: format.data_type,
-                num_descriptors: num_descriptors,
-                num_channels: format.channels as u16,
-                buffer_len: buffer_len,
-                period_len: period_len,
-                can_pause: can_pause,
-                is_paused: false,
-                resume_trigger: Trigger::new(),
-                buffer: vec![],
-            };
-
-            self.push_command(Command::NewStream(stream_inner));
-            Ok(new_stream_id)
-        }
-    }
-
-    #[inline]
-    fn push_command(&self, command: Command) {
-        // Safe to unwrap: sender outlives receiver.
-        self.commands.send(command).unwrap();
-        self.pending_command_trigger.wakeup();
-    }
-
-    #[inline]
-    fn destroy_stream(&self, stream_id: StreamId) {
-        self.push_command(Command::DestroyStream(stream_id));
-    }
-
-    #[inline]
-    fn play_stream(&self, stream_id: StreamId) -> Result<(), PlayStreamError> {
-        self.push_command(Command::PlayStream(stream_id));
-        Ok(())
-    }
-
-    #[inline]
-    fn pause_stream(&self, stream_id: StreamId) -> Result<(), PauseStreamError> {
-        self.push_command(Command::PauseStream(stream_id));
-        Ok(())
-    }
 }
 
-// Process any pending `Command`s within the `RunContext`'s queue.
-fn process_commands(run_context: &mut RunContext) {
-    for command in run_context.commands.try_iter() {
-        match command {
-            Command::DestroyStream(stream_id) => {
-                run_context.streams.retain(|s| s.id != stream_id);
-            },
-            Command::PlayStream(stream_id) => {
-                if let Some(stream) = run_context.streams.iter_mut()
-                    .find(|stream| stream.can_pause && stream.id == stream_id)
-                {
-                    unsafe {
-                        alsa::snd_pcm_pause(stream.channel, 0);
-                    }
-                    stream.is_paused = false;
-                }
-            },
-            Command::PauseStream(stream_id) => {
-                if let Some(stream) = run_context.streams.iter_mut()
-                    .find(|stream| stream.can_pause && stream.id == stream_id)
-                {
-                    unsafe {
-                        alsa::snd_pcm_pause(stream.channel, 1);
-                    }
-                    stream.is_paused = true;
-                }
-            },
-            Command::NewStream(stream_inner) => {
-                run_context.streams.push(stream_inner);
-            },
-        }
-    }
-}
-
-// Resets the descriptors so that only `pending_command_trigger.read_fd()` is contained.
-fn reset_descriptors_with_pending_command_trigger(
-    descriptors: &mut Vec<libc::pollfd>,
-    pending_command_trigger: &Trigger,
-) {
-    descriptors.clear();
-    descriptors.push(libc::pollfd {
-        fd: pending_command_trigger.read_fd(),
-        events: libc::POLLIN,
-        revents: 0,
+impl Stream {
+    fn new<D, E>(inner: Arc<StreamInner>, mut data_callback: D, mut error_callback: E) -> Stream
+        where D: FnMut(StreamData) + Send + 'static, E: FnMut(StreamError) + Send + 'static {
+        let (tx, rx) = trigger();
+        // Clone the handle for passing into worker thread.
+        let stream = inner.clone();
+        let thread = thread::spawn(move || {
+            stream_worker(rx, &*stream, &mut data_callback, &mut error_callback);
         });
+        Stream {
+            thread: Some(thread),
+            inner,
+            trigger: tx,
+        }
+    }
 }
 
-// Appends the `poll` descriptors for each stream onto the `RunContext`'s descriptor slice, ready
-// for a call to `libc::poll`.
-fn append_stream_poll_descriptors(run_context: &mut RunContext) {
-    for stream in run_context.streams.iter() {
-        run_context.descriptors.reserve(stream.num_descriptors);
-        let len = run_context.descriptors.len();
-        let filled = unsafe {
-            alsa::snd_pcm_poll_descriptors(
-                stream.channel,
-                run_context.descriptors.as_mut_ptr().offset(len as isize),
-                stream.num_descriptors as libc::c_uint,
-            )
-        };
-        debug_assert_eq!(filled, stream.num_descriptors as libc::c_int);
+impl Drop for Stream {
+    fn drop(&mut self) {
+        self.trigger.wakeup();
+        self.thread.take().unwrap().join().unwrap();
+    }
+}
+
+impl StreamTrait for Stream {
+    fn play(&self) -> Result<(), PlayStreamError> {
         unsafe {
-            run_context.descriptors.set_len(len + stream.num_descriptors);
+            alsa::snd_pcm_pause(self.inner.channel, 0);
         }
+        // TODO: error handling
+        Ok(())
     }
-}
-
-// Poll all descriptors within the given set.
-//
-// Returns `Ok(true)` if some event has occurred or `Ok(false)` if no events have
-// occurred.
-//
-// Returns an `Err` if `libc::poll` returns a negative value for some reason.
-fn poll_all_descriptors(descriptors: &mut [libc::pollfd]) -> Result<bool, BackendSpecificError> {
-    let res = unsafe {
-        // Don't timeout, wait forever.
-        libc::poll(descriptors.as_mut_ptr(), descriptors.len() as libc::nfds_t, -1)
-    };
-    if res < 0 {
-        let description = format!("`libc::poll()` failed: {}", res);
-        Err(BackendSpecificError { description })
-    } else if res == 0 {
-        Ok(false)
-    } else {
-        Ok(true)
+    fn pause(&self)-> Result<(), PauseStreamError> {
+        unsafe {
+            alsa::snd_pcm_pause(self.inner.channel, 1);
+        }
+        // TODO: error handling
+        Ok(())
     }
 }
 

src/host/asio/device.rs

@@ -3,7 +3,7 @@ pub type SupportedInputFormats = std::vec::IntoIter<SupportedFormat>;
 pub type SupportedOutputFormats = std::vec::IntoIter<SupportedFormat>;
 
 use std::hash::{Hash, Hasher};
-use std::sync::Arc;
+use std::sync::{Arc};
 use BackendSpecificError;
 use DefaultFormatError;
 use DeviceNameError;
@@ -14,12 +14,17 @@ use SampleRate;
 use SupportedFormat;
 use SupportedFormatsError;
 use super::sys;
+use super::parking_lot::Mutex;
 
 /// A ASIO Device
-#[derive(Debug)]
 pub struct Device {
     /// The driver represented by this device.
     pub driver: Arc<sys::Driver>,
+
+    // Input and/or Output stream.
+    // An driver can only have one of each.
+    // They need to be created at the same time.
+    pub asio_streams: Arc<Mutex<sys::AsioStreams>>,
 }
 
 /// All available devices.
@@ -148,7 +153,14 @@ impl Iterator for Devices {
         loop {
             match self.drivers.next() {
                 Some(name) => match self.asio.load_driver(&name) {
-                    Ok(driver) => return Some(Device { driver: Arc::new(driver) }),
+                    Ok(driver) => {
+                        let driver = Arc::new(driver);
+                        let asio_streams = Arc::new(Mutex::new(sys::AsioStreams {
+                            input: None,
+                            output: None,
+                        }));
+                        return Some(Device { driver, asio_streams });
+                    }
                     Err(_) => continue,
                 }
                 None => return None,

src/host/asio/mod.rs

@@ -1,4 +1,5 @@
 extern crate asio_sys as sys;
+extern crate parking_lot;
 
 use {
     BuildStreamError,
@@ -8,18 +9,18 @@ use {
     Format,
     PauseStreamError,
     PlayStreamError,
-    StreamDataResult,
     SupportedFormatsError,
+    StreamData,
+    StreamError,
 };
 use traits::{
     DeviceTrait,
-    EventLoopTrait,
     HostTrait,
-    StreamIdTrait,
+    StreamTrait,
 };
 
 pub use self::device::{Device, Devices, SupportedInputFormats, SupportedOutputFormats};
-pub use self::stream::{EventLoop, StreamId};
+pub use self::stream::Stream;
 use std::sync::Arc;
 
 mod device;
@@ -42,7 +43,6 @@ impl Host {
 impl HostTrait for Host {
     type Devices = Devices;
     type Device = Device;
-    type EventLoop = EventLoop;
 
     fn is_available() -> bool {
         true
@@ -62,15 +62,12 @@ impl HostTrait for Host {
         // ASIO has no concept of a default device, so just use the first.
         self.output_devices().ok().and_then(|mut ds| ds.next())
     }
-
-    fn event_loop(&self) -> Self::EventLoop {
-        EventLoop::new()
-    }
 }
 
 impl DeviceTrait for Device {
     type SupportedInputFormats = SupportedInputFormats;
     type SupportedOutputFormats = SupportedOutputFormats;
+    type Stream = Stream;
 
     fn name(&self) -> Result<String, DeviceNameError> {
         Device::name(self)
@@ -91,46 +88,28 @@ impl DeviceTrait for Device {
     fn default_output_format(&self) -> Result<Format, DefaultFormatError> {
         Device::default_output_format(self)
     }
-}
 
-impl EventLoopTrait for EventLoop {
-    type Device = Device;
-    type StreamId = StreamId;
-
-    fn build_input_stream(
-        &self,
-        device: &Self::Device,
-        format: &Format,
-    ) -> Result<Self::StreamId, BuildStreamError> {
-        EventLoop::build_input_stream(self, device, format)
-    }
-
-    fn build_output_stream(
-        &self,
-        device: &Self::Device,
-        format: &Format,
-    ) -> Result<Self::StreamId, BuildStreamError> {
-        EventLoop::build_output_stream(self, device, format)
-    }
-
-    fn play_stream(&self, stream: Self::StreamId) -> Result<(), PlayStreamError> {
-        EventLoop::play_stream(self, stream)
-    }
-
-    fn pause_stream(&self, stream: Self::StreamId) -> Result<(), PauseStreamError> {
-        EventLoop::pause_stream(self, stream)
-    }
-
-    fn destroy_stream(&self, stream: Self::StreamId) {
-        EventLoop::destroy_stream(self, stream)
-    }
-
-    fn run<F>(&self, callback: F) -> !
+    fn build_input_stream<D, E>(&self, format: &Format, data_callback: D, error_callback: E) -> Result<Self::Stream, BuildStreamError>
     where
-        F: FnMut(Self::StreamId, StreamDataResult) + Send,
+        D: FnMut(StreamData) + Send + 'static, E: FnMut(StreamError) + Send + 'static
     {
-        EventLoop::run(self, callback)
+        Device::build_input_stream(self, format, data_callback, error_callback)
+    }
+
+    fn build_output_stream<D, E>(&self, format: &Format, data_callback: D, error_callback: E) -> Result<Self::Stream, BuildStreamError>
+    where
+        D: FnMut(StreamData) + Send + 'static, E: FnMut(StreamError) + Send + 'static
+    {
+        Device::build_output_stream(self, format, data_callback, error_callback)
     }
 }
 
-impl StreamIdTrait for StreamId {}
+impl StreamTrait for Stream {
+    fn play(&self) -> Result<(), PlayStreamError> {
+        Stream::play(self)
+    }
+
+    fn pause(&self) -> Result<(), PauseStreamError> {
+        Stream::pause(self)
+    }
+}

src/host/asio/stream.rs

@@ -4,11 +4,9 @@ extern crate num_traits;
 use self::num_traits::PrimInt;
 use super::Device;
 use std;
-use std::mem;
-use std::sync::atomic::{AtomicUsize, Ordering};
-use std::sync::{Arc, Mutex};
-use std::thread;
-use std::time::Duration;
+use std::sync::atomic::{Ordering, AtomicBool};
+use std::sync::Arc;
+use super::parking_lot::Mutex;
 use BackendSpecificError;
 use BuildStreamError;
 use Format;
@@ -16,9 +14,9 @@ use PauseStreamError;
 use PlayStreamError;
 use SampleFormat;
 use StreamData;
-use StreamDataResult;
 use UnknownTypeInputBuffer;
 use UnknownTypeOutputBuffer;
+use StreamError;
 
 /// Sample types whose constant silent value is known.
 trait Silence {
@@ -34,35 +32,6 @@ trait InterleavedSample: Clone + Copy + Silence {
 /// Constraints on the ASIO sample types.
 trait AsioSample: Clone + Copy + Silence + std::ops::Add<Self, Output = Self> {}
 
-/// Controls all streams
-pub struct EventLoop {
-    /// The input and output ASIO streams
-    asio_streams: Arc<Mutex<sys::AsioStreams>>,
-    /// List of all CPAL streams
-    cpal_streams: Arc<Mutex<Vec<Option<Stream>>>>,
-    /// Total stream count.
-    stream_count: AtomicUsize,
-    /// The CPAL callback that the user gives to fill the buffers.
-    callbacks: Arc<Mutex<Option<&'static mut (FnMut(StreamId, StreamDataResult) + Send)>>>,
-}
-
-/// Id for each stream.
-/// Created depending on the number they are created.
-/// Starting at one! not zero.
-#[derive(Clone, Copy, Debug, PartialEq, Eq, Hash)]
-pub struct StreamId(usize);
-
-/// CPAL stream.
-/// This decouples the many cpal streams
-/// from the single input and single output
-/// ASIO streams.
-/// Each stream can be playing or paused.
-struct Stream {
-    playing: bool,
-    // The driver associated with this stream.
-    driver: Arc<sys::Driver>,
-}
-
 // Used to keep track of whether or not the current current asio stream buffer requires
 // being silencing before summing audio.
 #[derive(Default)]
@@ -71,114 +40,38 @@ struct SilenceAsioBuffer {
     second: bool,
 }
 
-impl EventLoop {
-    pub fn new() -> EventLoop {
-        EventLoop {
-            asio_streams: Arc::new(Mutex::new(sys::AsioStreams {
-                input: None,
-                output: None,
-            })),
-            cpal_streams: Arc::new(Mutex::new(Vec::new())),
-            // This is why the Id's count from one not zero
-            // because at this point there is no streams
-            stream_count: AtomicUsize::new(0),
-            callbacks: Arc::new(Mutex::new(None)),
-        }
+pub struct Stream {
+    playing: Arc<AtomicBool>,
+    // Ensure the `Driver` does not terminate until the last stream is dropped.
+    driver: Arc<sys::Driver>,
+    asio_streams: Arc<Mutex<sys::AsioStreams>>,
+    callback_id: sys::CallbackId,
+}
+
+impl Stream {
+    pub fn play(&self) -> Result<(), PlayStreamError> {
+        self.playing.store(true, Ordering::SeqCst);
+        Ok(())
     }
 
-    /// Create a new CPAL Input Stream.
-    ///
-    /// If there is no existing ASIO Input Stream it will be created.
-    ///
-    /// On success, the buffer size of the stream is returned.
-    fn get_or_create_input_stream(
-        &self,
-        driver: &sys::Driver,
-        format: &Format,
-        device: &Device,
-    ) -> Result<usize, BuildStreamError> {
-        match device.default_input_format() {
-            Ok(f) => {
-                let num_asio_channels = f.channels;
-                check_format(driver, format, num_asio_channels)
-            },
-            Err(_) => Err(BuildStreamError::FormatNotSupported),
-        }?;
-        let num_channels = format.channels as usize;
-        let ref mut streams = *self.asio_streams.lock().unwrap();
-        // Either create a stream if thers none or had back the
-        // size of the current one.
-        match streams.input {
-            Some(ref input) => Ok(input.buffer_size as usize),
-            None => {
-                let output = streams.output.take();
-                driver
-                    .prepare_input_stream(output, num_channels)
-                    .map(|new_streams| {
-                        let bs = match new_streams.input {
-                            Some(ref inp) => inp.buffer_size as usize,
-                            None => unreachable!(),
-                        };
-                        *streams = new_streams;
-                        bs
-                    }).map_err(|ref e| {
-                        println!("Error preparing stream: {}", e);
-                        BuildStreamError::DeviceNotAvailable
-                    })
-            }
-        }
+    pub fn pause(&self) -> Result<(), PauseStreamError> {
+        self.playing.store(false, Ordering::SeqCst);
+        Ok(())
     }
+}
 
-    /// Create a new CPAL Output Stream.
-    ///
-    /// If there is no existing ASIO Output Stream it will be created.
-    ///
-    /// On success, the buffer size of the stream is returned.
-    fn get_or_create_output_stream(
+impl Device {
+    pub fn build_input_stream<D, E>(
         &self,
-        driver: &sys::Driver,
         format: &Format,
-        device: &Device,
-    ) -> Result<usize, BuildStreamError> {
-        match device.default_output_format() {
-            Ok(f) => {
-                let num_asio_channels = f.channels;
-                check_format(driver, format, num_asio_channels)
-            },
-            Err(_) => Err(BuildStreamError::FormatNotSupported),
-        }?;
-        let num_channels = format.channels as usize;
-        let ref mut streams = *self.asio_streams.lock().unwrap();
-        // Either create a stream if there's none or return the size of the current one.
-        match streams.output {
-            Some(ref output) => Ok(output.buffer_size as usize),
-            None => {
-                let input = streams.input.take();
-                driver
-                    .prepare_output_stream(input, num_channels)
-                    .map(|new_streams| {
-                        let bs = match new_streams.output {
-                            Some(ref out) => out.buffer_size as usize,
-                            None => unreachable!(),
-                        };
-                        *streams = new_streams;
-                        bs
-                    }).map_err(|ref e| {
-                        println!("Error preparing stream: {}", e);
-                        BuildStreamError::DeviceNotAvailable
-                    })
-            }
-        }
-    }
-
-    /// Builds a new cpal input stream
-    pub fn build_input_stream(
-        &self,
-        device: &Device,
-        format: &Format,
-    ) -> Result<StreamId, BuildStreamError> {
-        let Device { driver, .. } = device;
-        let stream_type = driver.input_data_type().map_err(build_stream_err)?;
+        mut data_callback: D,
+        _error_callback: E,
+    ) -> Result<Stream, BuildStreamError>
+    where
+        D: FnMut(StreamData) + Send + 'static,
+        E: FnMut(StreamError) + Send + 'static
+    {
+        let stream_type = self.driver.input_data_type().map_err(build_stream_err)?;
 
         // Ensure that the desired sample type is supported.
         let data_type = super::device::convert_data_type(&stream_type)
@@ -188,48 +81,36 @@ impl EventLoop {
         }
 
         let num_channels = format.channels.clone();
-        let stream_buffer_size = self.get_or_create_input_stream(&driver, format, device)?;
-        let cpal_num_samples = stream_buffer_size * num_channels as usize;
-        let count = self.stream_count.fetch_add(1, Ordering::SeqCst);
-        let asio_streams = self.asio_streams.clone();
-        let cpal_streams = self.cpal_streams.clone();
-        let callbacks = self.callbacks.clone();
+        let buffer_size = self.get_or_create_input_stream(format)?;
+        let cpal_num_samples = buffer_size * num_channels as usize;
 
         // Create the buffer depending on the size of the data type.
-        let stream_id = StreamId(count);
         let len_bytes = cpal_num_samples * data_type.sample_size();
         let mut interleaved = vec![0u8; len_bytes];
 
+        let stream_playing = Arc::new(AtomicBool::new(false));
+        let playing = Arc::clone(&stream_playing);
+        let asio_streams = self.asio_streams.clone();
+
         // Set the input callback.
         // This is most performance critical part of the ASIO bindings.
-        driver.set_callback(move |buffer_index| unsafe {
+        let callback_id = self.driver.add_callback(move |buffer_index| unsafe {
             // If not playing return early.
-            // TODO: Don't assume `count` is valid - we should search for the matching `StreamId`.
-            if let Some(s) = cpal_streams.lock().unwrap().get(count) {
-                if let Some(s) = s {
-                    if !s.playing {
-                        return;
-                    }
-                }
+            if !playing.load(Ordering::SeqCst) {
+                return
             }
 
-            // Acquire the stream and callback.
-            let stream_lock = asio_streams.lock().unwrap();
+            // There is 0% chance of lock contention the host only locks when recreating streams.
+            let stream_lock = asio_streams.lock();
             let ref asio_stream = match stream_lock.input {
                 Some(ref asio_stream) => asio_stream,
                 None => return,
             };
-            let mut callbacks = callbacks.lock().unwrap();
-            let callback = match callbacks.as_mut() {
-                Some(callback) => callback,
-                None => return,
-            };
 
             /// 1. Write from the ASIO buffer to the interleaved CPAL buffer.
             /// 2. Deliver the CPAL buffer to the user callback.
-            unsafe fn process_input_callback<A, B, F, G>(
-                stream_id: StreamId,
-                callback: &mut (dyn FnMut(StreamId, StreamDataResult) + Send),
+            unsafe fn process_input_callback<A, B, D, F, G>(
+                callback: &mut D,
                 interleaved: &mut [u8],
                 asio_stream: &sys::AsioStream,
                 buffer_index: usize,
@@ -239,6 +120,7 @@ impl EventLoop {
             where
                 A: AsioSample,
                 B: InterleavedSample,
+                D: FnMut(StreamData) + Send + 'static,
                 F: Fn(A) -> A,
                 G: Fn(A) -> B,
             {
@@ -254,16 +136,14 @@ impl EventLoop {
 
                 // 2. Deliver the interleaved buffer to the callback.
                 callback(
-                    stream_id,
-                    Ok(StreamData::Input { buffer: B::unknown_type_input_buffer(interleaved) }),
+                    StreamData::Input { buffer: B::unknown_type_input_buffer(interleaved) },
                 );
             }
 
             match (&stream_type, data_type) {
                 (&sys::AsioSampleType::ASIOSTInt16LSB, SampleFormat::I16) => {
-                    process_input_callback::<i16, i16, _, _>(
-                        stream_id,
-                        callback,
+                    process_input_callback::<i16, i16, _, _, _>(
+                        &mut data_callback,
                         &mut interleaved,
                         asio_stream,
                         buffer_index as usize,
@@ -272,9 +152,8 @@ impl EventLoop {
                     );
                 }
                 (&sys::AsioSampleType::ASIOSTInt16MSB, SampleFormat::I16) => {
-                    process_input_callback::<i16, i16, _, _>(
-                        stream_id,
-                        callback,
+                    process_input_callback::<i16, i16, _, _, _>(
+                        &mut data_callback,
                         &mut interleaved,
                         asio_stream,
                         buffer_index as usize,
@@ -287,9 +166,8 @@ impl EventLoop {
                 // trait for the `to_le` and `to_be` methods, but this does not support floats.
                 (&sys::AsioSampleType::ASIOSTFloat32LSB, SampleFormat::F32) |
                 (&sys::AsioSampleType::ASIOSTFloat32MSB, SampleFormat::F32) => {
-                    process_input_callback::<f32, f32, _, _>(
-                        stream_id,
-                        callback,
+                    process_input_callback::<f32, f32, _, _, _>(
+                        &mut data_callback,
                         &mut interleaved,
                         asio_stream,
                         buffer_index as usize,
@@ -302,9 +180,8 @@ impl EventLoop {
                 // `process_output_callback` function above by removing the unnecessary sample
                 // conversion function.
                 (&sys::AsioSampleType::ASIOSTInt32LSB, SampleFormat::I16) => {
-                    process_input_callback::<i32, i16, _, _>(
-                        stream_id,
-                        callback,
+                    process_input_callback::<i32, i16, _, _, _>(
+                        &mut data_callback,
                         &mut interleaved,
                         asio_stream,
                         buffer_index as usize,
@@ -313,9 +190,8 @@ impl EventLoop {
                     );
                 }
                 (&sys::AsioSampleType::ASIOSTInt32MSB, SampleFormat::I16) => {
-                    process_input_callback::<i32, i16, _, _>(
-                        stream_id,
-                        callback,
+                    process_input_callback::<i32, i16, _, _, _>(
+                        &mut data_callback,
                         &mut interleaved,
                         asio_stream,
                         buffer_index as usize,
@@ -327,9 +203,8 @@ impl EventLoop {
                 // trait for the `to_le` and `to_be` methods, but this does not support floats.
                 (&sys::AsioSampleType::ASIOSTFloat64LSB, SampleFormat::F32) |
                 (&sys::AsioSampleType::ASIOSTFloat64MSB, SampleFormat::F32) => {
-                    process_input_callback::<f64, f32, _, _>(
-                        stream_id,
-                        callback,
+                    process_input_callback::<f64, f32, _, _, _>(
+                        &mut data_callback,
                         &mut interleaved,
                         asio_stream,
                         buffer_index as usize,
@@ -345,23 +220,31 @@ impl EventLoop {
             }
         });
 
-        // Create stream and set to paused
-        self.cpal_streams
-            .lock()
-            .unwrap()
-            .push(Some(Stream { driver: driver.clone(), playing: false }));
+        let driver = self.driver.clone();
+        let asio_streams = self.asio_streams.clone();
 
-        Ok(StreamId(count))
+        // Immediately start the device?
+        self.driver.start().map_err(build_stream_err)?;
+
+        Ok(Stream {
+            playing: stream_playing,
+            driver,
+            asio_streams,
+            callback_id,
+        })
     }
 
-    /// Create the an output cpal stream.
-    pub fn build_output_stream(
+    pub fn build_output_stream<D, E>(
         &self,
-        device: &Device,
         format: &Format,
-    ) -> Result<StreamId, BuildStreamError> {
-        let Device { driver, .. } = device;
-        let stream_type = driver.output_data_type().map_err(build_stream_err)?;
+        mut data_callback: D,
+        _error_callback: E,
+    ) -> Result<Stream, BuildStreamError>
+    where
+        D: FnMut(StreamData) + Send + 'static,
+        E: FnMut(StreamError) + Send + 'static,
+    {
+        let stream_type = self.driver.output_data_type().map_err(build_stream_err)?;
 
         // Ensure that the desired sample type is supported.
         let data_type = super::device::convert_data_type(&stream_type)
@@ -371,38 +254,30 @@ impl EventLoop {
         }
 
         let num_channels = format.channels.clone();
-        let stream_buffer_size = self.get_or_create_output_stream(&driver, format, device)?;
-        let cpal_num_samples = stream_buffer_size * num_channels as usize;
-        let count = self.stream_count.fetch_add(1, Ordering::SeqCst);
-        let asio_streams = self.asio_streams.clone();
-        let cpal_streams = self.cpal_streams.clone();
-        let callbacks = self.callbacks.clone();
+        let buffer_size = self.get_or_create_output_stream(format)?;
+        let cpal_num_samples = buffer_size * num_channels as usize;
 
         // Create buffers depending on data type.
-        let stream_id = StreamId(count);
         let len_bytes = cpal_num_samples * data_type.sample_size();
         let mut interleaved = vec![0u8; len_bytes];
         let mut silence_asio_buffer = SilenceAsioBuffer::default();
 
-        driver.set_callback(move |buffer_index| unsafe {
+        let stream_playing = Arc::new(AtomicBool::new(false));
+        let playing = Arc::clone(&stream_playing);
+        let asio_streams = self.asio_streams.clone();
+
+        let callback_id = self.driver.add_callback(move |buffer_index| unsafe {
             // If not playing, return early.
-            // TODO: Don't assume `count` is valid - we should search for the matching `StreamId`.
-            if let Some(s) = cpal_streams.lock().unwrap().get(count) {
-                if let Some(s) = s {
-                    if !s.playing {
-                        return ();
-                    }
-                }
+            if !playing.load(Ordering::SeqCst) {
+                return
             }
 
-            // Acquire the stream and callback.
-            let stream_lock = asio_streams.lock().unwrap();
+            // There is 0% chance of lock contention the host only locks when recreating streams.
+            let stream_lock = asio_streams.lock();
             let ref asio_stream = match stream_lock.output {
                 Some(ref asio_stream) => asio_stream,
                 None => return,
             };
-            let mut callbacks = callbacks.lock().unwrap();
-            let callback = callbacks.as_mut();
 
             // Silence the ASIO buffer that is about to be used.
             //
@@ -430,9 +305,8 @@ impl EventLoop {
             /// 2. If required, silence the ASIO buffer.
             /// 3. Finally, write the interleaved data to the non-interleaved ASIO buffer,
             ///    performing endianness conversions as necessary.
-            unsafe fn process_output_callback<A, B, F, G>(
-                stream_id: StreamId,
-                callback: Option<&mut &mut (dyn FnMut(StreamId, StreamDataResult) + Send)>,
+            unsafe fn process_output_callback<A, B, D, F, G>(
+                callback: &mut D,
                 interleaved: &mut [u8],
                 silence_asio_buffer: bool,
                 asio_stream: &sys::AsioStream,
@@ -443,18 +317,14 @@ impl EventLoop {
             where
                 A: InterleavedSample,
                 B: AsioSample,
+                D: FnMut(StreamData) + Send + 'static,
                 F: Fn(A) -> B,
                 G: Fn(B) -> B,
             {
                 // 1. Render interleaved buffer from callback.
                 let interleaved: &mut [A] = cast_slice_mut(interleaved);
-                match callback {
-                    None => interleaved.iter_mut().for_each(|s| *s = A::SILENCE),
-                    Some(callback) => {
                 let buffer = A::unknown_type_output_buffer(interleaved);
-                        callback(stream_id, Ok(StreamData::Output { buffer }));
-                    }
-                }
+                callback(StreamData::Output { buffer });
 
                 // 2. Silence ASIO channels if necessary.
                 let n_channels = interleaved.len() / asio_stream.buffer_size as usize;
@@ -478,9 +348,8 @@ impl EventLoop {
 
             match (data_type, &stream_type) {
                 (SampleFormat::I16, &sys::AsioSampleType::ASIOSTInt16LSB) => {
-                    process_output_callback::<i16, i16, _, _>(
-                        stream_id,
-                        callback,
+                    process_output_callback::<i16, i16, _, _, _>(
+                        &mut data_callback,
                         &mut interleaved,
                         silence,
                         asio_stream,
@@ -490,9 +359,8 @@ impl EventLoop {
                     );
                 }
                 (SampleFormat::I16, &sys::AsioSampleType::ASIOSTInt16MSB) => {
-                    process_output_callback::<i16, i16, _, _>(
-                        stream_id,
-                        callback,
+                    process_output_callback::<i16, i16, _, _, _>(
+                        &mut data_callback,
                         &mut interleaved,
                         silence,
                         asio_stream,
@@ -506,9 +374,8 @@ impl EventLoop {
                 // trait for the `to_le` and `to_be` methods, but this does not support floats.
                 (SampleFormat::F32, &sys::AsioSampleType::ASIOSTFloat32LSB) |
                 (SampleFormat::F32, &sys::AsioSampleType::ASIOSTFloat32MSB) => {
-                    process_output_callback::<f32, f32, _, _>(
-                        stream_id,
-                        callback,
+                    process_output_callback::<f32, f32, _, _, _>(
+                        &mut data_callback,
                         &mut interleaved,
                         silence,
                         asio_stream,
@@ -522,9 +389,8 @@ impl EventLoop {
                 // `process_output_callback` function above by removing the unnecessary sample
                 // conversion function.
                 (SampleFormat::I16, &sys::AsioSampleType::ASIOSTInt32LSB) => {
-                    process_output_callback::<i16, i32, _, _>(
-                        stream_id,
-                        callback,
+                    process_output_callback::<i16, i32, _, _, _>(
+                        &mut data_callback,
                         &mut interleaved,
                         silence,
                         asio_stream,
@@ -534,9 +400,8 @@ impl EventLoop {
                     );
                 }
                 (SampleFormat::I16, &sys::AsioSampleType::ASIOSTInt32MSB) => {
-                    process_output_callback::<i16, i32, _, _>(
-                        stream_id,
-                        callback,
+                    process_output_callback::<i16, i32, _, _, _>(
+                        &mut data_callback,
                         &mut interleaved,
                         silence,
                         asio_stream,
@@ -549,9 +414,8 @@ impl EventLoop {
                 // trait for the `to_le` and `to_be` methods, but this does not support floats.
                 (SampleFormat::F32, &sys::AsioSampleType::ASIOSTFloat64LSB) |
                 (SampleFormat::F32, &sys::AsioSampleType::ASIOSTFloat64MSB) => {
-                    process_output_callback::<f32, f64, _, _>(
-                        stream_id,
-                        callback,
+                    process_output_callback::<f32, f64, _, _, _>(
+                        &mut data_callback,
                         &mut interleaved,
                         silence,
                         asio_stream,
@@ -568,78 +432,104 @@ impl EventLoop {
             }
         });
 
-        // Create the stream paused
-        self.cpal_streams
-            .lock()
-            .unwrap()
-            .push(Some(Stream { driver: driver.clone(), playing: false }));
+        let driver = self.driver.clone();
+        let asio_streams = self.asio_streams.clone();
 
-        // Give the ID based on the stream count
-        Ok(StreamId(count))
+        // Immediately start the device?
+        self.driver.start().map_err(build_stream_err)?;
+
+        Ok(Stream {
+            playing: stream_playing,
+            driver,
+            asio_streams,
+            callback_id,
+        })
     }
 
-    /// Play the cpal stream for the given ID.
-    pub fn play_stream(&self, stream_id: StreamId) -> Result<(), PlayStreamError> {
-        let mut streams = self.cpal_streams.lock().unwrap();
-        if let Some(s) = streams.get_mut(stream_id.0).expect("Bad play stream index") {
-            s.playing = true;
-            // Calling play when already playing is a no-op
-            s.driver.start().map_err(play_stream_err)?;
-        }
-        Ok(())
-    }
-
-    /// Pause the cpal stream for the given ID.
+    /// Create a new CPAL Input Stream.
     ///
-    /// Pause the ASIO streams if there are no other CPAL streams playing, as ASIO only allows
-    /// stopping the entire driver.
-    pub fn pause_stream(&self, stream_id: StreamId) -> Result<(), PauseStreamError> {
-        let mut streams = self.cpal_streams.lock().unwrap();
-        let streams_playing = streams.iter()
-            .filter(|s| s.as_ref().map(|s| s.playing).unwrap_or(false))
-            .count();
-        if let Some(s) = streams.get_mut(stream_id.0).expect("Bad pause stream index") {
-            if streams_playing <= 1 {
-                s.driver.stop().map_err(pause_stream_err)?;
+    /// If there is no existing ASIO Input Stream it will be created.
+    ///
+    /// On success, the buffer size of the stream is returned.
+    fn get_or_create_input_stream(
+        &self,
+        format: &Format,
+    ) -> Result<usize, BuildStreamError> {
+        match self.default_input_format() {
+            Ok(f) => {
+                let num_asio_channels = f.channels;
+                check_format(&self.driver, format, num_asio_channels)
+            },
+            Err(_) => Err(BuildStreamError::FormatNotSupported),
+        }?;
+        let num_channels = format.channels as usize;
+        let ref mut streams = *self.asio_streams.lock();
+        // Either create a stream if thers none or had back the
+        // size of the current one.
+        match streams.input {
+            Some(ref input) => Ok(input.buffer_size as usize),
+            None => {
+                let output = streams.output.take();
+                self.driver
+                    .prepare_input_stream(output, num_channels)
+                    .map(|new_streams| {
+                        let bs = match new_streams.input {
+                            Some(ref inp) => inp.buffer_size as usize,
+                            None => unreachable!(),
+                        };
+                        *streams = new_streams;
+                        bs
+                    }).map_err(|ref e| {
+                        println!("Error preparing stream: {}", e);
+                        BuildStreamError::DeviceNotAvailable
+                    })
             }
-            s.playing = false;
         }
-        Ok(())
     }
 
-    /// Destroy the cpal stream based on the ID.
-    pub fn destroy_stream(&self, stream_id: StreamId) {
-        // TODO: Should we not also remove an ASIO stream here?
-        //       Yes, and we should update the logic in the callbacks to search for the stream with
-        //       the matching ID, rather than assuming the index associated with the ID is valid.
-        let mut streams = self.cpal_streams.lock().unwrap();
-        streams.get_mut(stream_id.0).take();
+    /// Create a new CPAL Output Stream.
+    ///
+    /// If there is no existing ASIO Output Stream it will be created.
+    fn get_or_create_output_stream(
+        &self,
+        format: &Format,
+    ) -> Result<usize, BuildStreamError> {
+        match self.default_output_format() {
+            Ok(f) => {
+                let num_asio_channels = f.channels;
+                check_format(&self.driver, format, num_asio_channels)
+            },
+            Err(_) => Err(BuildStreamError::FormatNotSupported),
+        }?;
+        let num_channels = format.channels as usize;
+        let ref mut streams = *self.asio_streams.lock();
+        // Either create a stream if thers none or had back the
+        // size of the current one.
+        match streams.output {
+            Some(ref output) => Ok(output.buffer_size as usize),
+            None => {
+                let output = streams.output.take();
+                self.driver
+                    .prepare_output_stream(output, num_channels)
+                    .map(|new_streams| {
+                        let bs = match new_streams.output {
+                            Some(ref out) => out.buffer_size as usize,
+                            None => unreachable!(),
+                        };
+                        *streams = new_streams;
+                        bs
+                    }).map_err(|ref e| {
+                        println!("Error preparing stream: {}", e);
+                        BuildStreamError::DeviceNotAvailable
+                    })
             }
-
-    /// Run the cpal callbacks
-    pub fn run<F>(&self, mut callback: F) -> !
-    where
-        F: FnMut(StreamId, StreamDataResult) + Send,
-    {
-        let callback: &mut (FnMut(StreamId, StreamDataResult) + Send) = &mut callback;
-        // Transmute needed to convince the compiler that the callback has a static lifetime
-        *self.callbacks.lock().unwrap() = Some(unsafe { mem::transmute(callback) });
-        loop {
-            // A sleep here to prevent the loop being
-            // removed in --release
-            thread::sleep(Duration::new(1u64, 0u32));
         }
     }
 }
 
-/// Clean up if event loop is dropped.
-/// Currently event loop is never dropped.
-impl Drop for EventLoop {
+impl Drop for Stream {
     fn drop(&mut self) {
-        *self.asio_streams.lock().unwrap() = sys::AsioStreams {
-            output: None,
-            input: None,
-        };
+        self.driver.remove_callback(self.callback_id);
     }
 }
 
@@ -790,25 +680,3 @@ fn build_stream_err(e: sys::AsioError) -> BuildStreamError {
         }
     }
 }
-
-fn pause_stream_err(e: sys::AsioError) -> PauseStreamError {
-    match e {
-        sys::AsioError::NoDrivers |
-        sys::AsioError::HardwareMalfunction => PauseStreamError::DeviceNotAvailable,
-        err => {
-            let description = format!("{}", err);
-            BackendSpecificError { description }.into()
-        }
-    }
-}
-
-fn play_stream_err(e: sys::AsioError) -> PlayStreamError {
-    match e {
-        sys::AsioError::NoDrivers |
-        sys::AsioError::HardwareMalfunction => PlayStreamError::DeviceNotAvailable,
-        err => {
-            let description = format!("{}", err);
-            BackendSpecificError { description }.into()
-        }
-    }
-}

src/host/coreaudio/mod.rs

@@ -14,20 +14,18 @@ use SupportedFormatsError;
 use SampleFormat;
 use SampleRate;
 use StreamData;
-use StreamDataResult;
+use StreamError;
 use SupportedFormat;
 use UnknownTypeInputBuffer;
 use UnknownTypeOutputBuffer;
-use traits::{DeviceTrait, EventLoopTrait, HostTrait, StreamIdTrait};
+use traits::{DeviceTrait, HostTrait, StreamTrait};
 
 use std::ffi::CStr;
 use std::fmt;
 use std::mem;
+use std::cell::RefCell;
 use std::os::raw::c_char;
 use std::ptr::null;
-use std::sync::{Arc, Condvar, Mutex};
-use std::thread;
-use std::time::Duration;
 use std::slice;
 
 use self::coreaudio::audio_unit::{AudioUnit, Scope, Element};
@@ -87,7 +85,6 @@ impl Host {
 impl HostTrait for Host {
     type Devices = Devices;
     type Device = Device;
-    type EventLoop = EventLoop;
 
     fn is_available() -> bool {
         // Assume coreaudio is always available on macOS and iOS.
@@ -105,15 +102,12 @@ impl HostTrait for Host {
     fn default_output_device(&self) -> Option<Self::Device> {
         default_output_device()
     }
-
-    fn event_loop(&self) -> Self::EventLoop {
-        EventLoop::new()
-    }
 }
 
 impl DeviceTrait for Device {
     type SupportedInputFormats = SupportedInputFormats;
     type SupportedOutputFormats = SupportedOutputFormats;
+	type Stream = Stream;
 
     fn name(&self) -> Result<String, DeviceNameError> {
         Device::name(self)
@@ -134,50 +128,16 @@ impl DeviceTrait for Device {
     fn default_output_format(&self) -> Result<Format, DefaultFormatError> {
         Device::default_output_format(self)
     }
-}
 
-impl EventLoopTrait for EventLoop {
-    type Device = Device;
-    type StreamId = StreamId;
-
-    fn build_input_stream(
-        &self,
-        device: &Self::Device,
-        format: &Format,
-    ) -> Result<Self::StreamId, BuildStreamError> {
-        EventLoop::build_input_stream(self, device, format)
+    fn build_input_stream<D, E>(&self, format: &Format, data_callback: D, error_callback: E) -> Result<Self::Stream, BuildStreamError> where D: FnMut(StreamData) + Send + 'static, E: FnMut(StreamError) + Send + 'static {
+        Device::build_input_stream(self, format, data_callback, error_callback)
     }
 
-    fn build_output_stream(
-        &self,
-        device: &Self::Device,
-        format: &Format,
-    ) -> Result<Self::StreamId, BuildStreamError> {
-        EventLoop::build_output_stream(self, device, format)
-    }
-
-    fn play_stream(&self, stream: Self::StreamId) -> Result<(), PlayStreamError> {
-        EventLoop::play_stream(self, stream)
-    }
-
-    fn pause_stream(&self, stream: Self::StreamId) -> Result<(), PauseStreamError> {
-        EventLoop::pause_stream(self, stream)
-    }
-
-    fn destroy_stream(&self, stream: Self::StreamId) {
-        EventLoop::destroy_stream(self, stream)
-    }
-
-    fn run<F>(&self, callback: F) -> !
-    where
-        F: FnMut(Self::StreamId, StreamDataResult) + Send,
-    {
-        EventLoop::run(self, callback)
+    fn build_output_stream<D, E>(&self, format: &Format, data_callback: D, error_callback: E) -> Result<Self::Stream, BuildStreamError> where D: FnMut(StreamData) + Send + 'static, E: FnMut(StreamError) + Send + 'static {
+        Device::build_output_stream(self, format, data_callback, error_callback)
     }
 }
 
-impl StreamIdTrait for StreamId {}
-
 #[derive(Clone, PartialEq, Eq)]
 pub struct Device {
     audio_device_id: AudioDeviceID,
@@ -420,31 +380,6 @@ impl fmt::Debug for Device {
     }
 }
 
-// The ID of a stream is its index within the `streams` array of the events loop.
-#[derive(Debug, Clone, PartialEq, Eq, Hash)]
-pub struct StreamId(usize);
-
-pub struct EventLoop {
-    // This `Arc` is shared with all the callbacks of coreaudio.
-    //
-    // TODO: Eventually, CPAL's API should be changed to allow for submitting a unique callback per
-    // stream to avoid streams blocking one another.
-    user_callback: Arc<Mutex<UserCallback>>,
-    streams: Mutex<Vec<Option<StreamInner>>>,
-    loop_cond: Arc<(Mutex<bool>, Condvar)>,
-}
-
-enum UserCallback {
-    // When `run` is called with a callback, that callback will be stored here.
-    //
-    // It is essential for the safety of the program that this callback is removed before `run`
-    // returns (not possible with the current CPAL API).
-    Active(&'static mut (dyn FnMut(StreamId, StreamDataResult) + Send)),
-    // A queue of events that have occurred but that have not yet been emitted to the user as we
-    // don't yet have a callback to do so.
-    Inactive,
-}
-
 struct StreamInner {
     playing: bool,
     audio_unit: AudioUnit,
@@ -540,75 +475,8 @@ fn audio_unit_from_device(device: &Device, input: bool) -> Result<AudioUnit, cor
     Ok(audio_unit)
 }
 
-impl EventLoop {
-    #[inline]
-    fn new() -> EventLoop {
-        EventLoop {
-            user_callback: Arc::new(Mutex::new(UserCallback::Inactive)),
-            streams: Mutex::new(Vec::new()),
-            loop_cond: Arc::new((Mutex::new(false), Condvar::new())),
-        }
-    }
-
-    #[inline]
-    fn run<F>(&self, mut callback: F) -> !
-        where F: FnMut(StreamId, StreamDataResult) + Send
-    {
-        {
-            let mut guard = self.user_callback.lock().unwrap();
-            if let UserCallback::Active(_) = *guard {
-                panic!("`EventLoop::run` was called when the event loop was already running");
-            }
-            let callback: &mut (dyn FnMut(StreamId, StreamDataResult) + Send) = &mut callback;
-            *guard = UserCallback::Active(unsafe { mem::transmute(callback) });
-        }
-
-        // Wait on a condvar to notify, which should never happen.
-        let &(ref lock, ref cvar) = &*self.loop_cond;
-        let mut running = lock.lock().unwrap();
-        *running = true;
-        while *running {
-            running = cvar.wait(running).unwrap();
-        }
-
-        unreachable!("current `EventLoop` API requires that `run` may not return");
-
-        // It is critical that we remove the callback before returning (currently not possible).
-        // *self.user_callback.lock().unwrap() = UserCallback::Inactive;
-    }
-
-    fn next_stream_id(&self) -> usize {
-        let streams_lock = self.streams.lock().unwrap();
-        let stream_id = streams_lock
-            .iter()
-            .position(|n| n.is_none())
-            .unwrap_or(streams_lock.len());
-        stream_id
-    }
-
-    // Add the stream to the list of streams within `self`.
-    fn add_stream(&self, stream_id: usize, au: AudioUnit, device_id: AudioDeviceID) {
-        let inner = StreamInner {
-            playing: true,
-            audio_unit: au,
-            device_id: device_id,
-        };
-
-        let mut streams_lock = self.streams.lock().unwrap();
-        if stream_id == streams_lock.len() {
-            streams_lock.push(Some(inner));
-        } else {
-            streams_lock[stream_id] = Some(inner);
-        }
-    }
-
-    #[inline]
-    fn build_input_stream(
-        &self,
-        device: &Device,
-        format: &Format,
-    ) -> Result<StreamId, BuildStreamError>
-    {
+impl Device {
+    fn build_input_stream<D, E>(&self, format: &Format, mut data_callback: D, _error_callback: E) -> Result<Stream, BuildStreamError> where D: FnMut(StreamData) + Send + 'static, E: FnMut(StreamError) + Send + 'static {
         // The scope and element for working with a device's input stream.
         let scope = Scope::Output;
         let element = Element::Input;
@@ -624,7 +492,7 @@ impl EventLoop {
             let sample_rate: f64 = 0.0;
             let data_size = mem::size_of::<f64>() as u32;
             let status = AudioObjectGetPropertyData(
-                device.audio_device_id,
+                self.audio_device_id,
                 &property_address as *const _,
                 0,
                 null(),
@@ -635,26 +503,11 @@ impl EventLoop {
 
             // If the requested sample rate is different to the device sample rate, update the device.
             if sample_rate as u32 != format.sample_rate.0 {
-
-                // In order to avoid breaking existing input streams we return an error if there is
-                // already an active input stream for this device with the actual sample rate.
-                for stream in &*self.streams.lock().unwrap() {
-                    if let Some(stream) = stream.as_ref() {
-                        if stream.device_id == device.audio_device_id {
-                            let description = "cannot change device sample rate for stream as an \
-                                existing stream is already running at the current sample rate"
-                                .into();
-                            let err = BackendSpecificError { description };
-                            return Err(err.into());
-                        }
-                    }
-                }
-
                 // Get available sample rate ranges.
                 property_address.mSelector = kAudioDevicePropertyAvailableNominalSampleRates;
                 let data_size = 0u32;
                 let status = AudioObjectGetPropertyDataSize(
-                    device.audio_device_id,
+                    self.audio_device_id,
                     &property_address as *const _,
                     0,
                     null(),
@@ -665,7 +518,7 @@ impl EventLoop {
                 let mut ranges: Vec<u8> = vec![];
                 ranges.reserve_exact(data_size as usize);
                 let status = AudioObjectGetPropertyData(
-                    device.audio_device_id,
+                    self.audio_device_id,
                     &property_address as *const _,
                     0,
                     null(),
@@ -719,7 +572,7 @@ impl EventLoop {
                 // Add our sample rate change listener callback.
                 let reported_rate: f64 = 0.0;
                 let status = AudioObjectAddPropertyListener(
-                    device.audio_device_id,
+                    self.audio_device_id,
                     &property_address as *const _,
                     Some(rate_listener),
                     &reported_rate as *const _ as *mut _,
@@ -729,7 +582,7 @@ impl EventLoop {
                 // Finally, set the sample rate.
                 let sample_rate = sample_rate as f64;
                 let status = AudioObjectSetPropertyData(
-                    device.audio_device_id,
+                    self.audio_device_id,
                     &property_address as *const _,
                     0,
                     null(),
@@ -753,7 +606,7 @@ impl EventLoop {
 
                 // Remove the `rate_listener` callback.
                 let status = AudioObjectRemovePropertyListener(
-                    device.audio_device_id,
+                    self.audio_device_id,
                     &property_address as *const _,
                     Some(rate_listener),
                     &reported_rate as *const _ as *mut _,
@@ -762,18 +615,14 @@ impl EventLoop {
             }
         }
 
-        let mut audio_unit = audio_unit_from_device(device, true)?;
+        let mut audio_unit = audio_unit_from_device(self, true)?;
 
         // Set the stream in interleaved mode.
         let asbd = asbd_from_format(format);
         audio_unit.set_property(kAudioUnitProperty_StreamFormat, scope, element, Some(&asbd))?;
 
-        // Determine the future ID of the stream.
-        let stream_id = self.next_stream_id();
-
         // Register the callback that is being called by coreaudio whenever it needs data to be
         // fed to the audio buffer.
-        let user_callback = self.user_callback.clone();
         let sample_format = format.data_type;
         let bytes_per_channel = format.data_type.sample_size();
         type Args = render_callback::Args<data::Raw>;
@@ -789,20 +638,14 @@ impl EventLoop {
                 mData: data
             } = buffers[0];
 
-            let mut user_callback = user_callback.lock().unwrap();
-
             // A small macro to simplify handling the callback for different sample types.
             macro_rules! try_callback {
                 ($SampleFormat:ident, $SampleType:ty) => {{
                     let data_len = (data_byte_size as usize / bytes_per_channel) as usize;
                     let data_slice = slice::from_raw_parts(data as *const $SampleType, data_len);
-                    let callback = match *user_callback {
-                        UserCallback::Active(ref mut cb) => cb,
-                        UserCallback::Inactive => return Ok(()),
-                    };
                     let unknown_type_buffer = UnknownTypeInputBuffer::$SampleFormat(::InputBuffer { buffer: data_slice });
                     let stream_data = StreamData::Input { buffer: unknown_type_buffer };
-                    callback(StreamId(stream_id), Ok(stream_data));
+                    data_callback(stream_data);
                 }};
             }
 
@@ -815,23 +658,17 @@ impl EventLoop {
             Ok(())
         })?;
 
-        // TODO: start playing now? is that consistent with the other backends?
         audio_unit.start()?;
 
-        // Add the stream to the list of streams within `self`.
-        self.add_stream(stream_id, audio_unit, device.audio_device_id);
-
-        Ok(StreamId(stream_id))
+        Ok(Stream::new(StreamInner {
+            playing: true,
+            audio_unit,
+            device_id: self.audio_device_id,
+        }))
     }
 
-    #[inline]
-    fn build_output_stream(
-        &self,
-        device: &Device,
-        format: &Format,
-    ) -> Result<StreamId, BuildStreamError>
-    {
-        let mut audio_unit = audio_unit_from_device(device, false)?;
+    fn build_output_stream<D, E>(&self, format: &Format, mut data_callback: D, _error_callback: E) -> Result<Stream, BuildStreamError> where D: FnMut(StreamData) + Send + 'static, E: FnMut(StreamError) + Send + 'static {
+        let mut audio_unit = audio_unit_from_device(self, false)?;
 
         // The scope and element for working with a device's output stream.
         let scope = Scope::Input;
@@ -841,12 +678,8 @@ impl EventLoop {
         let asbd = asbd_from_format(format);
         audio_unit.set_property(kAudioUnitProperty_StreamFormat, scope, element, Some(&asbd))?;
 
-        // Determine the future ID of the stream.
-        let stream_id = self.next_stream_id();
-
         // Register the callback that is being called by coreaudio whenever it needs data to be
         // fed to the audio buffer.
-        let user_callback = self.user_callback.clone();
         let sample_format = format.data_type;
         let bytes_per_channel = format.data_type.sample_size();
         type Args = render_callback::Args<data::Raw>;
@@ -860,25 +693,14 @@ impl EventLoop {
                 mData: data
             } = (*args.data.data).mBuffers[0];
 
-            let mut user_callback = user_callback.lock().unwrap();
-
             // A small macro to simplify handling the callback for different sample types.
             macro_rules! try_callback {
                 ($SampleFormat:ident, $SampleType:ty, $equilibrium:expr) => {{
                     let data_len = (data_byte_size as usize / bytes_per_channel) as usize;
                     let data_slice = slice::from_raw_parts_mut(data as *mut $SampleType, data_len);
-                    let callback = match *user_callback {
-                        UserCallback::Active(ref mut cb) => cb,
-                        UserCallback::Inactive => {
-                            for sample in data_slice.iter_mut() {
-                                *sample = $equilibrium;
-                            }
-                            return Ok(());
-                        }
-                    };
                     let unknown_type_buffer = UnknownTypeOutputBuffer::$SampleFormat(::OutputBuffer { buffer: data_slice });
                     let stream_data = StreamData::Output { buffer: unknown_type_buffer };
-                    callback(StreamId(stream_id), Ok(stream_data));
+                    data_callback(stream_data);
                 }};
             }
 
@@ -891,25 +713,31 @@ impl EventLoop {
             Ok(())
         })?;
 
-        // TODO: start playing now? is that consistent with the other backends?
         audio_unit.start()?;
 
-        // Add the stream to the list of streams within `self`.
-        self.add_stream(stream_id, audio_unit, device.audio_device_id);
-
-        Ok(StreamId(stream_id))
+        Ok(Stream::new(StreamInner {
+            playing: true,
+            audio_unit,
+            device_id: self.audio_device_id,
+        }))
     }
+}
 
-    fn destroy_stream(&self, stream_id: StreamId) {
-        {
-            let mut streams = self.streams.lock().unwrap();
-            streams[stream_id.0] = None;
+pub struct Stream {
+    inner: RefCell<StreamInner>,
+}
+
+impl Stream {
+    fn new(inner: StreamInner) -> Self {
+        Self {
+            inner: RefCell::new(inner),
         }
     }
+}
 
-    fn play_stream(&self, stream_id: StreamId) -> Result<(), PlayStreamError> {
-        let mut streams = self.streams.lock().unwrap();
-        let stream = streams[stream_id.0].as_mut().unwrap();
+impl StreamTrait for Stream {
+    fn play(&self) -> Result<(), PlayStreamError> {
+        let mut stream = self.inner.borrow_mut();
 
         if !stream.playing {
             if let Err(e) = stream.audio_unit.start() {
@@ -922,9 +750,8 @@ impl EventLoop {
         Ok(())
     }
 
-    fn pause_stream(&self, stream_id: StreamId) -> Result<(), PauseStreamError> {
-        let mut streams = self.streams.lock().unwrap();
-        let stream = streams[stream_id.0].as_mut().unwrap();
+    fn pause(&self) -> Result<(), PauseStreamError> {
+        let mut stream = self.inner.borrow_mut();
 
         if stream.playing {
             if let Err(e) = stream.audio_unit.stop() {

src/host/emscripten/mod.rs

@@ -1,7 +1,6 @@
 use std::mem;
 use std::os::raw::c_void;
 use std::slice::from_raw_parts;
-use std::sync::Mutex;
 use stdweb;
 use stdweb::Reference;
 use stdweb::unstable::TryInto;
@@ -17,25 +16,102 @@ use PauseStreamError;
 use PlayStreamError;
 use SupportedFormatsError;
 use StreamData;
-use StreamDataResult;
+use StreamError;
 use SupportedFormat;
 use UnknownTypeOutputBuffer;
-use traits::{DeviceTrait, EventLoopTrait, HostTrait, StreamIdTrait};
+use traits::{DeviceTrait, HostTrait, StreamTrait};
+
+// The emscripten backend currently works by instantiating an `AudioContext` object per `Stream`.
+// Creating a stream creates a new `AudioContext`. Destroying a stream destroys it. Creation of a
+// `Host` instance initializes the `stdweb` context.
 
 /// The default emscripten host type.
 #[derive(Debug)]
 pub struct Host;
 
+/// Content is false if the iterator is empty.
+pub struct Devices(bool);
+
+#[derive(Clone, Debug, PartialEq, Eq)]
+pub struct Device;
+
+pub struct Stream {
+    // A reference to an `AudioContext` object.
+    audio_ctxt_ref: Reference,
+}
+
+// Index within the `streams` array of the events loop.
+#[derive(Debug, Clone, PartialEq, Eq, Hash)]
+pub struct StreamId(usize);
+
+pub type SupportedInputFormats = ::std::vec::IntoIter<SupportedFormat>;
+pub type SupportedOutputFormats = ::std::vec::IntoIter<SupportedFormat>;
+
 impl Host {
     pub fn new() -> Result<Self, crate::HostUnavailable> {
+        stdweb::initialize();
         Ok(Host)
     }
 }
 
+impl Devices {
+    fn new() -> Result<Self, DevicesError> {
+        Ok(Self::default())
+    }
+}
+
+impl Device {
+    #[inline]
+    fn name(&self) -> Result<String, DeviceNameError> {
+        Ok("Default Device".to_owned())
+    }
+
+    #[inline]
+    fn supported_input_formats(&self) -> Result<SupportedInputFormats, SupportedFormatsError> {
+        unimplemented!();
+    }
+
+    #[inline]
+    fn supported_output_formats(&self) -> Result<SupportedOutputFormats, SupportedFormatsError> {
+        // TODO: right now cpal's API doesn't allow flexibility here
+        //       "44100" and "2" (channels) have also been hard-coded in the rest of the code ; if
+        //       this ever becomes more flexible, don't forget to change that
+        //       According to https://developer.mozilla.org/en-US/docs/Web/API/BaseAudioContext/createBuffer
+        //       browsers must support 1 to 32 channels at leats and 8,000 Hz to 96,000 Hz.
+        //
+        //       UPDATE: We can do this now. Might be best to use `crate::COMMON_SAMPLE_RATES` and
+        //       filter out those that lay outside the range specified above.
+        Ok(
+            vec![
+                SupportedFormat {
+                    channels: 2,
+                    min_sample_rate: ::SampleRate(44100),
+                    max_sample_rate: ::SampleRate(44100),
+                    data_type: ::SampleFormat::F32,
+                },
+            ].into_iter(),
+        )
+    }
+
+    fn default_input_format(&self) -> Result<Format, DefaultFormatError> {
+        unimplemented!();
+    }
+
+    fn default_output_format(&self) -> Result<Format, DefaultFormatError> {
+        // TODO: because it is hard coded, see supported_output_formats.
+        Ok(
+            Format {
+                channels: 2,
+                sample_rate: ::SampleRate(44100),
+                data_type: ::SampleFormat::F32,
+            },
+        )
+    }
+}
+
 impl HostTrait for Host {
     type Devices = Devices;
     type Device = Device;
-    type EventLoop = EventLoop;
 
     fn is_available() -> bool {
         // Assume this host is always available on emscripten.
@@ -53,15 +129,12 @@ impl HostTrait for Host {
     fn default_output_device(&self) -> Option<Self::Device> {
         default_output_device()
     }
-
-    fn event_loop(&self) -> Self::EventLoop {
-        EventLoop::new()
-    }
 }
 
 impl DeviceTrait for Device {
     type SupportedInputFormats = SupportedInputFormats;
     type SupportedOutputFormats = SupportedOutputFormats;
+    type Stream = Stream;
 
     fn name(&self) -> Result<String, DeviceNameError> {
         Device::name(self)
@@ -82,106 +155,87 @@ impl DeviceTrait for Device {
     fn default_output_format(&self) -> Result<Format, DefaultFormatError> {
         Device::default_output_format(self)
     }
-}
 
-impl EventLoopTrait for EventLoop {
-    type Device = Device;
-    type StreamId = StreamId;
-
-    fn build_input_stream(
+    fn build_input_stream<D, E>(
         &self,
-        device: &Self::Device,
-        format: &Format,
-    ) -> Result<Self::StreamId, BuildStreamError> {
-        EventLoop::build_input_stream(self, device, format)
-    }
-
-    fn build_output_stream(
-        &self,
-        device: &Self::Device,
-        format: &Format,
-    ) -> Result<Self::StreamId, BuildStreamError> {
-        EventLoop::build_output_stream(self, device, format)
-    }
-
-    fn play_stream(&self, stream: Self::StreamId) -> Result<(), PlayStreamError> {
-        EventLoop::play_stream(self, stream)
-    }
-
-    fn pause_stream(&self, stream: Self::StreamId) -> Result<(), PauseStreamError> {
-        EventLoop::pause_stream(self, stream)
-    }
-
-    fn destroy_stream(&self, stream: Self::StreamId) {
-        EventLoop::destroy_stream(self, stream)
-    }
-
-    fn run<F>(&self, callback: F) -> !
+        _format: &Format,
+        _data_callback: D,
+        _error_callback: E,
+    ) -> Result<Self::Stream, BuildStreamError>
     where
-        F: FnMut(Self::StreamId, StreamDataResult) + Send,
+        D: FnMut(StreamData) + Send + 'static,
+        E: FnMut(StreamError) + Send + 'static,
     {
-        EventLoop::run(self, callback)
+        unimplemented!()
+    }
+
+    fn build_output_stream<D, E>(
+        &self,
+        _format: &Format,
+        data_callback: D,
+        error_callback: E,
+    ) -> Result<Self::Stream, BuildStreamError>
+    where
+        D: FnMut(StreamData) + Send + 'static,
+        E: FnMut(StreamError) + Send + 'static,
+    {
+        // Create the stream.
+        let audio_ctxt_ref = js!(return new AudioContext()).into_reference().unwrap();
+        let stream = Stream { audio_ctxt_ref };
+
+        // Specify the callback.
+        let mut user_data = (self, data_callback, error_callback);
+        let user_data_ptr = &mut user_data as *mut (_, _, _);
+
+        // Use `set_timeout` to invoke a Rust callback repeatedly.
+        //
+        // The job of this callback is to fill the content of the audio buffers.
+        //
+        // See also: The call to `set_timeout` at the end of the `audio_callback_fn` which creates
+        // the loop.
+        set_timeout(|| audio_callback_fn::<D, E>(user_data_ptr as *mut c_void), 10);
+
+        Ok(stream)
     }
 }
 
-impl StreamIdTrait for StreamId {}
+impl StreamTrait for Stream {
+    fn play(&self) -> Result<(), PlayStreamError> {
+        let audio_ctxt = &self.audio_ctxt_ref;
+        js!(@{audio_ctxt}.resume());
+        Ok(())
+    }
 
-// The emscripten backend works by having a global variable named `_cpal_audio_contexts`, which
-// is an array of `AudioContext` objects. A stream ID corresponds to an entry in this array.
-//
-// Creating a stream creates a new `AudioContext`. Destroying a stream destroys it.
-
-// TODO: handle latency better ; right now we just use setInterval with the amount of sound data
-// that is in each buffer ; this is obviously bad, and also the schedule is too tight and there may
-// be underflows
-
-pub struct EventLoop {
-    streams: Mutex<Vec<Option<Reference>>>,
+    fn pause(&self) -> Result<(), PauseStreamError> {
+        let audio_ctxt = &self.audio_ctxt_ref;
+        js!(@{audio_ctxt}.suspend());
+        Ok(())
+    }
 }
 
-impl EventLoop {
-    #[inline]
-    pub fn new() -> EventLoop {
-        stdweb::initialize();
-        EventLoop {
-            streams: Mutex::new(Vec::new()),
-        }
-    }
-
-    #[inline]
-    fn run<F>(&self, callback: F) -> !
-        where F: FnMut(StreamId, StreamDataResult),
-    {
-        // The `run` function uses `set_timeout` to invoke a Rust callback repeatidely. The job
-        // of this callback is to fill the content of the audio buffers.
-
-        // The first argument of the callback function (a `void*`) is a casted pointer to `self`
-        // and to the `callback` parameter that was passed to `run`.
-
-        fn callback_fn<F>(user_data_ptr: *mut c_void)
-            where F: FnMut(StreamId, StreamDataResult)
-        {
+// The first argument of the callback function (a `void*`) is a casted pointer to `self`
+// and to the `callback` parameter that was passed to `run`.
+fn audio_callback_fn<D, E>(user_data_ptr: *mut c_void)
+where
+    D: FnMut(StreamData) + Send + 'static,
+    E: FnMut(StreamError) + Send + 'static,
+{
     unsafe {
-                let user_data_ptr2 = user_data_ptr as *mut (&EventLoop, F);
+        let user_data_ptr2 = user_data_ptr as *mut (&Stream, D, E);
         let user_data = &mut *user_data_ptr2;
-                let user_cb = &mut user_data.1;
-
-                let streams = user_data.0.streams.lock().unwrap().clone();
-                for (stream_id, stream) in streams.iter().enumerate() {
-                    let stream = match stream.as_ref() {
-                        Some(v) => v,
-                        None => continue,
-                    };
+        let (ref stream, ref mut data_cb, ref mut _err_cb) = user_data;
+        let audio_ctxt = &stream.audio_ctxt_ref;
 
+        // TODO: We should be re-using a buffer.
         let mut temporary_buffer = vec![0.0; 44100 * 2 / 3];
 
         {
             let buffer = UnknownTypeOutputBuffer::F32(::OutputBuffer { buffer: &mut temporary_buffer });
             let data = StreamData::Output { buffer: buffer };
-                        user_cb(StreamId(stream_id), Ok(data));
-                        // TODO: directly use a TypedArray<f32> once this is supported by stdweb
+            data_cb(data);
         }
 
+        // TODO: directly use a TypedArray<f32> once this is supported by stdweb
         let typed_array = {
             let f32_slice = temporary_buffer.as_slice();
             let u8_slice: &[u8] = from_raw_parts(
@@ -197,7 +251,7 @@ impl EventLoop {
 
         js!(
             var src_buffer = new Float32Array(@{typed_array}.buffer);
-                        var context = @{stream};
+            var context = @{audio_ctxt};
             var buf_len = @{temporary_buffer.len() as u32};
             var num_channels = @{num_channels};
 
@@ -214,92 +268,11 @@ impl EventLoop {
             node.connect(context.destination);
             node.start();
         );
-                }
 
-                set_timeout(|| callback_fn::<F>(user_data_ptr), 330);
-            }
-        }
-
-        let mut user_data = (self, callback);
-        let user_data_ptr = &mut user_data as *mut (_, _);
-
-        set_timeout(|| callback_fn::<F>(user_data_ptr as *mut _), 10);
-
-        stdweb::event_loop();
-    }
-
-    #[inline]
-    fn build_input_stream(&self, _: &Device, _format: &Format) -> Result<StreamId, BuildStreamError> {
-        unimplemented!();
-    }
-
-    #[inline]
-    fn build_output_stream(&self, _: &Device, _format: &Format) -> Result<StreamId, BuildStreamError> {
-        let stream = js!(return new AudioContext()).into_reference().unwrap();
-
-        let mut streams = self.streams.lock().unwrap();
-        let stream_id = if let Some(pos) = streams.iter().position(|v| v.is_none()) {
-            streams[pos] = Some(stream);
-            pos
-        } else {
-            let l = streams.len();
-            streams.push(Some(stream));
-            l
-        };
-
-        Ok(StreamId(stream_id))
-    }
-
-    #[inline]
-    fn destroy_stream(&self, stream_id: StreamId) {
-        self.streams.lock().unwrap()[stream_id.0] = None;
-    }
-
-    #[inline]
-    fn play_stream(&self, stream_id: StreamId) -> Result<(), PlayStreamError> {
-        let streams = self.streams.lock().unwrap();
-        let stream = streams
-            .get(stream_id.0)
-            .and_then(|v| v.as_ref())
-            .expect("invalid stream ID");
-        js!(@{stream}.resume());
-        Ok(())
-    }
-
-    #[inline]
-    fn pause_stream(&self, stream_id: StreamId) -> Result<(), PauseStreamError> {
-        let streams = self.streams.lock().unwrap();
-        let stream = streams
-            .get(stream_id.0)
-            .and_then(|v| v.as_ref())
-            .expect("invalid stream ID");
-        js!(@{stream}.suspend());
-        Ok(())
-    }
-}
-
-// Index within the `streams` array of the events loop.
-#[derive(Debug, Clone, PartialEq, Eq, Hash)]
-pub struct StreamId(usize);
-
-// Detects whether the `AudioContext` global variable is available.
-fn is_webaudio_available() -> bool {
-    stdweb::initialize();
-
-    js!(if (!AudioContext) {
-            return false;
-        } else {
-            return true;
-        }).try_into()
-        .unwrap()
-}
-
-// Content is false if the iterator is empty.
-pub struct Devices(bool);
-
-impl Devices {
-    fn new() -> Result<Self, DevicesError> {
-        Ok(Self::default())
+        // TODO: handle latency better ; right now we just use setInterval with the amount of sound
+        // data that is in each buffer ; this is obviously bad, and also the schedule is too tight
+        // and there may be underflows
+        set_timeout(|| audio_callback_fn::<D, E>(user_data_ptr), 330);
     }
 }
 
@@ -336,54 +309,13 @@ fn default_output_device() -> Option<Device> {
     }
 }
 
-#[derive(Clone, Debug, PartialEq, Eq)]
-pub struct Device;
-
-impl Device {
-    #[inline]
-    fn name(&self) -> Result<String, DeviceNameError> {
-        Ok("Default Device".to_owned())
-    }
-
-    #[inline]
-    fn supported_input_formats(&self) -> Result<SupportedInputFormats, SupportedFormatsError> {
-        unimplemented!();
-    }
-
-    #[inline]
-    fn supported_output_formats(&self) -> Result<SupportedOutputFormats, SupportedFormatsError> {
-        // TODO: right now cpal's API doesn't allow flexibility here
-        //       "44100" and "2" (channels) have also been hard-coded in the rest of the code ; if
-        //       this ever becomes more flexible, don't forget to change that
-        //       According to https://developer.mozilla.org/en-US/docs/Web/API/BaseAudioContext/createBuffer
-        //       browsers must support 1 to 32 channels at leats and 8,000 Hz to 96,000 Hz.
-        Ok(
-            vec![
-                SupportedFormat {
-                    channels: 2,
-                    min_sample_rate: ::SampleRate(44100),
-                    max_sample_rate: ::SampleRate(44100),
-                    data_type: ::SampleFormat::F32,
-                },
-            ].into_iter(),
-        )
-    }
-
-    fn default_input_format(&self) -> Result<Format, DefaultFormatError> {
-        unimplemented!();
-    }
-
-    fn default_output_format(&self) -> Result<Format, DefaultFormatError> {
-        // TODO: because it is hard coded, see supported_output_formats.
-        Ok(
-            Format {
-                channels: 2,
-                sample_rate: ::SampleRate(44100),
-                data_type: ::SampleFormat::F32,
-            },
-        )
-    }
+// Detects whether the `AudioContext` global variable is available.
+fn is_webaudio_available() -> bool {
+    stdweb::initialize();
+    js!(if (!AudioContext) {
+            return false;
+        } else {
+            return true;
+        }).try_into()
+        .unwrap()
 }
-
-pub type SupportedInputFormats = ::std::vec::IntoIter<SupportedFormat>;
-pub type SupportedOutputFormats = ::std::vec::IntoIter<SupportedFormat>;

src/host/null/mod.rs

@@ -1,5 +1,3 @@
-#![allow(dead_code)]
-
 use BuildStreamError;
 use DefaultFormatError;
 use DevicesError;
@@ -7,10 +5,11 @@ use DeviceNameError;
 use Format;
 use PauseStreamError;
 use PlayStreamError;
-use StreamDataResult;
+use StreamData;
+use StreamError;
 use SupportedFormatsError;
 use SupportedFormat;
-use traits::{DeviceTrait, EventLoopTrait, HostTrait, StreamIdTrait};
+use traits::{DeviceTrait, HostTrait, StreamTrait};
 
 #[derive(Default)]
 pub struct Devices;
@@ -18,17 +17,16 @@ pub struct Devices;
 #[derive(Clone, Debug, PartialEq, Eq)]
 pub struct Device;
 
-pub struct EventLoop;
-
 pub struct Host;
 
 #[derive(Debug, Clone, PartialEq, Eq, Hash)]
-pub struct StreamId;
+pub struct Stream;
 
 pub struct SupportedInputFormats;
 pub struct SupportedOutputFormats;
 
 impl Host {
+    #[allow(dead_code)]
     pub fn new() -> Result<Self, crate::HostUnavailable> {
         Ok(Host)
     }
@@ -40,15 +38,10 @@ impl Devices {
     }
 }
 
-impl EventLoop {
-    pub fn new() -> EventLoop {
-        EventLoop
-    }
-}
-
 impl DeviceTrait for Device {
     type SupportedInputFormats = SupportedInputFormats;
     type SupportedOutputFormats = SupportedOutputFormats;
+    type Stream = Stream;
 
     #[inline]
     fn name(&self) -> Result<String, DeviceNameError> {
@@ -74,49 +67,22 @@ impl DeviceTrait for Device {
     fn default_output_format(&self) -> Result<Format, DefaultFormatError> {
         unimplemented!()
     }
-}
 
-impl EventLoopTrait for EventLoop {
-    type Device = Device;
-    type StreamId = StreamId;
-
-    #[inline]
-    fn run<F>(&self, _callback: F) -> !
-        where F: FnMut(StreamId, StreamDataResult)
-    {
-        loop { /* TODO: don't spin */ }
-    }
-
-    #[inline]
-    fn build_input_stream(&self, _: &Device, _: &Format) -> Result<StreamId, BuildStreamError> {
-        Err(BuildStreamError::DeviceNotAvailable)
-    }
-
-    #[inline]
-    fn build_output_stream(&self, _: &Device, _: &Format) -> Result<StreamId, BuildStreamError> {
-        Err(BuildStreamError::DeviceNotAvailable)
-    }
-
-    #[inline]
-    fn destroy_stream(&self, _: StreamId) {
+    fn build_input_stream<D, E>(&self, _format: &Format, _data_callback: D, _error_callback: E) -> Result<Self::Stream, BuildStreamError>
+        where D: FnMut(StreamData) + Send + 'static, E: FnMut(StreamError) + Send + 'static {
         unimplemented!()
     }
 
-    #[inline]
-    fn play_stream(&self, _: StreamId) -> Result<(), PlayStreamError> {
-        panic!()
-    }
-
-    #[inline]
-    fn pause_stream(&self, _: StreamId) -> Result<(), PauseStreamError> {
-        panic!()
+    /// Create an output stream.
+    fn build_output_stream<D, E>(&self, _format: &Format, _data_callback: D, _error_callback: E) -> Result<Self::Stream, BuildStreamError>
+        where D: FnMut(StreamData) + Send + 'static, E: FnMut(StreamError) + Send + 'static{
+        unimplemented!()
     }
 }
 
 impl HostTrait for Host {
     type Device = Device;
     type Devices = Devices;
-    type EventLoop = EventLoop;
 
     fn is_available() -> bool {
         false
@@ -133,13 +99,17 @@ impl HostTrait for Host {
     fn default_output_device(&self) -> Option<Device> {
         None
     }
-
-    fn event_loop(&self) -> Self::EventLoop {
-        EventLoop::new()
-    }
 }
 
-impl StreamIdTrait for StreamId {}
+impl StreamTrait for Stream {
+    fn play(&self) -> Result<(), PlayStreamError> {
+        unimplemented!()
+    }
+
+    fn pause(&self) -> Result<(), PauseStreamError> {
+        unimplemented!()
+    }
+}
 
 impl Iterator for Devices {
     type Item = Device;

src/host/wasapi/com.rs

@@ -3,8 +3,8 @@
 use super::check_result;
 use std::ptr;
 
-use super::winapi::um::objbase::{COINIT_MULTITHREADED};
 use super::winapi::um::combaseapi::{CoInitializeEx, CoUninitialize};
+use super::winapi::um::objbase::COINIT_MULTITHREADED;
 
 thread_local!(static COM_INITIALIZED: ComInitialized = {
     unsafe {

src/host/wasapi/device.rs

@@ -7,66 +7,54 @@ use std::ops::{Deref, DerefMut};
 use std::os::windows::ffi::OsStringExt;
 use std::ptr;
 use std::slice;
-use std::sync::{Arc, Mutex, MutexGuard};
+use std::sync::{Arc, Mutex, MutexGuard, atomic::Ordering};
 
 use BackendSpecificError;
 use DefaultFormatError;
 use DeviceNameError;
 use DevicesError;
 use Format;
-use SupportedFormatsError;
 use SampleFormat;
 use SampleRate;
 use SupportedFormat;
+use SupportedFormatsError;
 use COMMON_SAMPLE_RATES;
 
 use super::check_result;
 use super::check_result_backend_specific;
 use super::com;
-use super::winapi::Interface;
 use super::winapi::ctypes::c_void;
 use super::winapi::shared::devpkey;
+use super::winapi::shared::guiddef::GUID;
 use super::winapi::shared::ksmedia;
-use super::winapi::shared::guiddef::{
-    GUID,
-};
-use super::winapi::shared::winerror;
-use super::winapi::shared::minwindef::{
-    DWORD,
-};
+use super::winapi::shared::minwindef::{DWORD, WORD};
 use super::winapi::shared::mmreg;
+use super::winapi::shared::winerror;
 use super::winapi::shared::wtypes;
+use super::winapi::Interface;
 // https://msdn.microsoft.com/en-us/library/cc230355.aspx
-use super::winapi::um::winnt::LPWSTR;
-use super::winapi::um::winnt::WCHAR;
-use super::winapi::um::coml2api;
 use super::winapi::um::audioclient::{
-    IAudioClient,
-    IID_IAudioClient,
-    AUDCLNT_E_DEVICE_INVALIDATED,
+    self, IAudioClient, IID_IAudioClient, AUDCLNT_E_DEVICE_INVALIDATED,
 };
 use super::winapi::um::audiosessiontypes::{
-    AUDCLNT_SHAREMODE_SHARED,
+    AUDCLNT_SHAREMODE_SHARED, AUDCLNT_STREAMFLAGS_EVENTCALLBACK,
 };
 use super::winapi::um::combaseapi::{
-    CoCreateInstance,
-    CoTaskMemFree,
-    CLSCTX_ALL,
-    PropVariantClear,
+    CoCreateInstance, CoTaskMemFree, PropVariantClear, CLSCTX_ALL,
 };
+use super::winapi::um::coml2api;
 use super::winapi::um::mmdeviceapi::{
-    eAll,
-    eCapture,
-    eConsole,
-    eRender,
-    CLSID_MMDeviceEnumerator,
-    DEVICE_STATE_ACTIVE,
-    EDataFlow,
-    IMMDevice,
-    IMMDeviceCollection,
-    IMMDeviceEnumerator,
-    IMMEndpoint,
+    eAll, eCapture, eConsole, eRender, CLSID_MMDeviceEnumerator, EDataFlow, IMMDevice,
+    IMMDeviceCollection, IMMDeviceEnumerator, IMMEndpoint, DEVICE_STATE_ACTIVE,
 };
+use super::winapi::um::winnt::LPWSTR;
+use super::winapi::um::winnt::WCHAR;
+
+use super::{
+    stream::{AudioClientFlow, Stream, StreamInner},
+    winapi::um::synchapi,
+};
+use crate::{traits::DeviceTrait, BuildStreamError, StreamData, StreamError};
 
 pub type SupportedInputFormats = std::vec::IntoIter<SupportedFormat>;
 pub type SupportedOutputFormats = std::vec::IntoIter<SupportedFormat>;
@@ -74,10 +62,8 @@ pub type SupportedOutputFormats = std::vec::IntoIter<SupportedFormat>;
 /// Wrapper because of that stupid decision to remove `Send` and `Sync` from raw pointers.
 #[derive(Copy, Clone)]
 struct IAudioClientWrapper(*mut IAudioClient);
-unsafe impl Send for IAudioClientWrapper {
-}
-unsafe impl Sync for IAudioClientWrapper {
-}
+unsafe impl Send for IAudioClientWrapper {}
+unsafe impl Sync for IAudioClientWrapper {}
 
 /// An opaque type that identifies an end point.
 pub struct Device {
@@ -87,6 +73,70 @@ pub struct Device {
     future_audio_client: Arc<Mutex<Option<IAudioClientWrapper>>>, // TODO: add NonZero around the ptr
 }
 
+impl DeviceTrait for Device {
+    type SupportedInputFormats = SupportedInputFormats;
+    type SupportedOutputFormats = SupportedOutputFormats;
+    type Stream = Stream;
+
+    fn name(&self) -> Result<String, DeviceNameError> {
+        Device::name(self)
+    }
+
+    fn supported_input_formats(
+        &self,
+    ) -> Result<Self::SupportedInputFormats, SupportedFormatsError> {
+        Device::supported_input_formats(self)
+    }
+
+    fn supported_output_formats(
+        &self,
+    ) -> Result<Self::SupportedOutputFormats, SupportedFormatsError> {
+        Device::supported_output_formats(self)
+    }
+
+    fn default_input_format(&self) -> Result<Format, DefaultFormatError> {
+        Device::default_input_format(self)
+    }
+
+    fn default_output_format(&self) -> Result<Format, DefaultFormatError> {
+        Device::default_output_format(self)
+    }
+
+    fn build_input_stream<D, E>(
+        &self,
+        format: &Format,
+        data_callback: D,
+        error_callback: E,
+    ) -> Result<Self::Stream, BuildStreamError>
+    where
+        D: FnMut(StreamData) + Send + 'static,
+        E: FnMut(StreamError) + Send + 'static,
+    {
+        Ok(Stream::new(
+            self.build_input_stream_inner(format)?,
+            data_callback,
+            error_callback,
+        ))
+    }
+
+    fn build_output_stream<D, E>(
+        &self,
+        format: &Format,
+        data_callback: D,
+        error_callback: E,
+    ) -> Result<Self::Stream, BuildStreamError>
+    where
+        D: FnMut(StreamData) + Send + 'static,
+        E: FnMut(StreamError) + Send + 'static,
+    {
+        Ok(Stream::new(
+            self.build_output_stream_inner(format)?,
+            data_callback,
+            error_callback,
+        ))
+    }
+}
+
 struct Endpoint {
     endpoint: *mut IMMEndpoint,
 }
@@ -107,7 +157,6 @@ impl Drop for WaveFormatExPtr {
     }
 }
 
-
 impl WaveFormat {
     // Given a pointer to some format, returns a valid copy of the format.
     pub fn copy_from_waveformatex_ptr(ptr: *const mmreg::WAVEFORMATEX) -> Option<Self> {
@@ -115,11 +164,11 @@ impl WaveFormat {
             match (*ptr).wFormatTag {
                 mmreg::WAVE_FORMAT_PCM | mmreg::WAVE_FORMAT_IEEE_FLOAT => {
                     Some(WaveFormat::Ex(*ptr))
-                },
+                }
                 mmreg::WAVE_FORMAT_EXTENSIBLE => {
                     let extensible_ptr = ptr as *const mmreg::WAVEFORMATEXTENSIBLE;
                     Some(WaveFormat::Extensible(*extensible_ptr))
-                },
+                }
                 _ => None,
             }
         }
@@ -150,10 +199,11 @@ impl DerefMut for WaveFormat {
     }
 }
 
-
 unsafe fn immendpoint_from_immdevice(device: *const IMMDevice) -> *mut IMMEndpoint {
     let mut endpoint: *mut IMMEndpoint = mem::uninitialized();
-    check_result((*device).QueryInterface(&IMMEndpoint::uuidof(), &mut endpoint as *mut _ as *mut _))
+    check_result(
+        (*device).QueryInterface(&IMMEndpoint::uuidof(), &mut endpoint as *mut _ as *mut _),
+    )
     .expect("could not query IMMDevice interface for IMMEndpoint");
     endpoint
 }
@@ -169,10 +219,7 @@ unsafe fn data_flow_from_immendpoint(endpoint: *const IMMEndpoint) -> EDataFlow
 pub unsafe fn is_format_supported(
     client: *const IAudioClient,
     waveformatex_ptr: *const mmreg::WAVEFORMATEX,
-) -> Result<bool, SupportedFormatsError>
-{
-
-
+) -> Result<bool, SupportedFormatsError> {
     /*
     // `IsFormatSupported` checks whether the format is supported and fills
     // a `WAVEFORMATEX`
@@ -205,7 +252,6 @@ pub unsafe fn is_format_supported(
     };
     */
 
-
     // Check if the given format is supported.
     let is_supported = |waveformatex_ptr, mut closest_waveformatex_ptr| {
         let result = (*client).IsFormatSupported(
@@ -217,17 +263,11 @@ pub unsafe fn is_format_supported(
         // has been found, but not an exact match) so we also treat this as unsupported.
         match (result, check_result(result)) {
             (_, Err(ref e)) if e.raw_os_error() == Some(AUDCLNT_E_DEVICE_INVALIDATED) => {
-                return Err(SupportedFormatsError::DeviceNotAvailable);
-            },
-            (_, Err(_)) => {
-                Ok(false)
-            },
-            (winerror::S_FALSE, _) => {
-                Ok(false)
-            },
-            (_, Ok(())) => {
-                Ok(true)
-            },
+                Err(SupportedFormatsError::DeviceNotAvailable)
+            }
+            (_, Err(_)) => Ok(false),
+            (winerror::S_FALSE, _) => Ok(false),
+            (_, Ok(())) => Ok(true),
         }
     };
 
@@ -240,34 +280,30 @@ pub unsafe fn is_format_supported(
             let mut closest_waveformatex = *waveformatex_ptr;
             let closest_waveformatex_ptr = &mut closest_waveformatex as *mut _;
             is_supported(waveformatex_ptr, closest_waveformatex_ptr)
-        },
+        }
         mmreg::WAVE_FORMAT_EXTENSIBLE => {
-            let waveformatextensible_ptr =
-                waveformatex_ptr as *const mmreg::WAVEFORMATEXTENSIBLE;
+            let waveformatextensible_ptr = waveformatex_ptr as *const mmreg::WAVEFORMATEXTENSIBLE;
             let mut closest_waveformatextensible = *waveformatextensible_ptr;
-            let closest_waveformatextensible_ptr =
-                &mut closest_waveformatextensible as *mut _;
+            let closest_waveformatextensible_ptr = &mut closest_waveformatextensible as *mut _;
             let closest_waveformatex_ptr =
                 closest_waveformatextensible_ptr as *mut mmreg::WAVEFORMATEX;
             is_supported(waveformatex_ptr, closest_waveformatex_ptr)
-        },
+        }
         _ => Ok(false),
     }
 }
 
-
 // Get a cpal Format from a WAVEFORMATEX.
 unsafe fn format_from_waveformatex_ptr(
     waveformatex_ptr: *const mmreg::WAVEFORMATEX,
-) -> Option<Format>
-{
+) -> Option<Format> {
     fn cmp_guid(a: &GUID, b: &GUID) -> bool {
-        a.Data1 == b.Data1
-            && a.Data2 == b.Data2
-            && a.Data3 == b.Data3
-            && a.Data4 == b.Data4
+        a.Data1 == b.Data1 && a.Data2 == b.Data2 && a.Data3 == b.Data3 && a.Data4 == b.Data4
     }
-    let data_type = match ((*waveformatex_ptr).wBitsPerSample, (*waveformatex_ptr).wFormatTag) {
+    let data_type = match (
+        (*waveformatex_ptr).wBitsPerSample,
+        (*waveformatex_ptr).wFormatTag,
+    ) {
         (16, mmreg::WAVE_FORMAT_PCM) => SampleFormat::I16,
         (32, mmreg::WAVE_FORMAT_IEEE_FLOAT) => SampleFormat::F32,
         (n_bits, mmreg::WAVE_FORMAT_EXTENSIBLE) => {
@@ -280,22 +316,20 @@ unsafe fn format_from_waveformatex_ptr(
             } else {
                 return None;
             }
-        },
+        }
         // Unknown data format returned by GetMixFormat.
         _ => return None,
     };
     let format = Format {
         channels: (*waveformatex_ptr).nChannels as _,
         sample_rate: SampleRate((*waveformatex_ptr).nSamplesPerSec),
-        data_type: data_type,
+        data_type,
     };
     Some(format)
 }
 
-unsafe impl Send for Device {
-}
-unsafe impl Sync for Device {
-}
+unsafe impl Send for Device {}
+unsafe impl Sync for Device {}
 
 impl Device {
     pub fn name(&self) -> Result<String, DeviceNameError> {
@@ -306,12 +340,10 @@ impl Device {
 
             // Get the endpoint's friendly-name property.
             let mut property_value = mem::zeroed();
-            if let Err(err) = check_result(
-                (*property_store).GetValue(
+            if let Err(err) = check_result((*property_store).GetValue(
                 &devpkey::DEVPKEY_Device_FriendlyName as *const _ as *const _,
-                    &mut property_value
-                )
-            ) {
+                &mut property_value,
+            )) {
                 let description = format!("failed to retrieve name from property store: {}", err);
                 let err = BackendSpecificError { description };
                 return Err(err.into());
@@ -319,13 +351,14 @@ impl Device {
 
             // Read the friendly-name from the union data field, expecting a *const u16.
             if property_value.vt != wtypes::VT_LPWSTR as _ {
-                let description =
-                    format!("property store produced invalid data: {:?}", property_value.vt);
+                let description = format!(
+                    "property store produced invalid data: {:?}",
+                    property_value.vt
+                );
                 let err = BackendSpecificError { description };
                 return Err(err.into());
             }
-            let ptr_usize: usize = *(&property_value.data as *const _ as *const usize);
-            let ptr_utf16 = ptr_usize as *const u16;
+            let ptr_utf16 = *(&property_value.data as *const _ as *const (*const u16));
 
             // Find the length of the friendly name.
             let mut len = 0;
@@ -351,14 +384,15 @@ impl Device {
     #[inline]
     fn from_immdevice(device: *mut IMMDevice) -> Self {
         Device {
-            device: device,
+            device,
             future_audio_client: Arc::new(Mutex::new(None)),
         }
     }
 
     /// Ensures that `future_audio_client` contains a `Some` and returns a locked mutex to it.
-    fn ensure_future_audio_client(&self)
-                                  -> Result<MutexGuard<Option<IAudioClientWrapper>>, IoError> {
+    fn ensure_future_audio_client(
+        &self,
+    ) -> Result<MutexGuard<Option<IAudioClientWrapper>>, IoError> {
         let mut lock = self.future_audio_client.lock().unwrap();
         if lock.is_some() {
             return Ok(lock);
@@ -366,10 +400,12 @@ impl Device {
 
         let audio_client: *mut IAudioClient = unsafe {
             let mut audio_client = mem::uninitialized();
-            let hresult = (*self.device).Activate(&IID_IAudioClient,
+            let hresult = (*self.device).Activate(
+                &IID_IAudioClient,
                 CLSCTX_ALL,
                 ptr::null_mut(),
-                                                  &mut audio_client);
+                &mut audio_client,
+            );
 
             // can fail if the device has been disconnected since we enumerated it, or if
             // the device doesn't support playback for some reason
@@ -416,7 +452,7 @@ impl Device {
                 let description = format!("{}", e);
                 let err = BackendSpecificError { description };
                 return Err(err.into());
-            },
+            }
         };
         let client = lock.unwrap().0;
 
@@ -427,16 +463,19 @@ impl Device {
                 Ok(()) => (),
                 Err(ref e) if e.raw_os_error() == Some(AUDCLNT_E_DEVICE_INVALIDATED) => {
                     return Err(SupportedFormatsError::DeviceNotAvailable);
-                },
+                }
                 Err(e) => {
                     let description = format!("{}", e);
                     let err = BackendSpecificError { description };
                     return Err(err.into());
-                },
+                }
             };
 
             // If the default format can't succeed we have no hope of finding other formats.
-            assert_eq!(try!(is_format_supported(client, default_waveformatex_ptr.0)), true);
+            assert_eq!(
+                is_format_supported(client, default_waveformatex_ptr.0)?,
+                true
+            );
 
             // Copy the format to use as a test format (as to avoid mutating the original format).
             let mut test_format = {
@@ -457,8 +496,8 @@ impl Device {
                 let rate = rate.0 as DWORD;
                 test_format.nSamplesPerSec = rate;
                 test_format.nAvgBytesPerSec =
-                    rate * (*default_waveformatex_ptr.0).nBlockAlign as DWORD;
-                if try!(is_format_supported(client, test_format.as_ptr())) {
+                    rate * u32::from((*default_waveformatex_ptr.0).nBlockAlign);
+                if is_format_supported(client, test_format.as_ptr())? {
                     supported_sample_rates.push(rate);
                 }
             }
@@ -503,7 +542,9 @@ impl Device {
         }
     }
 
-    pub fn supported_output_formats(&self) -> Result<SupportedOutputFormats, SupportedFormatsError> {
+    pub fn supported_output_formats(
+        &self,
+    ) -> Result<SupportedOutputFormats, SupportedFormatsError> {
         if self.data_flow() == eRender {
             self.supported_formats()
         // If it's an input device, assume no output formats.
@@ -538,12 +579,12 @@ impl Device {
             match check_result((*client).GetMixFormat(&mut format_ptr.0)) {
                 Err(ref e) if e.raw_os_error() == Some(AUDCLNT_E_DEVICE_INVALIDATED) => {
                     return Err(DefaultFormatError::DeviceNotAvailable);
-                },
+                }
                 Err(e) => {
                     let description = format!("{}", e);
                     let err = BackendSpecificError { description };
                     return Err(err.into());
-                },
+                }
                 Ok(()) => (),
             };
 
@@ -573,6 +614,295 @@ impl Device {
             Err(DefaultFormatError::StreamTypeNotSupported)
         }
     }
+
+    pub(crate) fn build_input_stream_inner(
+        &self,
+        format: &Format,
+    ) -> Result<StreamInner, BuildStreamError> {
+        unsafe {
+            // Making sure that COM is initialized.
+            // It's not actually sure that this is required, but when in doubt do it.
+            com::com_initialized();
+
+            // Obtaining a `IAudioClient`.
+            let audio_client = match self.build_audioclient() {
+                Ok(client) => client,
+                Err(ref e) if e.raw_os_error() == Some(AUDCLNT_E_DEVICE_INVALIDATED) => {
+                    return Err(BuildStreamError::DeviceNotAvailable)
+                }
+                Err(e) => {
+                    let description = format!("{}", e);
+                    let err = BackendSpecificError { description };
+                    return Err(err.into());
+                }
+            };
+
+            // Computing the format and initializing the device.
+            let waveformatex = {
+                let format_attempt = format_to_waveformatextensible(format)
+                    .ok_or(BuildStreamError::FormatNotSupported)?;
+                let share_mode = AUDCLNT_SHAREMODE_SHARED;
+
+                // Ensure the format is supported.
+                match super::device::is_format_supported(audio_client, &format_attempt.Format) {
+                    Ok(false) => return Err(BuildStreamError::FormatNotSupported),
+                    Err(_) => return Err(BuildStreamError::DeviceNotAvailable),
+                    _ => (),
+                }
+
+                // finally initializing the audio client
+                let hresult = (*audio_client).Initialize(
+                    share_mode,
+                    AUDCLNT_STREAMFLAGS_EVENTCALLBACK,
+                    0,
+                    0,
+                    &format_attempt.Format,
+                    ptr::null(),
+                );
+                match check_result(hresult) {
+                    Err(ref e) if e.raw_os_error() == Some(AUDCLNT_E_DEVICE_INVALIDATED) => {
+                        (*audio_client).Release();
+                        return Err(BuildStreamError::DeviceNotAvailable);
+                    }
+                    Err(e) => {
+                        (*audio_client).Release();
+                        let description = format!("{}", e);
+                        let err = BackendSpecificError { description };
+                        return Err(err.into());
+                    }
+                    Ok(()) => (),
+                };
+
+                format_attempt.Format
+            };
+
+            // obtaining the size of the samples buffer in number of frames
+            let max_frames_in_buffer = {
+                let mut max_frames_in_buffer = mem::uninitialized();
+                let hresult = (*audio_client).GetBufferSize(&mut max_frames_in_buffer);
+
+                match check_result(hresult) {
+                    Err(ref e) if e.raw_os_error() == Some(AUDCLNT_E_DEVICE_INVALIDATED) => {
+                        (*audio_client).Release();
+                        return Err(BuildStreamError::DeviceNotAvailable);
+                    }
+                    Err(e) => {
+                        (*audio_client).Release();
+                        let description = format!("{}", e);
+                        let err = BackendSpecificError { description };
+                        return Err(err.into());
+                    }
+                    Ok(()) => (),
+                };
+
+                max_frames_in_buffer
+            };
+
+            // Creating the event that will be signalled whenever we need to submit some samples.
+            let event = {
+                let event = synchapi::CreateEventA(ptr::null_mut(), 0, 0, ptr::null());
+                if event.is_null() {
+                    (*audio_client).Release();
+                    let description = "failed to create event".to_string();
+                    let err = BackendSpecificError { description };
+                    return Err(err.into());
+                }
+
+                if let Err(e) = check_result((*audio_client).SetEventHandle(event)) {
+                    (*audio_client).Release();
+                    let description = format!("failed to call SetEventHandle: {}", e);
+                    let err = BackendSpecificError { description };
+                    return Err(err.into());
+                }
+
+                event
+            };
+
+            // Building a `IAudioCaptureClient` that will be used to read captured samples.
+            let capture_client = {
+                let mut capture_client: *mut audioclient::IAudioCaptureClient =
+                    mem::uninitialized();
+                let hresult = (*audio_client).GetService(
+                    &audioclient::IID_IAudioCaptureClient,
+                    &mut capture_client as *mut *mut audioclient::IAudioCaptureClient as *mut _,
+                );
+
+                match check_result(hresult) {
+                    Err(ref e) if e.raw_os_error() == Some(AUDCLNT_E_DEVICE_INVALIDATED) => {
+                        (*audio_client).Release();
+                        return Err(BuildStreamError::DeviceNotAvailable);
+                    }
+                    Err(e) => {
+                        (*audio_client).Release();
+                        let description = format!("failed to build capture client: {}", e);
+                        let err = BackendSpecificError { description };
+                        return Err(err.into());
+                    }
+                    Ok(()) => (),
+                };
+
+                &mut *capture_client
+            };
+
+            // Once we built the `StreamInner`, we add a command that will be picked up by the
+            // `run()` method and added to the `RunContext`.
+            let client_flow = AudioClientFlow::Capture { capture_client };
+
+            Ok(StreamInner {
+                audio_client,
+                client_flow,
+                event,
+                playing: false,
+                max_frames_in_buffer,
+                bytes_per_frame: waveformatex.nBlockAlign,
+                sample_format: format.data_type,
+            })
+        }
+    }
+
+    pub(crate) fn build_output_stream_inner(
+        &self,
+        format: &Format,
+    ) -> Result<StreamInner, BuildStreamError> {
+        unsafe {
+            // Making sure that COM is initialized.
+            // It's not actually sure that this is required, but when in doubt do it.
+            com::com_initialized();
+
+            // Obtaining a `IAudioClient`.
+            let audio_client = match self.build_audioclient() {
+                Ok(client) => client,
+                Err(ref e) if e.raw_os_error() == Some(AUDCLNT_E_DEVICE_INVALIDATED) => {
+                    return Err(BuildStreamError::DeviceNotAvailable)
+                }
+                Err(e) => {
+                    let description = format!("{}", e);
+                    let err = BackendSpecificError { description };
+                    return Err(err.into());
+                }
+            };
+
+            // Computing the format and initializing the device.
+            let waveformatex = {
+                let format_attempt = format_to_waveformatextensible(format)
+                    .ok_or(BuildStreamError::FormatNotSupported)?;
+                let share_mode = AUDCLNT_SHAREMODE_SHARED;
+
+                // Ensure the format is supported.
+                match super::device::is_format_supported(audio_client, &format_attempt.Format) {
+                    Ok(false) => return Err(BuildStreamError::FormatNotSupported),
+                    Err(_) => return Err(BuildStreamError::DeviceNotAvailable),
+                    _ => (),
+                }
+
+                // finally initializing the audio client
+                let hresult = (*audio_client).Initialize(
+                    share_mode,
+                    AUDCLNT_STREAMFLAGS_EVENTCALLBACK,
+                    0,
+                    0,
+                    &format_attempt.Format,
+                    ptr::null(),
+                );
+                match check_result(hresult) {
+                    Err(ref e) if e.raw_os_error() == Some(AUDCLNT_E_DEVICE_INVALIDATED) => {
+                        (*audio_client).Release();
+                        return Err(BuildStreamError::DeviceNotAvailable);
+                    }
+                    Err(e) => {
+                        (*audio_client).Release();
+                        let description = format!("{}", e);
+                        let err = BackendSpecificError { description };
+                        return Err(err.into());
+                    }
+                    Ok(()) => (),
+                };
+
+                format_attempt.Format
+            };
+
+            // Creating the event that will be signalled whenever we need to submit some samples.
+            let event = {
+                let event = synchapi::CreateEventA(ptr::null_mut(), 0, 0, ptr::null());
+                if event.is_null() {
+                    (*audio_client).Release();
+                    let description = "failed to create event".to_string();
+                    let err = BackendSpecificError { description };
+                    return Err(err.into());
+                }
+
+                if let Err(e) = check_result((*audio_client).SetEventHandle(event)) {
+                    (*audio_client).Release();
+                    let description = format!("failed to call SetEventHandle: {}", e);
+                    let err = BackendSpecificError { description };
+                    return Err(err.into());
+                };
+
+                event
+            };
+
+            // obtaining the size of the samples buffer in number of frames
+            let max_frames_in_buffer = {
+                let mut max_frames_in_buffer = mem::uninitialized();
+                let hresult = (*audio_client).GetBufferSize(&mut max_frames_in_buffer);
+
+                match check_result(hresult) {
+                    Err(ref e) if e.raw_os_error() == Some(AUDCLNT_E_DEVICE_INVALIDATED) => {
+                        (*audio_client).Release();
+                        return Err(BuildStreamError::DeviceNotAvailable);
+                    }
+                    Err(e) => {
+                        (*audio_client).Release();
+                        let description = format!("failed to obtain buffer size: {}", e);
+                        let err = BackendSpecificError { description };
+                        return Err(err.into());
+                    }
+                    Ok(()) => (),
+                };
+
+                max_frames_in_buffer
+            };
+
+            // Building a `IAudioRenderClient` that will be used to fill the samples buffer.
+            let render_client = {
+                let mut render_client: *mut audioclient::IAudioRenderClient = mem::uninitialized();
+                let hresult = (*audio_client).GetService(
+                    &audioclient::IID_IAudioRenderClient,
+                    &mut render_client as *mut *mut audioclient::IAudioRenderClient as *mut _,
+                );
+
+                match check_result(hresult) {
+                    Err(ref e) if e.raw_os_error() == Some(AUDCLNT_E_DEVICE_INVALIDATED) => {
+                        (*audio_client).Release();
+                        return Err(BuildStreamError::DeviceNotAvailable);
+                    }
+                    Err(e) => {
+                        (*audio_client).Release();
+                        let description = format!("failed to build render client: {}", e);
+                        let err = BackendSpecificError { description };
+                        return Err(err.into());
+                    }
+                    Ok(()) => (),
+                };
+
+                &mut *render_client
+            };
+
+            // Once we built the `StreamInner`, we add a command that will be picked up by the
+            // `run()` method and added to the `RunContext`.
+            let client_flow = AudioClientFlow::Render { render_client };
+
+            Ok(StreamInner {
+                audio_client,
+                client_flow,
+                event,
+                playing: false,
+                max_frames_in_buffer,
+                bytes_per_frame: waveformatex.nBlockAlign,
+                sample_format: format.data_type,
+            })
+        }
+    }
 }
 
 impl PartialEq for Device {
@@ -586,38 +916,45 @@ impl PartialEq for Device {
         // In this code section we're trying to use the GetId method for the device comparison, cf.
         // https://docs.microsoft.com/en-us/windows/desktop/api/mmdeviceapi/nf-mmdeviceapi-immdevice-getid
         unsafe {
-            struct IdRAII (LPWSTR);
+            struct IdRAII(LPWSTR);
             /// RAII for device IDs.
             impl Drop for IdRAII {
                 fn drop(&mut self) {
-                    unsafe {CoTaskMemFree(self.0 as *mut c_void)}
+                    unsafe { CoTaskMemFree(self.0 as *mut c_void) }
                 }
             }
             let mut id1: LPWSTR = ptr::null_mut();
             let rc1 = (*self.device).GetId(&mut id1);
             // GetId only fails with E_OUTOFMEMORY and if it does, we're probably dead already.
             // Plus it won't do to change the device comparison logic unexpectedly.
-            if rc1 != winerror::S_OK {panic! ("cpal: GetId failure: {}", rc1)}
+            if rc1 != winerror::S_OK {
+                panic!("cpal: GetId failure: {}", rc1)
+            }
             let id1 = IdRAII(id1);
             let mut id2: LPWSTR = ptr::null_mut();
             let rc2 = (*other.device).GetId(&mut id2);
-            if rc2 != winerror::S_OK {panic! ("cpal: GetId failure: {}", rc1)}
+            if rc2 != winerror::S_OK {
+                panic!("cpal: GetId failure: {}", rc1)
+            }
             let id2 = IdRAII(id2);
             // 16-bit null-terminated comparison.
             let mut offset = 0;
             loop {
                 let w1: WCHAR = *id1.0.offset(offset);
                 let w2: WCHAR = *id2.0.offset(offset);
-                if w1 == 0 && w2 == 0 {return true}
-                if w1 != w2 {return false}
+                if w1 == 0 && w2 == 0 {
+                    return true;
+                }
+                if w1 != w2 {
+                    return false;
+                }
                 offset += 1;
             }
         }
     }
 }
 
-impl Eq for Device {
-}
+impl Eq for Device {}
 
 impl Clone for Device {
     #[inline]
@@ -669,16 +1006,14 @@ impl From<*const IMMDevice> for Endpoint {
     fn from(device: *const IMMDevice) -> Self {
         unsafe {
             let endpoint = immendpoint_from_immdevice(device);
-            Endpoint { endpoint: endpoint }
+            Endpoint { endpoint }
         }
     }
 }
 
 impl Endpoint {
     fn data_flow(&self) -> EDataFlow {
-        unsafe {
-            data_flow_from_immendpoint(self.endpoint)
-        }
+        unsafe { data_flow_from_immendpoint(self.endpoint) }
     }
 }
 
@@ -709,10 +1044,8 @@ lazy_static! {
 /// RAII object around `IMMDeviceEnumerator`.
 struct Enumerator(*mut IMMDeviceEnumerator);
 
-unsafe impl Send for Enumerator {
-}
-unsafe impl Sync for Enumerator {
-}
+unsafe impl Send for Enumerator {}
+unsafe impl Sync for Enumerator {}
 
 impl Drop for Enumerator {
     #[inline]
@@ -735,20 +1068,18 @@ impl Devices {
         unsafe {
             let mut collection: *mut IMMDeviceCollection = mem::uninitialized();
             // can fail because of wrong parameters (should never happen) or out of memory
-            check_result_backend_specific(
-                (*ENUMERATOR.0).EnumAudioEndpoints(
+            check_result_backend_specific((*ENUMERATOR.0).EnumAudioEndpoints(
                 eAll,
                 DEVICE_STATE_ACTIVE,
                 &mut collection,
-                )
-            )?;
+            ))?;
 
-            let mut count = mem::uninitialized();
+            let count = mem::uninitialized();
             // can fail if the parameter is null, which should never happen
-            check_result_backend_specific((*collection).GetCount(&mut count))?;
+            check_result_backend_specific((*collection).GetCount(&count))?;
 
             Ok(Devices {
-                collection: collection,
+                collection,
                 total_count: count,
                 next_item: 0,
             })
@@ -756,10 +1087,8 @@ impl Devices {
     }
 }
 
-unsafe impl Send for Devices {
-}
-unsafe impl Sync for Devices {
-}
+unsafe impl Send for Devices {}
+unsafe impl Sync for Devices {}
 
 impl Drop for Devices {
     #[inline]
@@ -799,8 +1128,7 @@ impl Iterator for Devices {
 fn default_device(data_flow: EDataFlow) -> Option<Device> {
     unsafe {
         let mut device = mem::uninitialized();
-        let hres = (*ENUMERATOR.0)
-            .GetDefaultAudioEndpoint(data_flow, eConsole, &mut device);
+        let hres = (*ENUMERATOR.0).GetDefaultAudioEndpoint(data_flow, eConsole, &mut device);
         if let Err(_err) = check_result(hres) {
             return None; // TODO: check specifically for `E_NOTFOUND`, and panic otherwise
         }
@@ -815,3 +1143,57 @@ pub fn default_input_device() -> Option<Device> {
 pub fn default_output_device() -> Option<Device> {
     default_device(eRender)
 }
+
+// Turns a `Format` into a `WAVEFORMATEXTENSIBLE`.
+//
+// Returns `None` if the WAVEFORMATEXTENSIBLE does not support the given format.
+fn format_to_waveformatextensible(format: &Format) -> Option<mmreg::WAVEFORMATEXTENSIBLE> {
+    let format_tag = match format.data_type {
+        SampleFormat::I16 => mmreg::WAVE_FORMAT_PCM,
+        SampleFormat::F32 => mmreg::WAVE_FORMAT_EXTENSIBLE,
+        SampleFormat::U16 => return None,
+    };
+    let channels = format.channels as WORD;
+    let sample_rate = format.sample_rate.0 as DWORD;
+    let sample_bytes = format.data_type.sample_size() as WORD;
+    let avg_bytes_per_sec = u32::from(channels) * sample_rate * u32::from(sample_bytes);
+    let block_align = channels * sample_bytes;
+    let bits_per_sample = 8 * sample_bytes;
+    let cb_size = match format.data_type {
+        SampleFormat::I16 => 0,
+        SampleFormat::F32 => {
+            let extensible_size = mem::size_of::<mmreg::WAVEFORMATEXTENSIBLE>();
+            let ex_size = mem::size_of::<mmreg::WAVEFORMATEX>();
+            (extensible_size - ex_size) as WORD
+        }
+        SampleFormat::U16 => return None,
+    };
+    let waveformatex = mmreg::WAVEFORMATEX {
+        wFormatTag: format_tag,
+        nChannels: channels,
+        nSamplesPerSec: sample_rate,
+        nAvgBytesPerSec: avg_bytes_per_sec,
+        nBlockAlign: block_align,
+        wBitsPerSample: bits_per_sample,
+        cbSize: cb_size,
+    };
+
+    // CPAL does not care about speaker positions, so pass audio straight through.
+    // TODO: This constant should be defined in winapi but is missing.
+    const KSAUDIO_SPEAKER_DIRECTOUT: DWORD = 0;
+    let channel_mask = KSAUDIO_SPEAKER_DIRECTOUT;
+
+    let sub_format = match format.data_type {
+        SampleFormat::I16 => ksmedia::KSDATAFORMAT_SUBTYPE_PCM,
+        SampleFormat::F32 => ksmedia::KSDATAFORMAT_SUBTYPE_IEEE_FLOAT,
+        SampleFormat::U16 => return None,
+    };
+    let waveformatextensible = mmreg::WAVEFORMATEXTENSIBLE {
+        Format: waveformatex,
+        Samples: bits_per_sample as WORD,
+        dwChannelMask: channel_mask,
+        SubFormat: sub_format,
+    };
+
+    Some(waveformatextensible)
+}

src/host/wasapi/mod.rs

@@ -1,20 +1,15 @@
 extern crate winapi;
 
-use BackendSpecificError;
-use BuildStreamError;
-use DefaultFormatError;
-use DeviceNameError;
-use DevicesError;
-use Format;
-use PlayStreamError;
-use PauseStreamError;
-use StreamDataResult;
-use SupportedFormatsError;
+pub use self::device::{
+    default_input_device, default_output_device, Device, Devices, SupportedInputFormats,
+    SupportedOutputFormats,
+};
+pub use self::stream::Stream;
 use self::winapi::um::winnt::HRESULT;
 use std::io::Error as IoError;
-use traits::{DeviceTrait, EventLoopTrait, HostTrait, StreamIdTrait};
-pub use self::device::{Device, Devices, SupportedInputFormats, SupportedOutputFormats, default_input_device, default_output_device};
-pub use self::stream::{EventLoop, StreamId};
+use traits::HostTrait;
+use BackendSpecificError;
+use DevicesError;
 
 mod com;
 mod device;
@@ -37,7 +32,6 @@ impl Host {
 impl HostTrait for Host {
     type Devices = Devices;
     type Device = Device;
-    type EventLoop = EventLoop;
 
     fn is_available() -> bool {
         // Assume WASAPI is always available on windows.
@@ -55,79 +49,8 @@ impl HostTrait for Host {
     fn default_output_device(&self) -> Option<Self::Device> {
         default_output_device()
     }
-
-    fn event_loop(&self) -> Self::EventLoop {
-        EventLoop::new()
-    }
 }
 
-impl DeviceTrait for Device {
-    type SupportedInputFormats = SupportedInputFormats;
-    type SupportedOutputFormats = SupportedOutputFormats;
-
-    fn name(&self) -> Result<String, DeviceNameError> {
-        Device::name(self)
-    }
-
-    fn supported_input_formats(&self) -> Result<Self::SupportedInputFormats, SupportedFormatsError> {
-        Device::supported_input_formats(self)
-    }
-
-    fn supported_output_formats(&self) -> Result<Self::SupportedOutputFormats, SupportedFormatsError> {
-        Device::supported_output_formats(self)
-    }
-
-    fn default_input_format(&self) -> Result<Format, DefaultFormatError> {
-        Device::default_input_format(self)
-    }
-
-    fn default_output_format(&self) -> Result<Format, DefaultFormatError> {
-        Device::default_output_format(self)
-    }
-}
-
-impl EventLoopTrait for EventLoop {
-    type Device = Device;
-    type StreamId = StreamId;
-
-    fn build_input_stream(
-        &self,
-        device: &Self::Device,
-        format: &Format,
-    ) -> Result<Self::StreamId, BuildStreamError> {
-        EventLoop::build_input_stream(self, device, format)
-    }
-
-    fn build_output_stream(
-        &self,
-        device: &Self::Device,
-        format: &Format,
-    ) -> Result<Self::StreamId, BuildStreamError> {
-        EventLoop::build_output_stream(self, device, format)
-    }
-
-    fn play_stream(&self, stream: Self::StreamId) -> Result<(), PlayStreamError> {
-        EventLoop::play_stream(self, stream)
-    }
-
-    fn pause_stream(&self, stream: Self::StreamId) -> Result<(), PauseStreamError> {
-        EventLoop::pause_stream(self, stream)
-    }
-
-    fn destroy_stream(&self, stream: Self::StreamId) {
-        EventLoop::destroy_stream(self, stream)
-    }
-
-    fn run<F>(&self, callback: F) -> !
-    where
-        F: FnMut(Self::StreamId, StreamDataResult) + Send,
-    {
-        EventLoop::run(self, callback)
-    }
-}
-
-impl StreamIdTrait for StreamId {}
-
 #[inline]
 fn check_result(result: HRESULT) -> Result<(), IoError> {
     if result < 0 {
@@ -140,9 +63,8 @@ fn check_result(result: HRESULT) -> Result<(), IoError> {
 fn check_result_backend_specific(result: HRESULT) -> Result<(), BackendSpecificError> {
     match check_result(result) {
         Ok(()) => Ok(()),
-        Err(err) => {
-            let description = format!("{}", err);
-            return Err(BackendSpecificError { description });
-        }
+        Err(err) => Err(BackendSpecificError {
+            description: format!("{}", err),
+        }),
     }
 }

src/lib.rs

@@ -7,25 +7,21 @@
 //!   least one [**DefaultHost**](./struct.Host.html) that is guaranteed to be available.
 //! - A [**Device**](./struct.Device.html) is an audio device that may have any number of input and
 //!   output streams.
-//! - A stream is an open flow of audio data. Input streams allow you to receive audio data, output
-//!   streams allow you to play audio data. You must choose which **Device** will run your stream
-//!   before you can create one. Often, a default device can be retrieved via the **Host**.
-//! - An [**EventLoop**](./struct.EventLoop.html) is a collection of streams being run by one or
-//!   more **Device**s under a single **Host**. Each stream must belong to an **EventLoop**, and
-//!   all the streams that belong to an **EventLoop** are managed together.
+//! - A [**Stream**](./trait.Stream.html) is an open flow of audio data. Input streams allow you to
+//!   receive audio data, output streams allow you to play audio data. You must choose which
+//!   **Device** will run your stream before you can create one. Often, a default device can be
+//!   retrieved via the **Host**.
 //!
-//! The first step is to initialise the `Host` (for accessing audio devices) and create an
-//! `EventLoop`:
+//! The first step is to initialise the `Host`:
 //!
 //! ```
 //! use cpal::traits::HostTrait;
 //! let host = cpal::default_host();
-//! let event_loop = host.event_loop();
 //! ```
 //!
-//! Then choose a `Device`. The easiest way is to use the default input or output `Device` via the
-//! `default_input_device()` or `default_output_device()` functions. Alternatively you can
-//! enumerate all the available devices with the `devices()` function. Beware that the
+//! Then choose an available `Device`. The easiest way is to use the default input or output
+//! `Device` via the `default_input_device()` or `default_output_device()` functions. Alternatively
+//! you can enumerate all the available devices with the `devices()` function. Beware that the
 //! `default_*_device()` functions return an `Option` in case no device is available for that
 //! stream type on the system.
 //!
@@ -56,68 +52,51 @@
 //!     .with_max_sample_rate();
 //! ```
 //!
-//! Now that we have everything for the stream, we can create it from our event loop:
+//! Now that we have everything for the stream, we are ready to create it from our selected device:
 //!
 //! ```no_run
-//! use cpal::traits::{DeviceTrait, EventLoopTrait, HostTrait};
+//! use cpal::traits::{DeviceTrait, HostTrait, StreamTrait};
 //! # let host = cpal::default_host();
-//! # let event_loop = host.event_loop();
 //! # let device = host.default_output_device().unwrap();
-//! # let format = device.supported_output_formats().unwrap().next().unwrap().with_max_sample_rate();
-//! let stream_id = event_loop.build_output_stream(&device, &format).unwrap();
+//! # let format = device.default_output_format().unwrap();
+//! let stream = device.build_output_stream(
+//!     &format,
+//!     move |data| {
+//!         // react to stream events and read or write stream data here.
+//!     },
+//!     move |err| {
+//!         // react to errors here.
+//!     },
+//! );
 //! ```
 //!
-//! The value returned by `build_output_stream()` is of type `StreamId` and is an identifier that
-//! will allow you to control the stream.
+//! While the stream is running, the selected audio device will periodically call the data callback
+//! that was passed to the function. The callback is passed an instance of type `StreamData` that
+//! represents the data that must be read from or written to. The inner `UnknownTypeOutputBuffer`
+//! can be one of `I16`, `U16` or `F32` depending on the format that was passed to
+//! `build_output_stream`.
 //!
-//! Now we must start the stream. This is done with the `play_stream()` method on the event loop.
-//!
-//! ```no_run
-//! # use cpal::traits::{EventLoopTrait, HostTrait};
-//! # let host = cpal::default_host();
-//! # let event_loop = host.event_loop();
-//! # let stream_id = unimplemented!();
-//! event_loop.play_stream(stream_id).expect("failed to play_stream");
-//! ```
-//!
-//! Now everything is ready! We call `run()` on the `event_loop` to begin processing.
-//!
-//! ```no_run
-//! # use cpal::traits::{EventLoopTrait, HostTrait};
-//! # let host = cpal::default_host();
-//! # let event_loop = host.event_loop();
-//! event_loop.run(move |_stream_id, _stream_result| {
-//!     // react to stream events and read or write stream data here
-//! });
-//! ```
-//!
-//! > **Note**: Calling `run()` will block the thread forever, so it's usually best done in a
-//! > separate thread.
-//!
-//! While `run()` is running, the audio device of the user will from time to time call the callback
-//! that you passed to this function. The callback gets passed the stream ID and an instance of type
-//! `StreamData` that represents the data that must be read from or written to. The inner
-//! `UnknownTypeOutputBuffer` can be one of `I16`, `U16` or `F32` depending on the format that was
-//! passed to `build_output_stream`.
+//! > **Note**: Creating and running a stream will *not* block the thread. On modern platforms, the
+//! > given callback is called by a dedicated, high-priority thread responsible for delivering
+//! > audio data to the system's audio device in a timely manner. On older platforms that only
+//! > provide a blocking API (e.g. ALSA), CPAL will create a thread in order to consistently
+//! > provide non-blocking behaviour (currently this is a thread per stream, but this may change to
+//! > use a single thread for all streams). *If this is an issue for your platform or design,
+//! > please share your issue and use-case with the CPAL team on the github issue tracker for
+//! > consideration.*
 //!
 //! In this example, we simply fill the given output buffer with zeroes.
 //!
 //! ```no_run
 //! use cpal::{StreamData, UnknownTypeOutputBuffer};
-//! use cpal::traits::{EventLoopTrait, HostTrait};
+//! use cpal::traits::{DeviceTrait, HostTrait, StreamTrait};
 //! # let host = cpal::default_host();
-//! # let event_loop = host.event_loop();
-//! event_loop.run(move |stream_id, stream_result| {
-//!     let stream_data = match stream_result {
-//!         Ok(data) => data,
-//!         Err(err) => {
-//!             eprintln!("an error occurred on stream {:?}: {}", stream_id, err);
-//!             return;
-//!         }
-//!         _ => return,
-//!     };
-//!
-//!     match stream_data {
+//! # let device = host.default_output_device().unwrap();
+//! # let format = device.default_output_format().unwrap();
+//! let stream = device.build_output_stream(
+//!     &format,
+//!     move |data| {
+//!         match data {
 //!             StreamData::Output { buffer: UnknownTypeOutputBuffer::U16(mut buffer) } => {
 //!                 for elem in buffer.iter_mut() {
 //!                     *elem = u16::max_value() / 2;
@@ -135,8 +114,35 @@
 //!             },
 //!             _ => (),
 //!         }
-//! });
+//!     },
+//!     move |err| {
+//!         eprintln!("an error occurred on the output audio stream: {}", err);
+//!     },
+//! );
 //! ```
+//!
+//! Not all platforms automatically run the stream upon creation. To ensure the stream has started,
+//! we can use `Stream::play`.
+//!
+//! ```no_run
+//! # use cpal::traits::{DeviceTrait, HostTrait, StreamTrait};
+//! # let host = cpal::default_host();
+//! # let device = host.default_output_device().unwrap();
+//! # let format = device.default_output_format().unwrap();
+//! # let stream = device.build_output_stream(&format, move |_data| {}, move |_err| {}).unwrap();
+//! stream.play().unwrap();
+//! ```
+//!
+//! Some devices support pausing the audio stream. This can be useful for saving energy in moments
+//! of silence.
+//!
+//! ```no_run
+//! # use cpal::traits::{DeviceTrait, HostTrait, StreamTrait};
+//! # let host = cpal::default_host();
+//! # let device = host.default_output_device().unwrap();
+//! # let format = device.default_output_format().unwrap();
+//! # let stream = device.build_output_stream(&format, move |_data| {}, move |_err| {}).unwrap();
+//! stream.pause().unwrap();
 
 #![recursion_limit = "512"]
 
@@ -151,11 +157,10 @@ extern crate thiserror;
 
 pub use error::*;
 pub use platform::{
-    ALL_HOSTS, Device, Devices, EventLoop, Host, HostId, SupportedInputFormats,
-    SupportedOutputFormats, StreamId, available_hosts, default_host, host_from_id,
+    ALL_HOSTS, available_hosts, default_host, Device, Devices, Host, host_from_id,
+    HostId, Stream, SupportedInputFormats, SupportedOutputFormats,
 };
 pub use samples_formats::{Sample, SampleFormat};
-
 use std::ops::{Deref, DerefMut};
 
 mod error;
@@ -198,6 +203,7 @@ pub struct SupportedFormat {
 }
 
 /// Stream data passed to the `EventLoop::run` callback.
+#[derive(Debug)]
 pub enum StreamData<'a> {
     Input {
         buffer: UnknownTypeInputBuffer<'a>,
@@ -207,16 +213,13 @@ pub enum StreamData<'a> {
     },
 }
 
-/// Stream data passed to the `EventLoop::run` callback, or an error in the case that the device
-/// was invalidated or some backend-specific error occurred.
-pub type StreamDataResult<'a> = Result<StreamData<'a>, StreamError>;
-
 /// Represents a buffer containing audio data that may be read.
 ///
 /// This struct implements the `Deref` trait targeting `[T]`. Therefore this buffer can be read the
 /// same way as reading from a `Vec` or any other kind of Rust array.
 // TODO: explain audio stuff in general
 // TODO: remove the wrapper and just use slices in next major version
+#[derive(Debug)]
 pub struct InputBuffer<'a, T: 'a>
 where
     T: Sample,
@@ -232,6 +235,7 @@ where
 // TODO: explain audio stuff in general
 // TODO: remove the wrapper and just use slices
 #[must_use]
+#[derive(Debug)]
 pub struct OutputBuffer<'a, T: 'a>
 where
     T: Sample,
@@ -242,6 +246,7 @@ where
 /// This is the struct that is provided to you by cpal when you want to read samples from a buffer.
 ///
 /// Since the type of data is only known at runtime, you have to read the right buffer.
+#[derive(Debug)]
 pub enum UnknownTypeInputBuffer<'a> {
     /// Samples whose format is `u16`.
     U16(InputBuffer<'a, u16>),
@@ -254,6 +259,7 @@ pub enum UnknownTypeInputBuffer<'a> {
 /// This is the struct that is provided to you by cpal when you want to write samples to a buffer.
 ///
 /// Since the type of data is only known at runtime, you have to fill the right buffer.
+#[derive(Debug)]
 pub enum UnknownTypeOutputBuffer<'a> {
     /// Samples whose format is `u16`.
     U16(OutputBuffer<'a, u16>),

src/platform/mod.rs

@@ -58,14 +58,14 @@ macro_rules! impl_platform_host {
         /// type.
         pub struct Devices(DevicesInner);
 
-        /// The **EventLoop** implementation associated with the platform's dynamically dispatched
+        /// The **Stream** implementation associated with the platform's dynamically dispatched
         /// **Host** type.
-        pub struct EventLoop(EventLoopInner);
-
-        /// The **StreamId** implementation associated with the platform's dynamically dispatched
-        /// **Host** type.
-        #[derive(Clone, Debug, Eq, Hash, PartialEq)]
-        pub struct StreamId(StreamIdInner);
+        // Streams cannot be `Send` or `Sync` if we plan to support Android's AAudio API. This is
+        // because the stream API is not thread-safe, and the API prohibits calling certain
+        // functions within the callback.
+        //
+        // TODO: Confirm this and add more specific detail and references.
+        pub struct Stream(StreamInner, crate::platform::NotSendSyncAcrossAllPlatforms);
 
         /// The **SupportedInputFormats** iterator associated with the platform's dynamically
         /// dispatched **Host** type.
@@ -95,22 +95,15 @@ macro_rules! impl_platform_host {
             )*
         }
 
-        enum EventLoopInner {
-            $(
-                $HostVariant(crate::host::$host_mod::EventLoop),
-            )*
-        }
-
         enum HostInner {
             $(
                 $HostVariant(crate::host::$host_mod::Host),
             )*
         }
 
-        #[derive(Clone, Debug, Eq, Hash, PartialEq)]
-        enum StreamIdInner {
+        enum StreamInner {
             $(
-                $HostVariant(crate::host::$host_mod::StreamId),
+                $HostVariant(crate::host::$host_mod::Stream),
             )*
         }
 
@@ -154,7 +147,7 @@ macro_rules! impl_platform_host {
                 match self.0 {
                     $(
                         DevicesInner::$HostVariant(ref mut d) => {
-                            d.next().map(DeviceInner::$HostVariant).map(Device)
+                            d.next().map(DeviceInner::$HostVariant).map(Device::from)
                         }
                     )*
                 }
@@ -212,6 +205,7 @@ macro_rules! impl_platform_host {
         impl crate::traits::DeviceTrait for Device {
             type SupportedInputFormats = SupportedInputFormats;
             type SupportedOutputFormats = SupportedOutputFormats;
+            type Stream = Stream;
 
             fn name(&self) -> Result<String, crate::DeviceNameError> {
                 match self.0 {
@@ -260,96 +254,25 @@ macro_rules! impl_platform_host {
                     )*
                 }
             }
-        }
 
-        impl crate::traits::EventLoopTrait for EventLoop {
-            type StreamId = StreamId;
-            type Device = Device;
-
-            #[allow(unreachable_patterns)]
-            fn build_input_stream(
-                &self,
-                device: &Self::Device,
-                format: &crate::Format,
-            ) -> Result<Self::StreamId, crate::BuildStreamError> {
-                match (&self.0, &device.0) {
-                    $(
-                        (&EventLoopInner::$HostVariant(ref e), &DeviceInner::$HostVariant(ref d)) => {
-                            e.build_input_stream(d, format)
-                                .map(StreamIdInner::$HostVariant)
-                                .map(StreamId)
-                        }
-                    )*
-                    _ => panic!("tried to build a stream with a device from another host"),
-                }
-            }
-
-            #[allow(unreachable_patterns)]
-            fn build_output_stream(
-                &self,
-                device: &Self::Device,
-                format: &crate::Format,
-            ) -> Result<Self::StreamId, crate::BuildStreamError> {
-                match (&self.0, &device.0) {
-                    $(
-                        (&EventLoopInner::$HostVariant(ref e), &DeviceInner::$HostVariant(ref d)) => {
-                            e.build_output_stream(d, format)
-                                .map(StreamIdInner::$HostVariant)
-                                .map(StreamId)
-                        }
-                    )*
-                    _ => panic!("tried to build a stream with a device from another host"),
-                }
-            }
-
-            #[allow(unreachable_patterns)]
-            fn play_stream(&self, stream: Self::StreamId) -> Result<(), crate::PlayStreamError> {
-                match (&self.0, stream.0) {
-                    $(
-                        (&EventLoopInner::$HostVariant(ref e), StreamIdInner::$HostVariant(ref s)) => {
-                            e.play_stream(s.clone())
-                        }
-                    )*
-                    _ => panic!("tried to play a stream with an ID associated with another host"),
-                }
-            }
-
-            #[allow(unreachable_patterns)]
-            fn pause_stream(&self, stream: Self::StreamId) -> Result<(), crate::PauseStreamError> {
-                match (&self.0, stream.0) {
-                    $(
-                        (&EventLoopInner::$HostVariant(ref e), StreamIdInner::$HostVariant(ref s)) => {
-                            e.pause_stream(s.clone())
-                        }
-                    )*
-                    _ => panic!("tried to pause a stream with an ID associated with another host"),
-                }
-            }
-
-            #[allow(unreachable_patterns)]
-            fn destroy_stream(&self, stream: Self::StreamId) {
-                match (&self.0, stream.0) {
-                    $(
-                        (&EventLoopInner::$HostVariant(ref e), StreamIdInner::$HostVariant(ref s)) => {
-                            e.destroy_stream(s.clone())
-                        }
-                    )*
-                    _ => panic!("tried to destroy a stream with an ID associated with another host"),
-                }
-            }
-
-            fn run<F>(&self, mut callback: F) -> !
-            where
-                F: FnMut(Self::StreamId, crate::StreamDataResult) + Send
-            {
+            fn build_input_stream<D, E>(&self, format: &crate::Format, data_callback: D, error_callback: E) -> Result<Self::Stream, crate::BuildStreamError>
+                where D: FnMut(crate::StreamData) + Send + 'static, E: FnMut(crate::StreamError) + Send + 'static {
                 match self.0 {
                     $(
-                        EventLoopInner::$HostVariant(ref e) => {
-                            e.run(|id, result| {
-                                let result = result;
-                                callback(StreamId(StreamIdInner::$HostVariant(id)), result);
-                            });
-                        },
+                        DeviceInner::$HostVariant(ref d) => d.build_input_stream(format, data_callback, error_callback)
+                            .map(StreamInner::$HostVariant)
+                            .map(Stream::from),
+                    )*
+                }
+            }
+
+            fn build_output_stream<D, E>(&self, format: &crate::Format, data_callback: D, error_callback: E) -> Result<Self::Stream, crate::BuildStreamError>
+                where D: FnMut(crate::StreamData) + Send + 'static, E: FnMut(crate::StreamError) + Send + 'static {
+                match self.0 {
+                    $(
+                        DeviceInner::$HostVariant(ref d) => d.build_output_stream(format, data_callback, error_callback)
+                            .map(StreamInner::$HostVariant)
+                            .map(Stream::from),
                     )*
                 }
             }
@@ -358,7 +281,6 @@ macro_rules! impl_platform_host {
         impl crate::traits::HostTrait for Host {
             type Devices = Devices;
             type Device = Device;
-            type EventLoop = EventLoop;
 
             fn is_available() -> bool {
                 $( crate::host::$host_mod::Host::is_available() ||)* false
@@ -368,7 +290,7 @@ macro_rules! impl_platform_host {
                 match self.0 {
                     $(
                         HostInner::$HostVariant(ref h) => {
-                            h.devices().map(DevicesInner::$HostVariant).map(Devices)
+                            h.devices().map(DevicesInner::$HostVariant).map(Devices::from)
                         }
                     )*
                 }
@@ -378,7 +300,7 @@ macro_rules! impl_platform_host {
                 match self.0 {
                     $(
                         HostInner::$HostVariant(ref h) => {
-                            h.default_input_device().map(DeviceInner::$HostVariant).map(Device)
+                            h.default_input_device().map(DeviceInner::$HostVariant).map(Device::from)
                         }
                     )*
                 }
@@ -388,53 +310,81 @@ macro_rules! impl_platform_host {
                 match self.0 {
                     $(
                         HostInner::$HostVariant(ref h) => {
-                            h.default_output_device().map(DeviceInner::$HostVariant).map(Device)
+                            h.default_output_device().map(DeviceInner::$HostVariant).map(Device::from)
                         }
                     )*
                 }
             }
+        }
 
-            fn event_loop(&self) -> Self::EventLoop {
+        impl crate::traits::StreamTrait for Stream {
+            fn play(&self) -> Result<(), crate::PlayStreamError> {
                 match self.0 {
                     $(
-                        HostInner::$HostVariant(ref h) => {
-                            EventLoop(EventLoopInner::$HostVariant(h.event_loop()))
+                        StreamInner::$HostVariant(ref s) => {
+                            s.play()
+                        }
+                    )*
+                }
+            }
+
+            fn pause(&self) -> Result<(), crate::PauseStreamError> {
+                match self.0 {
+                    $(
+                        StreamInner::$HostVariant(ref s) => {
+                            s.pause()
                         }
                     )*
                 }
             }
         }
 
-        impl crate::traits::StreamIdTrait for StreamId {}
+        impl From<DeviceInner> for Device {
+            fn from(d: DeviceInner) -> Self {
+                Device(d)
+            }
+        }
+
+        impl From<DevicesInner> for Devices {
+            fn from(d: DevicesInner) -> Self {
+                Devices(d)
+            }
+        }
+
+        impl From<HostInner> for Host {
+            fn from(h: HostInner) -> Self {
+                Host(h)
+            }
+        }
+
+        impl From<StreamInner> for Stream {
+            fn from(s: StreamInner) -> Self {
+                Stream(s, Default::default())
+            }
+        }
 
         $(
             impl From<crate::host::$host_mod::Device> for Device {
                 fn from(h: crate::host::$host_mod::Device) -> Self {
-                    Device(DeviceInner::$HostVariant(h))
+                    DeviceInner::$HostVariant(h).into()
                 }
             }
 
             impl From<crate::host::$host_mod::Devices> for Devices {
                 fn from(h: crate::host::$host_mod::Devices) -> Self {
-                    Devices(DevicesInner::$HostVariant(h))
-                }
-            }
-
-            impl From<crate::host::$host_mod::EventLoop> for EventLoop {
-                fn from(h: crate::host::$host_mod::EventLoop) -> Self {
-                    EventLoop(EventLoopInner::$HostVariant(h))
+                    DevicesInner::$HostVariant(h).into()
                 }
             }
 
             impl From<crate::host::$host_mod::Host> for Host {
                 fn from(h: crate::host::$host_mod::Host) -> Self {
-                    Host(HostInner::$HostVariant(h))
+                    HostInner::$HostVariant(h).into()
                 }
             }
 
-            impl From<crate::host::$host_mod::StreamId> for StreamId {
-                fn from(h: crate::host::$host_mod::StreamId) -> Self {
-                    StreamId(StreamIdInner::$HostVariant(h))
+            impl From<crate::host::$host_mod::Stream> for Stream {
+                fn from(h: crate::host::$host_mod::Stream) -> Self {
+                    StreamInner::$HostVariant(h).into()
                 }
             }
         )*
@@ -457,7 +407,7 @@ macro_rules! impl_platform_host {
                     HostId::$HostVariant => {
                         crate::host::$host_mod::Host::new()
                             .map(HostInner::$HostVariant)
-                            .map(Host)
+                            .map(Host::from)
                     }
                 )*
             }
@@ -471,9 +421,8 @@ mod platform_impl {
     pub use crate::host::alsa::{
         Device as AlsaDevice,
         Devices as AlsaDevices,
-        EventLoop as AlsaEventLoop,
         Host as AlsaHost,
-        StreamId as AlsaStreamId,
+        Stream as AlsaStream,
         SupportedInputFormats as AlsaSupportedInputFormats,
         SupportedOutputFormats as AlsaSupportedOutputFormats,
     };
@@ -494,9 +443,8 @@ mod platform_impl {
     pub use crate::host::coreaudio::{
         Device as CoreAudioDevice,
         Devices as CoreAudioDevices,
-        EventLoop as CoreAudioEventLoop,
         Host as CoreAudioHost,
-        StreamId as CoreAudioStreamId,
+        Stream as CoreAudioStream,
         SupportedInputFormats as CoreAudioSupportedInputFormats,
         SupportedOutputFormats as CoreAudioSupportedOutputFormats,
     };
@@ -516,9 +464,8 @@ mod platform_impl {
     pub use crate::host::emscripten::{
         Device as EmscriptenDevice,
         Devices as EmscriptenDevices,
-        EventLoop as EmscriptenEventLoop,
         Host as EmscriptenHost,
-        StreamId as EmscriptenStreamId,
+        Stream as EmscriptenStream,
         SupportedInputFormats as EmscriptenSupportedInputFormats,
         SupportedOutputFormats as EmscriptenSupportedOutputFormats,
     };
@@ -539,18 +486,16 @@ mod platform_impl {
     pub use crate::host::asio::{
         Device as AsioDevice,
         Devices as AsioDevices,
-        EventLoop as AsioEventLoop,
+        Stream as AsioStream,
         Host as AsioHost,
-        StreamId as AsioStreamId,
         SupportedInputFormats as AsioSupportedInputFormats,
         SupportedOutputFormats as AsioSupportedOutputFormats,
     };
     pub use crate::host::wasapi::{
         Device as WasapiDevice,
         Devices as WasapiDevices,
-        EventLoop as WasapiEventLoop,
+        Stream as WasapiStream,
         Host as WasapiHost,
-        StreamId as WasapiStreamId,
         SupportedInputFormats as WasapiSupportedInputFormats,
         SupportedOutputFormats as WasapiSupportedOutputFormats,
     };
@@ -591,3 +536,19 @@ mod platform_impl {
             .into()
     }
 }
+
+// The following zero-sized types are for applying Send/Sync restrictions to ensure
+// consistent behaviour across different platforms. These verbosely named types are used
+// (rather than using the markers directly) in the hope of making the compile errors
+// slightly more helpful.
+//
+// TODO: Remove these in favour of using negative trait bounds if they stabilise.
+
+// A marker used to remove the `Send` and `Sync` traits.
+struct NotSendSyncAcrossAllPlatforms(std::marker::PhantomData<*mut ()>);
+
+impl Default for NotSendSyncAcrossAllPlatforms {
+    fn default() -> Self {
+        NotSendSyncAcrossAllPlatforms(std::marker::PhantomData)
+    }
+}

src/traits.rs

@@ -10,7 +10,8 @@ use {
     OutputDevices,
     PauseStreamError,
     PlayStreamError,
-    StreamDataResult,
+    StreamData,
+    StreamError,
     SupportedFormat,
     SupportedFormatsError,
 };
@@ -39,8 +40,6 @@ pub trait HostTrait {
     type Devices: Iterator<Item = Self::Device>;
     /// The `Device` type yielded by the host.
     type Device: DeviceTrait;
-    /// The event loop type used by the `Host`
-    type EventLoop: EventLoopTrait<Device = Self::Device>;
 
     /// Whether or not the host is available on the system.
     fn is_available() -> bool;
@@ -60,9 +59,6 @@ pub trait HostTrait {
     /// Returns `None` if no output device is available.
     fn default_output_device(&self) -> Option<Self::Device>;
 
-    /// Initialise the event loop, ready for managing audio streams.
-    fn event_loop(&self) -> Self::EventLoop;
-
     /// An iterator yielding all `Device`s currently available to the system that support one or more
     /// input stream formats.
     ///
@@ -99,6 +95,8 @@ pub trait DeviceTrait {
     type SupportedInputFormats: Iterator<Item = SupportedFormat>;
     /// The iterator type yielding supported output stream formats.
     type SupportedOutputFormats: Iterator<Item = SupportedFormat>;
+    /// The stream type created by `build_input_stream` and `build_output_stream`.
+    type Stream: StreamTrait;
 
     /// The human-readable name of the device.
     fn name(&self) -> Result<String, DeviceNameError>;
@@ -118,81 +116,28 @@ pub trait DeviceTrait {
 
     /// The default output stream format for the device.
     fn default_output_format(&self) -> Result<Format, DefaultFormatError>;
+
+    /// Create an input stream.
+    fn build_input_stream<D, E>(&self, format: &Format, data_callback: D, error_callback: E) -> Result<Self::Stream, BuildStreamError>
+        where D: FnMut(StreamData) + Send + 'static, E: FnMut(StreamError) + Send + 'static;
+
+    /// Create an output stream.
+    fn build_output_stream<D, E>(&self, format: &Format, data_callback: D, error_callback: E) -> Result<Self::Stream, BuildStreamError>
+        where D: FnMut(StreamData) + Send + 'static, E: FnMut(StreamError) + Send + 'static;
 }
 
-/// Collection of streams managed together.
-///
-/// Created with the `Host::event_loop` method.
-pub trait EventLoopTrait {
-    /// The `Device` type yielded by the host.
-    type Device: DeviceTrait;
-    /// The type used to uniquely distinguish between streams.
-    type StreamId: StreamIdTrait;
+/// A stream created from `Device`, with methods to control playback.
+pub trait StreamTrait {
+    /// Run the stream.
+    ///
+    /// Note: Not all platforms automatically run the stream upon creation, so it is important to
+    /// call `play` after creation if it is expected that the stream should run immediately.
+    fn play(&self) -> Result<(), PlayStreamError>;
 
-    /// Creates a new input stream that will run from the given device and with the given format.
+    /// Some devices support pausing the audio stream. This can be useful for saving energy in
+    /// moments of silence.
     ///
-    /// On success, returns an identifier for the stream.
-    ///
-    /// Can return an error if the device is no longer valid, or if the input stream format is not
-    /// supported by the device.
-    fn build_input_stream(
-        &self,
-        device: &Self::Device,
-        format: &Format,
-    ) -> Result<Self::StreamId, BuildStreamError>;
-
-    /// Creates a new output stream that will play on the given device and with the given format.
-    ///
-    /// On success, returns an identifier for the stream.
-    ///
-    /// Can return an error if the device is no longer valid, or if the output stream format is not
-    /// supported by the device.
-    fn build_output_stream(
-        &self,
-        device: &Self::Device,
-        format: &Format,
-    ) -> Result<Self::StreamId, BuildStreamError>;
-
-    /// Instructs the audio device that it should start playing the stream with the given ID.
-    ///
-    /// Has no effect is the stream was already playing.
-    ///
-    /// Only call this after you have submitted some data, otherwise you may hear some glitches.
-    ///
-    /// # Panic
-    ///
-    /// If the stream does not exist, this function can either panic or be a no-op.
-    fn play_stream(&self, stream: Self::StreamId) -> Result<(), PlayStreamError>;
-
-    /// Instructs the audio device that it should stop playing the stream with the given ID.
-    ///
-    /// Has no effect is the stream was already paused.
-    ///
-    /// If you call `play` afterwards, the playback will resume where it was.
-    ///
-    /// # Panic
-    ///
-    /// If the stream does not exist, this function can either panic or be a no-op.
-    fn pause_stream(&self, stream: Self::StreamId) -> Result<(), PauseStreamError>;
-
-    /// Destroys an existing stream.
-    ///
-    /// # Panic
-    ///
-    /// If the stream does not exist, this function can either panic or be a no-op.
-    fn destroy_stream(&self, stream: Self::StreamId);
-
-    /// Takes control of the current thread and begins the stream processing.
-    ///
-    /// > **Note**: Since it takes control of the thread, this method is best called on a separate
-    /// > thread.
-    ///
-    /// Whenever a stream needs to be fed some data, the closure passed as parameter is called.
-    /// You can call the other methods of `EventLoop` without getting a deadlock.
-    fn run<F>(&self, callback: F) -> !
-    where
-        F: FnMut(Self::StreamId, StreamDataResult) + Send;
+    /// Note: Not all devices support suspending the stream at the hardware level. This method may
+    /// fail in these cases.
+    fn pause(&self) -> Result<(), PauseStreamError>;
 }
-
-/// The set of required bounds for host `StreamId` types.
-pub trait StreamIdTrait: Clone + std::fmt::Debug + std::hash::Hash + PartialEq + Eq {}