Remove EventLoop and port the ALSA backend

examples/beep.rs

@@ -1,37 +1,34 @@
 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 stream = device.build_output_stream(&format, move |result| {
         let data = match result {
             Ok(data) => data,
             Err(err) => {
-                eprintln!("an error occurred on stream {:?}: {}", id, err);
+                eprintln!("an error occurred on stream: {}", err);
                 return;
             }
         };
 
         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 +36,7 @@ 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 +44,7 @@ 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 +53,10 @@ fn main() -> Result<(), anyhow::Error> {
             },
             _ => (),
         }
-    });
+    })?;
+    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 input_device = host
-    let output_device = host.default_output_device().expect("failed to get default output device");
+        .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 +47,21 @@ fn main() -> Result<(), anyhow::Error> {
         producer.push(0.0).unwrap();
     }
 
-    // Play the streams.
+    // Build streams.
-    println!("Starting the input and output streams with `{}` milliseconds of latency.", LATENCY_MS);
+    println!("Attempting to build both streams with `{:?}`.", format);
-    event_loop.play_stream(input_stream_id.clone())?;
+    let input_stream = input_device.build_input_stream(&format, move |result| {
-    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);
+                eprintln!("an error occurred on input stream: {}", err);
                 return;
-                }
+            },
         };
 
         match data {
-                cpal::StreamData::Input { buffer: cpal::UnknownTypeInputBuffer::F32(buffer) } => {
+            cpal::StreamData::Input {
-                    assert_eq!(id, input_stream_id);
+                buffer: cpal::UnknownTypeInputBuffer::F32(buffer),
+            } => {
                 let mut output_fell_behind = false;
                 for &sample in buffer.iter() {
                     if producer.push(sample).is_err() {
@@ -79,8 +72,21 @@ fn main() -> Result<(), anyhow::Error> {
                     eprintln!("output stream fell behind: try increasing latency");
                 }
             },
-                cpal::StreamData::Output { buffer: cpal::UnknownTypeOutputBuffer::F32(mut buffer) } => {
+            _ => panic!("Expected input with f32 data"),
-                    assert_eq!(id, output_stream_id);
+        }
+    })?;
+    let output_stream = output_device.build_output_stream(&format, move |result| {
+        let data = match result {
+            Ok(data) => data,
+            Err(err) => {
+                eprintln!("an error occurred on output stream: {}", err);
+                return;
+            },
+        };
+        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 +97,28 @@ fn main() -> Result<(), anyhow::Error> {
                         },
                     };
                 }
-                    if let Some(_) = input_fell_behind {
+                if let Some(err) = input_fell_behind {
-                        eprintln!("input stream fell behind: try increasing latency");
+                    eprintln!("input stream fell behind: {:?}: try increasing latency", err);
                 }
             },
-                _ => panic!("we're expecting f32 data"),
+            _ => panic!("Expected output with f32 data"),
         }
-        });
+    })?;
-    });
+    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 +29,23 @@ 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();
+    let stream = device.build_input_stream(&format, move |event| {
-    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);
+                eprintln!("an error occurred on stream: {}", err);
                 return;
-                }
+            },
         };
 
-            // If we're done recording, return early.
-            if !recording_2.load(std::sync::atomic::Ordering::Relaxed) {
-                return;
-            }
         // 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 +55,9 @@ 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 +66,9 @@ 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 +79,12 @@ fn main() -> Result<(), anyhow::Error> {
             },
             _ => (),
         }
-        });
+    })?;
-    });
+    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};
+pub use self::enumerate::{default_input_device, default_output_device, Devices};
-use std::sync::Mutex;
-use std::sync::mpsc::{channel, Sender, Receiver};
-use std::sync::atomic::{AtomicUsize, Ordering};
-use std::vec::IntoIter as VecIntoIter;
 
 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 {
+    fn build_input_stream<F>(&self, format: &Format, callback: F) -> Result<Self::Stream, BuildStreamError> where F: FnMut(StreamDataResult) + Send + 'static {
-    type Device = Device;
+        Ok(Stream::new(Arc::new(self.build_stream_inner(format, alsa::SND_PCM_STREAM_CAPTURE)?), callback))
-    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(
+    fn build_output_stream<F>(&self, format: &Format, callback: F) -> Result<Self::Stream, BuildStreamError> where F: FnMut(StreamDataResult) + Send + 'static {
-        &self,
+        Ok(Stream::new(Arc::new(self.build_stream_inner(format, alsa::SND_PCM_STREAM_PLAYBACK)?), callback))
-        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 {}
 
-struct Trigger {
+struct TriggerSender(libc::c_int);
-    // [read fd, write fd]
-    fds: [libc::c_int; 2],
-}
 
-impl Trigger {
+struct TriggerReceiver(libc::c_int);
-    fn new() -> Self {
+
-        let mut fds = [0, 0];
+impl TriggerSender {
-        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]
-    }
     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.0);
-            libc::close(self.fds[1]);
         }
     }
 }
 
+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 +503,174 @@ 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)]
+// Assume that the ALSA library is built with thread safe option.
-pub struct StreamId(usize);
+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 {
+    /// Handle to the underlying stream for playback controls.
-    #[inline]
+    inner: Arc<StreamInner>,
-    fn new() -> EventLoop {
-        let pending_command_trigger = Trigger::new();
 
-        let mut initial_descriptors = vec![];
+    /// Used to signal to stop processing.
-        reset_descriptors_with_pending_command_trigger(
+    trigger: TriggerSender,
-            &mut initial_descriptors,
+}
-            &pending_command_trigger,
-        );
 
-        let (tx, rx) = channel();
+/// The inner body of the audio processing thread. Takes the polymorphic
-
+/// callback to avoid generating too much generic code.
-        let run_context = Mutex::new(RunContext {
+fn stream_worker(rx: TriggerReceiver, stream: &StreamInner, callback: &mut (dyn FnMut(StreamDataResult) + Send + 'static)) {
-                                         descriptors: initial_descriptors,
+    let mut descriptors = Vec::new();
-                                         streams: Vec::new(),
+    let mut buffer = Vec::new();
-                                         commands: rx,
+    loop {
+        descriptors.clear();
+        // Add the self-pipe for signaling termination.
+        descriptors.push(libc::pollfd {
+            fd: rx.0,
+            events: libc::POLLIN,
+            revents: 0,
         });
 
-        EventLoop {
+        // Add ALSA polling fds.
-            next_stream_id: AtomicUsize::new(0),
+        descriptors.reserve(stream.num_descriptors);
-            pending_command_trigger: pending_command_trigger,
+        let len = descriptors.len();
-            run_context,
+        let filled = unsafe {
-            commands: tx,
+            alsa::snd_pcm_poll_descriptors(
-        }
+                stream.channel,
-    }
+                descriptors[len..].as_mut_ptr(),
-
+                stream.num_descriptors as libc::c_uint,
-    #[inline]
+            )
-    fn run<F>(&self, mut callback: F) -> !
+        };
-        where F: FnMut(StreamId, StreamDataResult)
+        debug_assert_eq!(filled, stream.num_descriptors as libc::c_int);
-    {
-        self.run_inner(&mut callback)
-    }
-
-    fn run_inner(&self, callback: &mut dyn FnMut(StreamId, StreamDataResult)) -> ! {
         unsafe {
-            let mut run_context = self.run_context.lock().unwrap();
+            descriptors.set_len(len + stream.num_descriptors);
-            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;
-                    }
         }
 
-                // If the `pending_command_trigger` was signaled, we need to process the comands.
+        let res = unsafe {
-                if run_context.descriptors[0].revents != 0 {
+            // Don't timeout, wait forever.
-                    run_context.descriptors[0].revents = 0;
+            libc::poll(descriptors.as_mut_ptr(), descriptors.len() as libc::nfds_t, -1)
-                    self.pending_command_trigger.clear_pipe();
+        };
+        if res < 0 {
+            let description = format!("`libc::poll()` failed: {}", io::Error::last_os_error());
+            callback(Err(BackendSpecificError { description }.into()));
+            continue;
+        } else if res == 0 {
+            let description = String::from("`libc::poll()` spuriously returned");
+            callback(Err(BackendSpecificError { description }.into()));
+            continue;
         }
 
-                // The set of streams that error within the following loop and should be removed.
+        if descriptors[0].revents != 0 {
-                let mut streams_to_remove: Vec<(StreamId, StreamError)> = vec![];
+            // The stream has been requested to be destroyed.
+            rx.clear_pipe();
+            return;
+        }
 
-                // Iterate over each individual stream/descriptor.
+        let stream_type = match check_for_pollout_or_pollin(stream, descriptors[1..].as_mut_ptr()) {
-                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) {
             Ok(Some(ty)) => ty,
             Ok(None) => {
-                            i_descriptor += stream.num_descriptors as isize;
+                // Nothing to process, poll again
-                            i_stream += 1;
                 continue;
             },
             Err(err) => {
-                            streams_to_remove.push((stream.id, err.into()));
+                // TODO: signal errors
-                            i_descriptor += stream.num_descriptors as isize;
-                            i_stream += 1;
                 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()));
+                let description = format!("Failed to query the number of available samples: {}", err);
-                            i_descriptor += stream.num_descriptors as isize;
+                callback(Err(BackendSpecificError { description }.into()));
-                            i_stream += 1;
                 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 };
+                    callback(Err(BackendSpecificError { description }.into()));
-                                streams_to_remove.push((stream.id, err.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));
+                callback(Ok(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));
+                    callback(Ok(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 };
+                        callback(Err(BackendSpecificError { description }.into()));
-                                    streams_to_remove.push((stream.id, err.into()));
                         continue;
                     } else if result as usize != available_frames {
                         let description = format!(
@@ -724,8 +679,7 @@ impl EventLoop {
                             available_frames,
                             result,
                         );
-                                    let err = BackendSpecificError { description };
+                        callback(Err(BackendSpecificError { description }.into()));
-                                    streams_to_remove.push((stream.id, err.into()));
                         continue;
                     } else {
                         break;
@@ -734,280 +688,45 @@ 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.
+impl Stream {
-fn process_commands(run_context: &mut RunContext) {
+    fn new<F>(inner: Arc<StreamInner>, mut callback: F) -> Stream where F: FnMut(StreamDataResult) + Send + 'static {
-    for command in run_context.commands.try_iter() {
+        let (tx, rx) = trigger();
-        match command {
+        // Clone the handle for passing into worker thread.
-            Command::DestroyStream(stream_id) => {
+        let stream = inner.clone();
-                run_context.streams.retain(|s| s.id != stream_id);
+        let thread = thread::spawn(move || {
-            },
+            stream_worker(rx, &*stream, &mut callback);
-            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,
         });
+        Stream {
+            thread: Some(thread),
+            inner,
+            trigger: tx,
+        }
+    }
 }
 
-// Appends the `poll` descriptors for each stream onto the `RunContext`'s descriptor slice, ready
+impl Drop for Stream {
-// for a call to `libc::poll`.
+    fn drop(&mut self) {
-fn append_stream_poll_descriptors(run_context: &mut RunContext) {
+        self.trigger.wakeup();
-    for stream in run_context.streams.iter() {
+        self.thread.take().unwrap().join().unwrap();
-        run_context.descriptors.reserve(stream.num_descriptors);
+    }
-        let len = run_context.descriptors.len();
+}
-        let filled = unsafe {
+
-            alsa::snd_pcm_poll_descriptors(
+impl StreamTrait for Stream {
-                stream.channel,
+    fn play(&self) -> Result<(), PlayStreamError> {
-                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);
         unsafe {
-            run_context.descriptors.set_len(len + stream.num_descriptors);
+            alsa::snd_pcm_pause(self.inner.channel, 0);
         }
+        // TODO: error handling
+        Ok(())
     }
-}
+    fn pause(&self)-> Result<(), PauseStreamError> {
-
+        unsafe {
-// Poll all descriptors within the given set.
+            alsa::snd_pcm_pause(self.inner.channel, 1);
-//
+        }
-// Returns `Ok(true)` if some event has occurred or `Ok(false)` if no events have
+        // TODO: error handling
-// occurred.
+        Ok(())
-//
-// 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)
     }
 }
 

src/host/null/mod.rs

@@ -10,7 +10,7 @@ use PlayStreamError;
 use StreamDataResult;
 use SupportedFormatsError;
 use SupportedFormat;
-use traits::{DeviceTrait, EventLoopTrait, HostTrait, StreamIdTrait};
+use traits::{DeviceTrait, HostTrait, StreamTrait};
 
 #[derive(Default)]
 pub struct Devices;
@@ -23,7 +23,7 @@ pub struct EventLoop;
 pub struct Host;
 
 #[derive(Debug, Clone, PartialEq, Eq, Hash)]
-pub struct StreamId;
+pub struct Stream;
 
 pub struct SupportedInputFormats;
 pub struct SupportedOutputFormats;
@@ -49,6 +49,7 @@ impl EventLoop {
 impl DeviceTrait for Device {
     type SupportedInputFormats = SupportedInputFormats;
     type SupportedOutputFormats = SupportedOutputFormats;
+    type Stream = Stream;
 
     #[inline]
     fn name(&self) -> Result<String, DeviceNameError> {
@@ -74,49 +75,22 @@ impl DeviceTrait for Device {
     fn default_output_format(&self) -> Result<Format, DefaultFormatError> {
         unimplemented!()
     }
-}
 
-impl EventLoopTrait for EventLoop {
+    fn build_input_stream<F>(&self, format: &Format, callback: F) -> Result<Self::Stream, BuildStreamError>
-    type Device = Device;
+        where F: FnMut(StreamDataResult) + Send + 'static {
-    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) {
         unimplemented!()
     }
 
-    #[inline]
+    /// Create an output stream.
-    fn play_stream(&self, _: StreamId) -> Result<(), PlayStreamError> {
+    fn build_output_stream<F>(&self, format: &Format, callback: F) -> Result<Self::Stream, BuildStreamError>
-        panic!()
+        where F: FnMut(StreamDataResult) + Send + 'static{
-    }
+        unimplemented!()
-
-    #[inline]
-    fn pause_stream(&self, _: StreamId) -> Result<(), PauseStreamError> {
-        panic!()
     }
 }
 
 impl HostTrait for Host {
     type Device = Device;
     type Devices = Devices;
-    type EventLoop = EventLoop;
 
     fn is_available() -> bool {
         false
@@ -133,13 +107,17 @@ 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/lib.rs

@@ -151,11 +151,10 @@ extern crate thiserror;
 
 pub use error::*;
 pub use platform::{
-    ALL_HOSTS, Device, Devices, EventLoop, Host, HostId, SupportedInputFormats,
+    ALL_HOSTS, available_hosts, default_host, Device, Devices, Host, host_from_id,
-    SupportedOutputFormats, StreamId, available_hosts, default_host, host_from_id,
+    HostId, Stream, SupportedInputFormats, SupportedOutputFormats,
 };
 pub use samples_formats::{Sample, SampleFormat};
-
 use std::ops::{Deref, DerefMut};
 
 mod error;

src/platform/mod.rs

@@ -58,14 +58,9 @@ 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);
+        pub struct Stream(StreamInner);
-
-        /// The **StreamId** implementation associated with the platform's dynamically dispatched
-        /// **Host** type.
-        #[derive(Clone, Debug, Eq, Hash, PartialEq)]
-        pub struct StreamId(StreamIdInner);
 
         /// The **SupportedInputFormats** iterator associated with the platform's dynamically
         /// dispatched **Host** type.
@@ -95,22 +90,15 @@ 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 StreamInner {
-        enum StreamIdInner {
             $(
-                $HostVariant(crate::host::$host_mod::StreamId),
+                $HostVariant(crate::host::$host_mod::Stream),
             )*
         }
 
@@ -212,6 +200,7 @@ 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 +249,25 @@ macro_rules! impl_platform_host {
                     )*
                 }
             }
-        }
 
-        impl crate::traits::EventLoopTrait for EventLoop {
+            fn build_input_stream<F>(&self, format: &crate::Format, callback: F) -> Result<Self::Stream, crate::BuildStreamError>
-            type StreamId = StreamId;
+                where F: FnMut(crate::StreamDataResult) + Send + 'static {
-            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
-            {
                 match self.0 {
                     $(
-                        EventLoopInner::$HostVariant(ref e) => {
+                        DeviceInner::$HostVariant(ref d) => d.build_input_stream(format, callback)
-                            e.run(|id, result| {
+                            .map(StreamInner::$HostVariant)
-                                let result = result;
+                            .map(Stream),
-                                callback(StreamId(StreamIdInner::$HostVariant(id)), result);
+                    )*
-                            });
+                }
-                        },
+            }
+
+            fn build_output_stream<F>(&self, format: &crate::Format, callback: F) -> Result<Self::Stream, crate::BuildStreamError>
+                where F: FnMut(crate::StreamDataResult) + Send + 'static {
+                match self.0 {
+                    $(
+                        DeviceInner::$HostVariant(ref d) => d.build_output_stream(format, callback)
+                            .map(StreamInner::$HostVariant)
+                            .map(Stream),
                     )*
                 }
             }
@@ -358,7 +276,6 @@ 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
@@ -393,20 +310,30 @@ macro_rules! impl_platform_host {
                     )*
                 }
             }
+        }
 
-            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) => {
+                        StreamInner::$HostVariant(ref s) => {
-                            EventLoop(EventLoopInner::$HostVariant(h.event_loop()))
+                            s.play()
+                        }
+                    )*
+                }
+            }
+
+            fn pause(&self) -> Result<(), crate::PauseStreamError> {
+                match self.0 {
+                    $(
+                        StreamInner::$HostVariant(ref s) => {
+                            s.pause()
                         }
                     )*
                 }
             }
         }
 
-        impl crate::traits::StreamIdTrait for StreamId {}
-
         $(
             impl From<crate::host::$host_mod::Device> for Device {
                 fn from(h: crate::host::$host_mod::Device) -> Self {
@@ -420,21 +347,15 @@ macro_rules! impl_platform_host {
                 }
             }
 
-            impl From<crate::host::$host_mod::EventLoop> for EventLoop {
-                fn from(h: crate::host::$host_mod::EventLoop) -> Self {
-                    EventLoop(EventLoopInner::$HostVariant(h))
-                }
-            }
-
             impl From<crate::host::$host_mod::Host> for Host {
                 fn from(h: crate::host::$host_mod::Host) -> Self {
                     Host(HostInner::$HostVariant(h))
                 }
             }
 
-            impl From<crate::host::$host_mod::StreamId> for StreamId {
+            impl From<crate::host::$host_mod::Stream> for Stream {
-                fn from(h: crate::host::$host_mod::StreamId) -> Self {
+                fn from(h: crate::host::$host_mod::Stream) -> Self {
-                    StreamId(StreamIdInner::$HostVariant(h))
+                    Stream(StreamInner::$HostVariant(h))
                 }
             }
         )*
@@ -471,9 +392,8 @@ 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,
     };

src/traits.rs

@@ -39,8 +39,6 @@ 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 +58,6 @@ 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 +94,8 @@ 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 +115,19 @@ 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<F>(&self, format: &Format, callback: F) -> Result<Self::Stream, BuildStreamError>
+        where F: FnMut(StreamDataResult) + Send + 'static;
+
+    /// Create an output stream.
+    fn build_output_stream<F>(&self, format: &Format, callback: F) -> Result<Self::Stream, BuildStreamError>
+        where F: FnMut(StreamDataResult) + Send + 'static;
 }
 
-/// Collection of streams managed together.
+/// A stream created from `Device`, with methods to control playback.
-///
+pub trait StreamTrait {
-/// Created with the `Host::event_loop` method.
+    fn play(&self) -> Result<(), PlayStreamError>;
-pub trait EventLoopTrait {
-    /// The `Device` type yielded by the host.
-    type Device: DeviceTrait;
-    /// The type used to uniquely distinguish between streams.
-    type StreamId: StreamIdTrait;
 
-    /// Creates a new input stream that will run from the given device and with the given format.
+    fn pause(&self) -> Result<(), PauseStreamError>;
-    ///
-    /// 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;
 }
-
-/// The set of required bounds for host `StreamId` types.
-pub trait StreamIdTrait: Clone + std::fmt::Debug + std::hash::Hash + PartialEq + Eq {}