Merge pull request #289 from mitchmindtree/host

Introduce a `Host` API

README.md

@@ -6,6 +6,7 @@ Low-level library for audio input and output in pure Rust.
 
 This library currently supports the following:
 
+- Enumerate supported audio hosts.
 - Enumerate all available audio devices.
 - Get the current default input and output devices.
 - Enumerate known supported input and output stream formats for a device.

examples/beep.rs

@@ -1,10 +1,13 @@
 extern crate cpal;
 extern crate failure;
 
+use cpal::{Device, EventLoop, Host};
+
 fn main() -> Result<(), failure::Error> {
-    let device = cpal::default_output_device().expect("failed to find a default output device");
+    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 = cpal::EventLoop::new();
+    let event_loop = host.event_loop();
     let stream_id = event_loop.build_output_stream(&device, &format)?;
     event_loop.play_stream(stream_id.clone())?;
 

examples/enumerate.rs

@@ -1,50 +1,60 @@
 extern crate cpal;
 extern crate failure;
 
+use cpal::{Device, Host};
+
 fn main() -> Result<(), failure::Error> {
-    let default_in = cpal::default_input_device().map(|e| e.name().unwrap());
+    println!("Supported hosts:\n  {:?}", cpal::ALL_HOSTS);
-    let default_out = cpal::default_output_device().map(|e| e.name().unwrap());
+    let available_hosts = cpal::available_hosts();
-    println!("Default Input Device:\n  {:?}", default_in);
+    println!("Available hosts:\n  {:?}", available_hosts);
-    println!("Default Output Device:\n  {:?}", default_out);
 
-    let devices = cpal::devices()?;
+    for host_id in available_hosts {
-    println!("Devices: ");
+        println!("{:?}", host_id);
-    for (device_index, device) in devices.enumerate() {
+        let host = cpal::host_from_id(host_id)?;
-        println!("{}. \"{}\"", device_index + 1, device.name()?);
+        let default_in = host.default_input_device().map(|e| e.name().unwrap());
+        let default_out = host.default_output_device().map(|e| e.name().unwrap());
+        println!("  Default Input Device:\n    {:?}", default_in);
+        println!("  Default Output Device:\n    {:?}", default_out);
 
-        // Input formats
+        let devices = host.devices()?;
-        if let Ok(fmt) = device.default_input_format() {
+        println!("  Devices: ");
-            println!("  Default input stream format:\n    {:?}", fmt);
+        for (device_index, device) in devices.enumerate() {
-        }
+            println!("  {}. \"{}\"", device_index + 1, device.name()?);
-        let mut input_formats = match device.supported_input_formats() {
+
-            Ok(f) => f.peekable(),
+            // Input formats
-            Err(e) => {
+            if let Ok(fmt) = device.default_input_format() {
-                println!("Error: {:?}", e);
+                println!("    Default input stream format:\n      {:?}", fmt);
-                continue;
+            }
-            },
+            let mut input_formats = match device.supported_input_formats() {
-        };
+                Ok(f) => f.peekable(),
-        if input_formats.peek().is_some() {
+                Err(e) => {
-            println!("  All supported input stream formats:");
+                    println!("Error: {:?}", e);
-            for (format_index, format) in input_formats.enumerate() {
+                    continue;
-                println!("    {}.{}. {:?}", device_index + 1, format_index + 1, format);
+                },
+            };
+            if input_formats.peek().is_some() {
+                println!("    All supported input stream formats:");
+                for (format_index, format) in input_formats.enumerate() {
+                    println!("      {}.{}. {:?}", device_index + 1, format_index + 1, format);
+                }
             }
-        }
 
-        // Output formats
+            // Output formats
-        if let Ok(fmt) = device.default_output_format() {
+            if let Ok(fmt) = device.default_output_format() {
-            println!("  Default output stream format:\n    {:?}", fmt);
+                println!("    Default output stream format:\n      {:?}", fmt);
-        }
+            }
-        let mut output_formats = match device.supported_output_formats() {
+            let mut output_formats = match device.supported_output_formats() {
-            Ok(f) => f.peekable(),
+                Ok(f) => f.peekable(),
-            Err(e) => {
+                Err(e) => {
-                println!("Error: {:?}", e);
+                    println!("Error: {:?}", e);
-                continue;
+                    continue;
-            },
+                },
-        };
+            };
-        if output_formats.peek().is_some() {
+            if output_formats.peek().is_some() {
-            println!("  All supported output stream formats:");
+                println!("    All supported output stream formats:");
-            for (format_index, format) in output_formats.enumerate() {
+                for (format_index, format) in output_formats.enumerate() {
-                println!("    {}.{}. {:?}", device_index + 1, format_index + 1, format);
+                    println!("      {}.{}. {:?}", device_index + 1, format_index + 1, format);
+                }
             }
         }
     }

examples/feedback.rs

@@ -9,14 +9,17 @@
 extern crate cpal;
 extern crate failure;
 
+use cpal::{Device, EventLoop, Host};
+
 const LATENCY_MS: f32 = 150.0;
 
 fn main() -> Result<(), failure::Error> {
-    let event_loop = cpal::EventLoop::new();
+    let host = cpal::default_host();
+    let event_loop = host.event_loop();
 
     // Default devices.
-    let input_device = cpal::default_input_device().expect("failed to get default input device");
+    let input_device = host.default_input_device().expect("failed to get default input device");
-    let output_device = cpal::default_output_device().expect("failed to get default output 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()?);
 

examples/record_wav.rs

@@ -6,13 +6,18 @@ extern crate cpal;
 extern crate failure;
 extern crate hound;
 
+use cpal::{Device, EventLoop, Host};
+
 fn main() -> Result<(), failure::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 = cpal::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");
     println!("Default input format: {:?}", format);
-    let event_loop = cpal::EventLoop::new();
+    let event_loop = host.event_loop();
     let stream_id = event_loop.build_input_stream(&device, &format)?;
     event_loop.play_stream(stream_id)?;
 

src/host/alsa/mod.rs

@@ -7,16 +7,21 @@ use ChannelCount;
 use BackendSpecificError;
 use BuildStreamError;
 use DefaultFormatError;
+use Device as DeviceTrait;
 use DeviceNameError;
+use DevicesError;
+use EventLoop as EventLoopTrait;
 use Format;
+use Host as HostTrait;
 use PauseStreamError;
 use PlayStreamError;
-use SupportedFormatsError;
 use SampleFormat;
 use SampleRate;
+use SupportedFormatsError;
 use StreamData;
 use StreamDataResult;
 use StreamError;
+use StreamId as StreamIdTrait;
 use SupportedFormat;
 use UnknownTypeInputBuffer;
 use UnknownTypeOutputBuffer;
@@ -32,6 +37,109 @@ pub type SupportedOutputFormats = VecIntoIter<SupportedFormat>;
 
 mod enumerate;
 
+/// The default linux and freebsd host type.
+#[derive(Debug)]
+pub struct Host;
+
+impl Host {
+    pub fn new() -> Result<Self, crate::HostUnavailable> {
+        Ok(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.
+        true
+    }
+
+    fn devices(&self) -> Result<Self::Devices, DevicesError> {
+        Devices::new()
+    }
+
+    fn default_input_device(&self) -> Option<Self::Device> {
+        default_input_device()
+    }
+
+    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 {}
 
 struct Trigger {
     // [read fd, write fd]
@@ -79,7 +187,7 @@ pub struct Device(String);
 
 impl Device {
     #[inline]
-    pub fn name(&self) -> Result<String, DeviceNameError> {
+    fn name(&self) -> Result<String, DeviceNameError> {
         Ok(self.0.clone())
     }
 
@@ -287,13 +395,13 @@ impl Device {
         Ok(output.into_iter())
     }
 
-    pub fn supported_input_formats(&self) -> Result<SupportedInputFormats, SupportedFormatsError> {
+    fn supported_input_formats(&self) -> Result<SupportedInputFormats, SupportedFormatsError> {
         unsafe {
             self.supported_formats(alsa::SND_PCM_STREAM_CAPTURE)
         }
     }
 
-    pub fn supported_output_formats(&self) -> Result<SupportedOutputFormats, SupportedFormatsError> {
+    fn supported_output_formats(&self) -> Result<SupportedOutputFormats, SupportedFormatsError> {
         unsafe {
             self.supported_formats(alsa::SND_PCM_STREAM_PLAYBACK)
         }
@@ -340,11 +448,11 @@ impl Device {
         }
     }
 
-    pub fn default_input_format(&self) -> Result<Format, DefaultFormatError> {
+    fn default_input_format(&self) -> Result<Format, DefaultFormatError> {
         self.default_format(alsa::SND_PCM_STREAM_CAPTURE)
     }
 
-    pub fn default_output_format(&self) -> Result<Format, DefaultFormatError> {
+    fn default_output_format(&self) -> Result<Format, DefaultFormatError> {
         self.default_format(alsa::SND_PCM_STREAM_PLAYBACK)
     }
 }
@@ -434,7 +542,7 @@ enum StreamType { Input, Output }
 
 impl EventLoop {
     #[inline]
-    pub fn new() -> EventLoop {
+    fn new() -> EventLoop {
         let pending_command_trigger = Trigger::new();
 
         let mut initial_descriptors = vec![];
@@ -460,7 +568,7 @@ impl EventLoop {
     }
 
     #[inline]
-    pub fn run<F>(&self, mut callback: F) -> !
+    fn run<F>(&self, mut callback: F) -> !
         where F: FnMut(StreamId, StreamDataResult)
     {
         self.run_inner(&mut callback)
@@ -645,7 +753,7 @@ impl EventLoop {
         panic!("`cpal::EventLoop::run` API currently disallows returning");
     }
 
-    pub fn build_input_stream(
+    fn build_input_stream(
         &self,
         device: &Device,
         format: &Format,
@@ -724,7 +832,7 @@ impl EventLoop {
         }
     }
 
-    pub fn build_output_stream(
+    fn build_output_stream(
         &self,
         device: &Device,
         format: &Format,
@@ -805,18 +913,18 @@ impl EventLoop {
     }
 
     #[inline]
-    pub fn destroy_stream(&self, stream_id: StreamId) {
+    fn destroy_stream(&self, stream_id: StreamId) {
         self.push_command(Command::DestroyStream(stream_id));
     }
 
     #[inline]
-    pub fn play_stream(&self, stream_id: StreamId) -> Result<(), PlayStreamError> {
+    fn play_stream(&self, stream_id: StreamId) -> Result<(), PlayStreamError> {
         self.push_command(Command::PlayStream(stream_id));
         Ok(())
     }
 
     #[inline]
-    pub fn pause_stream(&self, stream_id: StreamId) -> Result<(), PauseStreamError> {
+    fn pause_stream(&self, stream_id: StreamId) -> Result<(), PauseStreamError> {
         self.push_command(Command::PauseStream(stream_id));
         Ok(())
     }

src/host/coreaudio/mod.rs

@@ -5,8 +5,12 @@ use ChannelCount;
 use BackendSpecificError;
 use BuildStreamError;
 use DefaultFormatError;
+use Device as DeviceTrait;
 use DeviceNameError;
+use DevicesError;
+use EventLoop as EventLoopTrait;
 use Format;
+use Host as HostTrait;
 use PauseStreamError;
 use PlayStreamError;
 use SupportedFormatsError;
@@ -14,6 +18,7 @@ use SampleFormat;
 use SampleRate;
 use StreamData;
 use StreamDataResult;
+use StreamId as StreamIdTrait;
 use SupportedFormat;
 use UnknownTypeInputBuffer;
 use UnknownTypeOutputBuffer;
@@ -72,13 +77,117 @@ mod enumerate;
 
 pub use self::enumerate::{Devices, SupportedInputFormats, SupportedOutputFormats, default_input_device, default_output_device};
 
+/// Coreaudio host, the default host on macOS and iOS.
+#[derive(Debug)]
+pub struct Host;
+
+impl Host {
+    pub fn new() -> Result<Self, crate::HostUnavailable> {
+        Ok(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.
+        true
+    }
+
+    fn devices(&self) -> Result<Self::Devices, DevicesError> {
+        Devices::new()
+    }
+
+    fn default_input_device(&self) -> Option<Self::Device> {
+        default_input_device()
+    }
+
+    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 {}
+
 #[derive(Clone, PartialEq, Eq)]
 pub struct Device {
     audio_device_id: AudioDeviceID,
 }
 
 impl Device {
-    pub fn name(&self) -> Result<String, DeviceNameError> {
+    fn name(&self) -> Result<String, DeviceNameError> {
         let property_address = AudioObjectPropertyAddress {
             mSelector: kAudioDevicePropertyDeviceNameCFString,
             mScope: kAudioDevicePropertyScopeOutput,
@@ -212,11 +321,11 @@ impl Device {
         }
     }
 
-    pub fn supported_input_formats(&self) -> Result<SupportedOutputFormats, SupportedFormatsError> {
+    fn supported_input_formats(&self) -> Result<SupportedOutputFormats, SupportedFormatsError> {
         self.supported_formats(kAudioObjectPropertyScopeInput)
     }
 
-    pub fn supported_output_formats(&self) -> Result<SupportedOutputFormats, SupportedFormatsError> {
+    fn supported_output_formats(&self) -> Result<SupportedOutputFormats, SupportedFormatsError> {
         self.supported_formats(kAudioObjectPropertyScopeOutput)
     }
 
@@ -296,11 +405,11 @@ impl Device {
         }
     }
 
-    pub fn default_input_format(&self) -> Result<Format, DefaultFormatError> {
+    fn default_input_format(&self) -> Result<Format, DefaultFormatError> {
         self.default_format(kAudioObjectPropertyScopeInput)
     }
 
-    pub fn default_output_format(&self) -> Result<Format, DefaultFormatError> {
+    fn default_output_format(&self) -> Result<Format, DefaultFormatError> {
         self.default_format(kAudioObjectPropertyScopeOutput)
     }
 }
@@ -435,7 +544,7 @@ fn audio_unit_from_device(device: &Device, input: bool) -> Result<AudioUnit, cor
 
 impl EventLoop {
     #[inline]
-    pub fn new() -> EventLoop {
+    fn new() -> EventLoop {
         EventLoop {
             user_callback: Arc::new(Mutex::new(UserCallback::Inactive)),
             streams: Mutex::new(Vec::new()),
@@ -443,7 +552,7 @@ impl EventLoop {
     }
 
     #[inline]
-    pub fn run<F>(&self, mut callback: F) -> !
+    fn run<F>(&self, mut callback: F) -> !
         where F: FnMut(StreamId, StreamDataResult) + Send
     {
         {
@@ -490,7 +599,7 @@ impl EventLoop {
     }
 
     #[inline]
-    pub fn build_input_stream(
+    fn build_input_stream(
         &self,
         device: &Device,
         format: &Format,
@@ -712,7 +821,7 @@ impl EventLoop {
     }
 
     #[inline]
-    pub fn build_output_stream(
+    fn build_output_stream(
         &self,
         device: &Device,
         format: &Format,
@@ -787,14 +896,14 @@ impl EventLoop {
         Ok(StreamId(stream_id))
     }
 
-    pub fn destroy_stream(&self, stream_id: StreamId) {
+    fn destroy_stream(&self, stream_id: StreamId) {
         {
             let mut streams = self.streams.lock().unwrap();
             streams[stream_id.0] = None;
         }
     }
 
-    pub fn play_stream(&self, stream_id: StreamId) -> Result<(), PlayStreamError> {
+    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();
 
@@ -809,7 +918,7 @@ impl EventLoop {
         Ok(())
     }
 
-    pub fn pause_stream(&self, stream_id: StreamId) -> Result<(), PauseStreamError> {
+    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();
 

src/host/emscripten/mod.rs

@@ -10,17 +10,125 @@ use stdweb::web::set_timeout;
 
 use BuildStreamError;
 use DefaultFormatError;
+use Device as DeviceTrait;
 use DeviceNameError;
 use DevicesError;
+use EventLoop as EventLoopTrait;
 use Format;
+use Host as HostTrait;
 use PauseStreamError;
 use PlayStreamError;
 use SupportedFormatsError;
 use StreamData;
 use StreamDataResult;
+use StreamId as StreamIdTrait;
 use SupportedFormat;
 use UnknownTypeOutputBuffer;
 
+/// The default emscripten host type.
+#[derive(Debug)]
+pub struct Host;
+
+impl Host {
+    pub fn new() -> Result<Self, crate::HostUnavailable> {
+        Ok(Host)
+    }
+}
+
+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.
+        true
+    }
+
+    fn devices(&self) -> Result<Self::Devices, DevicesError> {
+        Devices::new()
+    }
+
+    fn default_input_device(&self) -> Option<Self::Device> {
+        default_input_device()
+    }
+
+    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 {}
+
 // 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.
 //
@@ -44,7 +152,7 @@ impl EventLoop {
     }
 
     #[inline]
-    pub fn run<F>(&self, callback: F) -> !
+    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
@@ -124,12 +232,12 @@ impl EventLoop {
     }
 
     #[inline]
-    pub fn build_input_stream(&self, _: &Device, _format: &Format) -> Result<StreamId, BuildStreamError> {
+    fn build_input_stream(&self, _: &Device, _format: &Format) -> Result<StreamId, BuildStreamError> {
         unimplemented!();
     }
 
     #[inline]
-    pub fn build_output_stream(&self, _: &Device, _format: &Format) -> Result<StreamId, BuildStreamError> {
+    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();
@@ -146,12 +254,12 @@ impl EventLoop {
     }
 
     #[inline]
-    pub fn destroy_stream(&self, stream_id: StreamId) {
+    fn destroy_stream(&self, stream_id: StreamId) {
         self.streams.lock().unwrap()[stream_id.0] = None;
     }
 
     #[inline]
-    pub fn play_stream(&self, stream_id: StreamId) -> Result<(), PlayStreamError> {
+    fn play_stream(&self, stream_id: StreamId) -> Result<(), PlayStreamError> {
         let streams = self.streams.lock().unwrap();
         let stream = streams
             .get(stream_id.0)
@@ -162,7 +270,7 @@ impl EventLoop {
     }
 
     #[inline]
-    pub fn pause_stream(&self, stream_id: StreamId) -> Result<(), PauseStreamError> {
+    fn pause_stream(&self, stream_id: StreamId) -> Result<(), PauseStreamError> {
         let streams = self.streams.lock().unwrap();
         let stream = streams
             .get(stream_id.0)
@@ -193,7 +301,7 @@ fn is_webaudio_available() -> bool {
 pub struct Devices(bool);
 
 impl Devices {
-    pub fn new() -> Result<Self, DevicesError> {
+    fn new() -> Result<Self, DevicesError> {
         Ok(Self::default())
     }
 }
@@ -218,12 +326,12 @@ impl Iterator for Devices {
 }
 
 #[inline]
-pub fn default_input_device() -> Option<Device> {
+fn default_input_device() -> Option<Device> {
     unimplemented!();
 }
 
 #[inline]
-pub fn default_output_device() -> Option<Device> {
+fn default_output_device() -> Option<Device> {
     if is_webaudio_available() {
         Some(Device)
     } else {
@@ -236,17 +344,17 @@ pub struct Device;
 
 impl Device {
     #[inline]
-    pub fn name(&self) -> Result<String, DeviceNameError> {
+    fn name(&self) -> Result<String, DeviceNameError> {
         Ok("Default Device".to_owned())
     }
 
     #[inline]
-    pub fn supported_input_formats(&self) -> Result<SupportedInputFormats, SupportedFormatsError> {
+    fn supported_input_formats(&self) -> Result<SupportedInputFormats, SupportedFormatsError> {
         unimplemented!();
     }
 
     #[inline]
-    pub fn supported_output_formats(&self) -> Result<SupportedOutputFormats, SupportedFormatsError> {
+    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
@@ -264,11 +372,11 @@ impl Device {
         )
     }
 
-    pub fn default_input_format(&self) -> Result<Format, DefaultFormatError> {
+    fn default_input_format(&self) -> Result<Format, DefaultFormatError> {
         unimplemented!();
     }
 
-    pub fn default_output_format(&self) -> Result<Format, DefaultFormatError> {
+    fn default_output_format(&self) -> Result<Format, DefaultFormatError> {
         // TODO: because it is hard coded, see supported_output_formats.
         Ok(
             Format {

src/host/mod.rs

@@ -0,0 +1,9 @@
+#[cfg(any(target_os = "linux", target_os = "freebsd"))]
+pub(crate) mod alsa;
+#[cfg(any(target_os = "macos", target_os = "ios"))]
+pub(crate) mod coreaudio;
+#[cfg(target_os = "emscripten")]
+pub(crate) mod emscripten;
+pub(crate) mod null;
+#[cfg(windows)]
+pub(crate) mod wasapi;

src/host/null/mod.rs

@@ -0,0 +1,172 @@
+#![allow(dead_code)]
+
+use BuildStreamError;
+use DefaultFormatError;
+use Device as DeviceTrait;
+use DevicesError;
+use DeviceNameError;
+use EventLoop as EventLoopTrait;
+use Format;
+use Host as HostTrait;
+use PauseStreamError;
+use PlayStreamError;
+use StreamDataResult;
+use SupportedFormatsError;
+use StreamId as StreamIdTrait;
+use SupportedFormat;
+
+#[derive(Default)]
+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 SupportedInputFormats;
+pub struct SupportedOutputFormats;
+
+impl Host {
+    pub fn new() -> Result<Self, crate::HostUnavailable> {
+        Ok(Host)
+    }
+}
+
+impl Devices {
+    pub fn new() -> Result<Self, DevicesError> {
+        Ok(Devices)
+    }
+}
+
+impl EventLoop {
+    pub fn new() -> EventLoop {
+        EventLoop
+    }
+}
+
+impl DeviceTrait for Device {
+    type SupportedInputFormats = SupportedInputFormats;
+    type SupportedOutputFormats = SupportedOutputFormats;
+
+    #[inline]
+    fn name(&self) -> Result<String, DeviceNameError> {
+        Ok("null".to_owned())
+    }
+
+    #[inline]
+    fn supported_input_formats(&self) -> Result<SupportedInputFormats, SupportedFormatsError> {
+        unimplemented!()
+    }
+
+    #[inline]
+    fn supported_output_formats(&self) -> Result<SupportedOutputFormats, SupportedFormatsError> {
+        unimplemented!()
+    }
+
+    #[inline]
+    fn default_input_format(&self) -> Result<Format, DefaultFormatError> {
+        unimplemented!()
+    }
+
+    #[inline]
+    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) {
+        unimplemented!()
+    }
+
+    #[inline]
+    fn play_stream(&self, _: StreamId) -> Result<(), PlayStreamError> {
+        panic!()
+    }
+
+    #[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
+    }
+
+    fn devices(&self) -> Result<Self::Devices, DevicesError> {
+        Devices::new()
+    }
+
+    fn default_input_device(&self) -> Option<Device> {
+        None
+    }
+
+    fn default_output_device(&self) -> Option<Device> {
+        None
+    }
+
+    fn event_loop(&self) -> Self::EventLoop {
+        EventLoop::new()
+    }
+}
+
+impl StreamIdTrait for StreamId {}
+
+impl Iterator for Devices {
+    type Item = Device;
+
+    #[inline]
+    fn next(&mut self) -> Option<Device> {
+        None
+    }
+}
+
+impl Iterator for SupportedInputFormats {
+    type Item = SupportedFormat;
+
+    #[inline]
+    fn next(&mut self) -> Option<SupportedFormat> {
+        None
+    }
+}
+
+impl Iterator for SupportedOutputFormats {
+    type Item = SupportedFormat;
+
+    #[inline]
+    fn next(&mut self) -> Option<SupportedFormat> {
+        None
+    }
+}

src/host/wasapi/mod.rs

@@ -0,0 +1,147 @@
+extern crate winapi;
+
+use BackendSpecificError;
+use BuildStreamError;
+use DefaultFormatError;
+use Device as DeviceTrait;
+use DeviceNameError;
+use DevicesError;
+use EventLoop as EventLoopTrait;
+use Format;
+use Host as HostTrait;
+use PlayStreamError;
+use PauseStreamError;
+use StreamDataResult;
+use StreamId as StreamIdTrait;
+use SupportedFormatsError;
+use self::winapi::um::winnt::HRESULT;
+use std::io::Error as IoError;
+pub use self::device::{Device, Devices, SupportedInputFormats, SupportedOutputFormats, default_input_device, default_output_device};
+pub use self::stream::{EventLoop, StreamId};
+
+mod com;
+mod device;
+mod stream;
+
+/// The WASAPI host, the default windows host type.
+#[derive(Debug)]
+pub struct Host;
+
+impl Host {
+    pub fn new() -> Result<Self, crate::HostUnavailable> {
+        Ok(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.
+        true
+    }
+
+    fn devices(&self) -> Result<Self::Devices, DevicesError> {
+        Devices::new()
+    }
+
+    fn default_input_device(&self) -> Option<Self::Device> {
+        default_input_device()
+    }
+
+    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 {
+        Err(IoError::from_raw_os_error(result))
+    } else {
+        Ok(())
+    }
+}
+
+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 });
+        }
+    }
+}

src/host/wasapi/stream.rs

@@ -115,7 +115,7 @@ impl EventLoop {
         }
     }
 
-    pub fn build_input_stream(
+    pub(crate) fn build_input_stream(
         &self,
         device: &Device,
         format: &Format,
@@ -276,7 +276,7 @@ impl EventLoop {
         }
     }
 
-    pub fn build_output_stream(
+    pub(crate) fn build_output_stream(
         &self,
         device: &Device,
         format: &Format,
@@ -439,12 +439,12 @@ impl EventLoop {
     }
 
     #[inline]
-    pub fn destroy_stream(&self, stream_id: StreamId) {
+    pub(crate) fn destroy_stream(&self, stream_id: StreamId) {
         self.push_command(Command::DestroyStream(stream_id));
     }
 
     #[inline]
-    pub fn run<F>(&self, mut callback: F) -> !
+    pub(crate) fn run<F>(&self, mut callback: F) -> !
         where F: FnMut(StreamId, StreamDataResult)
     {
         self.run_inner(&mut callback);
@@ -617,13 +617,13 @@ impl EventLoop {
     }
 
     #[inline]
-    pub fn play_stream(&self, stream: StreamId) -> Result<(), PlayStreamError> {
+    pub(crate) fn play_stream(&self, stream: StreamId) -> Result<(), PlayStreamError> {
         self.push_command(Command::PlayStream(stream));
         Ok(())
     }
 
     #[inline]
-    pub fn pause_stream(&self, stream: StreamId) -> Result<(), PauseStreamError> {
+    pub(crate) fn pause_stream(&self, stream: StreamId) -> Result<(), PauseStreamError> {
         self.push_command(Command::PauseStream(stream));
         Ok(())
     }

src/lib.rs

@@ -2,19 +2,25 @@
 //!
 //! Here are some concepts cpal exposes:
 //!
-//! - A `Device` is an audio device that may have any number of input and output streams.
+//! - A [**Host**](./trait.Host.html) provides access to the available audio devices on the system.
-//! - A stream is an open audio channel. Input streams allow you to receive audio data, output
+//!   Some platforms have more than one host available, but every platform supported by CPAL has at
-//!   streams allow you to play audio data. You must choose which `Device` runs your stream before
+//!   least one [**DefaultHost**](./trait.Host.html) that is guaranteed to be available.
-//!   you create one.
+//! - A [**Device**](./trait.Device.html) is an audio device that may have any number of input and
-//! - An `EventLoop` is a collection of streams being run by one or more `Device`. Each stream must
+//!   output streams.
-//!   belong to an `EventLoop`, and all the streams that belong to an `EventLoop` are managed
+//! - A stream is an open flow of audio data. Input streams allow you to receive audio data, output
-//!   together.
+//!   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**](./trait.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.
 //!
-//! The first step is to create an `EventLoop`:
+//! The first step is to initialise the `Host` (for accessing audio devices) and create an
+//! `EventLoop`:
 //!
 //! ```
-//! use cpal::EventLoop;
+//! use cpal::Host;
-//! let event_loop = EventLoop::new();
+//! 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
@@ -24,7 +30,9 @@
 //! stream type on the system.
 //!
 //! ```no_run
-//! let device = cpal::default_output_device().expect("no output device available");
+//! # use cpal::Host;
+//! # let host = cpal::default_host();
+//! let device = host.default_output_device().expect("no output device available");
 //! ```
 //!
 //! Before we can create a stream, we must decide what the format of the audio samples is going to
@@ -38,7 +46,9 @@
 //! > has been disconnected.
 //!
 //! ```no_run
-//! # let device = cpal::default_output_device().unwrap();
+//! use cpal::{Device, Host};
+//! # let host = cpal::default_host();
+//! # let device = host.default_output_device().unwrap();
 //! let mut supported_formats_range = device.supported_output_formats()
 //!     .expect("error while querying formats");
 //! let format = supported_formats_range.next()
@@ -49,9 +59,11 @@
 //! Now that we have everything for the stream, we can create it from our event loop:
 //!
 //! ```no_run
-//! # let device = cpal::default_output_device().unwrap();
+//! use cpal::{Device, EventLoop, Host};
+//! # 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 event_loop = cpal::EventLoop::new();
 //! let stream_id = event_loop.build_output_stream(&device, &format).unwrap();
 //! ```
 //!
@@ -61,15 +73,19 @@
 //! Now we must start the stream. This is done with the `play_stream()` method on the event loop.
 //!
 //! ```no_run
-//! # let event_loop: cpal::EventLoop = return;
+//! # use cpal::{EventLoop, Host};
-//! # let stream_id: cpal::StreamId = return;
+//! # 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
-//! # let event_loop = cpal::EventLoop::new();
+//! # use cpal::{EventLoop, Host};
+//! # 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
 //! });
@@ -87,9 +103,9 @@
 //! In this example, we simply fill the given output buffer with zeroes.
 //!
 //! ```no_run
-//! use cpal::{StreamData, UnknownTypeOutputBuffer};
+//! use cpal::{EventLoop, Host, StreamData, UnknownTypeOutputBuffer};
-//!
+//! # let host = cpal::default_host();
-//! # let event_loop = cpal::EventLoop::new();
+//! # let event_loop = host.event_loop();
 //! event_loop.run(move |stream_id, stream_result| {
 //!     let stream_data = match stream_result {
 //!         Ok(data) => data,
@@ -132,51 +148,203 @@ extern crate lazy_static;
 #[macro_use]
 extern crate stdweb;
 
+pub use platform::{ALL_HOSTS, HostId, available_hosts, default_host, host_from_id};
 pub use samples_formats::{Sample, SampleFormat};
 
-#[cfg(not(any(windows, target_os = "linux", target_os = "freebsd",
-              target_os = "macos", target_os = "ios", target_os = "emscripten")))]
-use null as cpal_impl;
-
 use failure::Fail;
-use std::fmt;
-use std::iter;
 use std::ops::{Deref, DerefMut};
 
-mod null;
+mod host;
+pub mod platform;
 mod samples_formats;
 
-#[cfg(any(target_os = "linux", target_os = "freebsd"))]
+/// A **Host** provides access to the available audio devices on the system.
-#[path = "alsa/mod.rs"]
+///
-mod cpal_impl;
+/// Each platform may have a number of available hosts depending on the system, each with their own
+/// pros and cons.
+///
+/// For example, WASAPI is the standard audio host API that ships with the Windows operating
+/// system. However, due to historical limitations with respect to performance and flexibility,
+/// Steinberg created the ASIO API providing better audio device support for pro audio and
+/// low-latency applications. As a result, it is common for some devices and device capabilities to
+/// only be available via ASIO, while others are only available via WASAPI.
+///
+/// Another great example is the Linux platform. While the ALSA host API is the lowest-level API
+/// available to almost all distributions of Linux, its flexibility is limited as it requires that
+/// each process have exclusive access to the devices with which they establish streams. PulseAudio
+/// is another popular host API that aims to solve this issue by providing user-space mixing,
+/// however it has its own limitations w.r.t. low-latency and high-performance audio applications.
+/// JACK is yet another host API that is more suitable to pro-audio applications, however it is
+/// less readily available by default in many Linux distributions and is known to be tricky to
+/// setup.
+pub trait Host {
+    /// The type used for enumerating available devices by the host.
+    type Devices: Iterator<Item = Self::Device>;
+    /// The `Device` type yielded by the host.
+    type Device: Device;
+    /// The event loop type used by the `Host`
+    type EventLoop: EventLoop<Device = Self::Device>;
 
-#[cfg(windows)]
+    /// Whether or not the host is available on the system.
-#[path = "wasapi/mod.rs"]
+    fn is_available() -> bool;
-mod cpal_impl;
 
-#[cfg(any(target_os = "macos", target_os = "ios"))]
+    /// An iterator yielding all `Device`s currently available to the host on the system.
-#[path = "coreaudio/mod.rs"]
+    ///
-mod cpal_impl;
+    /// Can be empty if the system does not support audio in general.
+    fn devices(&self) -> Result<Self::Devices, DevicesError>;
 
-#[cfg(target_os = "emscripten")]
+    /// The default input audio device on the system.
-#[path = "emscripten/mod.rs"]
+    ///
-mod cpal_impl;
+    /// Returns `None` if no input device is available.
+    fn default_input_device(&self) -> Option<Self::Device>;
 
-/// An opaque type that identifies a device that is capable of either audio input or output.
+    /// The default output audio device on the system.
+    ///
+    /// 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.
+    ///
+    /// Can be empty if the system does not support audio input.
+    fn input_devices(&self) -> Result<InputDevices<Self::Devices>, DevicesError> {
+        fn supports_input<D: Device>(device: &D) -> bool {
+            device.supported_input_formats()
+                .map(|mut iter| iter.next().is_some())
+                .unwrap_or(false)
+        }
+        Ok(self.devices()?.filter(supports_input::<Self::Device>))
+    }
+
+    /// An iterator yielding all `Device`s currently available to the system that support one or more
+    /// output stream formats.
+    ///
+    /// Can be empty if the system does not support audio output.
+    fn output_devices(&self) -> Result<OutputDevices<Self::Devices>, DevicesError> {
+        fn supports_output<D: Device>(device: &D) -> bool {
+            device.supported_output_formats()
+                .map(|mut iter| iter.next().is_some())
+                .unwrap_or(false)
+        }
+        Ok(self.devices()?.filter(supports_output::<Self::Device>))
+    }
+}
+
+/// A device that is capable of audio input and/or output.
 ///
 /// Please note that `Device`s may become invalid if they get disconnected. Therefore all the
-/// methods that involve a device return a `Result`.
+/// methods that involve a device return a `Result` allowing the user to handle this case.
-#[derive(Clone, PartialEq, Eq)]
+pub trait Device {
-pub struct Device(cpal_impl::Device);
+    /// The iterator type yielding supported input stream formats.
+    type SupportedInputFormats: Iterator<Item = SupportedFormat>;
+    /// The iterator type yielding supported output stream formats.
+    type SupportedOutputFormats: Iterator<Item = SupportedFormat>;
+
+    /// The human-readable name of the device.
+    fn name(&self) -> Result<String, DeviceNameError>;
+
+    /// An iterator yielding formats that are supported by the backend.
+    ///
+    /// Can return an error if the device is no longer valid (eg. it has been disconnected).
+    fn supported_input_formats(&self) -> Result<Self::SupportedInputFormats, SupportedFormatsError>;
+
+    /// An iterator yielding output stream formats that are supported by the device.
+    ///
+    /// Can return an error if the device is no longer valid (eg. it has been disconnected).
+    fn supported_output_formats(&self) -> Result<Self::SupportedOutputFormats, SupportedFormatsError>;
+
+    /// The default input stream format for the device.
+    fn default_input_format(&self) -> Result<Format, DefaultFormatError>;
+
+    /// The default output stream format for the device.
+    fn default_output_format(&self) -> Result<Format, DefaultFormatError>;
+}
 
 /// Collection of streams managed together.
 ///
-/// Created with the [`new`](struct.EventLoop.html#method.new) method.
+/// Created with the `Host::event_loop` method.
-pub struct EventLoop(cpal_impl::EventLoop);
+pub trait EventLoop {
+    /// The `Device` type yielded by the host.
+    type Device: Device;
+    /// The type used to uniquely distinguish between streams.
+    type StreamId: StreamId;
 
-/// Identifier of a stream within the `EventLoop`.
+    /// Creates a new input stream that will run from the given device and with the given format.
-#[derive(Debug, Clone, PartialEq, Eq, Hash)]
+    ///
-pub struct StreamId(cpal_impl::StreamId);
+    /// 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 StreamId: Clone + std::fmt::Debug + PartialEq + Eq {}
+
+/// A host's device iterator yielding only *input* devices.
+pub type InputDevices<I> = std::iter::Filter<I, fn(&<I as Iterator>::Item) -> bool>;
+
+/// A host's device iterator yielding only *output* devices.
+pub type OutputDevices<I> = std::iter::Filter<I, fn(&<I as Iterator>::Item) -> bool>;
 
 /// Number of channels.
 pub type ChannelCount = u16;
@@ -271,26 +439,10 @@ pub enum UnknownTypeOutputBuffer<'a> {
     F32(OutputBuffer<'a, f32>),
 }
 
-/// An iterator yielding all `Device`s currently available to the system.
+/// The requested host, although supported on this platform, is unavailable.
-///
+#[derive(Clone, Debug, Fail)]
-/// See [`devices()`](fn.devices.html).
+#[fail(display = "the requested host is unavailable")]
-pub struct Devices(cpal_impl::Devices);
+pub struct HostUnavailable;
-
-/// A `Devices` yielding only *input* devices.
-pub type InputDevices = iter::Filter<Devices, fn(&Device) -> bool>;
-
-/// A `Devices` yielding only *output* devices.
-pub type OutputDevices = iter::Filter<Devices, fn(&Device) -> bool>;
-
-/// An iterator that produces a list of input stream formats supported by the device.
-///
-/// See [`Device::supported_input_formats()`](struct.Device.html#method.supported_input_formats).
-pub struct SupportedInputFormats(cpal_impl::SupportedInputFormats);
-
-/// An iterator that produces a list of output stream formats supported by the device.
-///
-/// See [`Device::supported_output_formats()`](struct.Device.html#method.supported_output_formats).
-pub struct SupportedOutputFormats(cpal_impl::SupportedOutputFormats);
 
 /// Some error has occurred that is specific to the backend from which it was produced.
 ///
@@ -440,191 +592,6 @@ pub enum StreamError {
     }
 }
 
-/// An iterator yielding all `Device`s currently available to the system.
-///
-/// Can be empty if the system does not support audio in general.
-#[inline]
-pub fn devices() -> Result<Devices, DevicesError> {
-    Ok(Devices(cpal_impl::Devices::new()?))
-}
-
-/// An iterator yielding all `Device`s currently available to the system that support one or more
-/// input stream formats.
-///
-/// Can be empty if the system does not support audio input.
-pub fn input_devices() -> Result<InputDevices, DevicesError> {
-    fn supports_input(device: &Device) -> bool {
-        device.supported_input_formats()
-            .map(|mut iter| iter.next().is_some())
-            .unwrap_or(false)
-    }
-    Ok(devices()?.filter(supports_input))
-}
-
-/// An iterator yielding all `Device`s currently available to the system that support one or more
-/// output stream formats.
-///
-/// Can be empty if the system does not support audio output.
-pub fn output_devices() -> Result<OutputDevices, DevicesError> {
-    fn supports_output(device: &Device) -> bool {
-        device.supported_output_formats()
-            .map(|mut iter| iter.next().is_some())
-            .unwrap_or(false)
-    }
-    Ok(devices()?.filter(supports_output))
-}
-
-/// The default input audio device on the system.
-///
-/// Returns `None` if no input device is available.
-pub fn default_input_device() -> Option<Device> {
-    cpal_impl::default_input_device().map(Device)
-}
-
-/// The default output audio device on the system.
-///
-/// Returns `None` if no output device is available.
-pub fn default_output_device() -> Option<Device> {
-    cpal_impl::default_output_device().map(Device)
-}
-
-impl Device {
-    /// The human-readable name of the device.
-    #[inline]
-    pub fn name(&self) -> Result<String, DeviceNameError> {
-        self.0.name()
-    }
-
-    /// An iterator yielding formats that are supported by the backend.
-    ///
-    /// Can return an error if the device is no longer valid (eg. it has been disconnected).
-    #[inline]
-    pub fn supported_input_formats(&self) -> Result<SupportedInputFormats, SupportedFormatsError> {
-        Ok(SupportedInputFormats(self.0.supported_input_formats()?))
-    }
-
-    /// An iterator yielding output stream formats that are supported by the device.
-    ///
-    /// Can return an error if the device is no longer valid (eg. it has been disconnected).
-    #[inline]
-    pub fn supported_output_formats(&self) -> Result<SupportedOutputFormats, SupportedFormatsError> {
-        Ok(SupportedOutputFormats(self.0.supported_output_formats()?))
-    }
-
-    /// The default input stream format for the device.
-    #[inline]
-    pub fn default_input_format(&self) -> Result<Format, DefaultFormatError> {
-        self.0.default_input_format()
-    }
-
-    /// The default output stream format for the device.
-    #[inline]
-    pub fn default_output_format(&self) -> Result<Format, DefaultFormatError> {
-        self.0.default_output_format()
-    }
-}
-
-impl fmt::Debug for Device {
-    fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
-        fmt::Debug::fmt(&self.0, f)
-    }
-}
-
-impl EventLoop {
-    /// Initializes a new events loop.
-    #[inline]
-    pub fn new() -> EventLoop {
-        EventLoop(cpal_impl::EventLoop::new())
-    }
-
-    /// Creates a new input stream that will run from 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 input stream format is not
-    /// supported by the device.
-    #[inline]
-    pub fn build_input_stream(
-        &self,
-        device: &Device,
-        format: &Format,
-    ) -> Result<StreamId, BuildStreamError>
-    {
-        self.0.build_input_stream(&device.0, format).map(StreamId)
-    }
-
-    /// 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.
-    #[inline]
-    pub fn build_output_stream(
-        &self,
-        device: &Device,
-        format: &Format,
-    ) -> Result<StreamId, BuildStreamError>
-    {
-        self.0.build_output_stream(&device.0, format).map(StreamId)
-    }
-
-    /// 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.
-    ///
-    #[inline]
-    pub fn play_stream(&self, stream: StreamId) -> Result<(), PlayStreamError> {
-        self.0.play_stream(stream.0)
-    }
-
-    /// 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.
-    ///
-    #[inline]
-    pub fn pause_stream(&self, stream: StreamId) -> Result<(), PauseStreamError> {
-        self.0.pause_stream(stream.0)
-    }
-
-    /// Destroys an existing stream.
-    ///
-    /// # Panic
-    ///
-    /// If the stream does not exist, this function can either panic or be a no-op.
-    ///
-    #[inline]
-    pub fn destroy_stream(&self, stream_id: StreamId) {
-        self.0.destroy_stream(stream_id.0)
-    }
-
-    /// 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.
-    #[inline]
-    pub fn run<F>(&self, mut callback: F) -> !
-        where F: FnMut(StreamId, StreamDataResult) + Send
-    {
-        self.0.run(move |id, data| callback(StreamId(id), data))
-    }
-}
-
 impl SupportedFormat {
     /// Turns this `SupportedFormat` into a `Format` corresponding to the maximum samples rate.
     #[inline]
@@ -775,48 +742,6 @@ impl From<Format> for SupportedFormat {
     }
 }
 
-impl Iterator for Devices {
-    type Item = Device;
-
-    #[inline]
-    fn next(&mut self) -> Option<Self::Item> {
-        self.0.next().map(Device)
-    }
-
-    #[inline]
-    fn size_hint(&self) -> (usize, Option<usize>) {
-        self.0.size_hint()
-    }
-}
-
-impl Iterator for SupportedInputFormats {
-    type Item = SupportedFormat;
-
-    #[inline]
-    fn next(&mut self) -> Option<SupportedFormat> {
-        self.0.next()
-    }
-
-    #[inline]
-    fn size_hint(&self) -> (usize, Option<usize>) {
-        self.0.size_hint()
-    }
-}
-
-impl Iterator for SupportedOutputFormats {
-    type Item = SupportedFormat;
-
-    #[inline]
-    fn next(&mut self) -> Option<SupportedFormat> {
-        self.0.next()
-    }
-
-    #[inline]
-    fn size_hint(&self) -> (usize, Option<usize>) {
-        self.0.size_hint()
-    }
-}
-
 impl From<BackendSpecificError> for DevicesError {
     fn from(err: BackendSpecificError) -> Self {
         DevicesError::BackendSpecific { err }

src/null/mod.rs

@@ -1,145 +0,0 @@
-#![allow(dead_code)]
-
-use std::marker::PhantomData;
-
-use BuildStreamError;
-use DefaultFormatError;
-use DevicesError;
-use DeviceNameError;
-use Format;
-use PauseStreamError;
-use PlayStreamError;
-use StreamDataResult;
-use SupportedFormatsError;
-use SupportedFormat;
-
-pub struct EventLoop;
-
-impl EventLoop {
-    #[inline]
-    pub fn new() -> EventLoop {
-        EventLoop
-    }
-
-    #[inline]
-    pub fn run<F>(&self, _callback: F) -> !
-        where F: FnMut(StreamId, StreamDataResult)
-    {
-        loop { /* TODO: don't spin */ }
-    }
-
-    #[inline]
-    pub fn build_input_stream(&self, _: &Device, _: &Format) -> Result<StreamId, BuildStreamError> {
-        Err(BuildStreamError::DeviceNotAvailable)
-    }
-
-    #[inline]
-    pub fn build_output_stream(&self, _: &Device, _: &Format) -> Result<StreamId, BuildStreamError> {
-        Err(BuildStreamError::DeviceNotAvailable)
-    }
-
-    #[inline]
-    pub fn destroy_stream(&self, _: StreamId) {
-        unimplemented!()
-    }
-
-    #[inline]
-    pub fn play_stream(&self, _: StreamId) -> Result<(), PlayStreamError> {
-        panic!()
-    }
-
-    #[inline]
-    pub fn pause_stream(&self, _: StreamId) -> Result<(), PauseStreamError> {
-        panic!()
-    }
-}
-
-#[derive(Debug, Clone, PartialEq, Eq, Hash)]
-pub struct StreamId;
-
-#[derive(Default)]
-pub struct Devices;
-
-impl Devices {
-    pub fn new() -> Result<Self, DevicesError> {
-        Ok(Devices)
-    }
-}
-
-impl Iterator for Devices {
-    type Item = Device;
-
-    #[inline]
-    fn next(&mut self) -> Option<Device> {
-        None
-    }
-}
-
-#[inline]
-pub fn default_input_device() -> Option<Device> {
-    None
-}
-
-#[inline]
-pub fn default_output_device() -> Option<Device> {
-    None
-}
-
-#[derive(Clone, Debug, PartialEq, Eq)]
-pub struct Device;
-
-impl Device {
-    #[inline]
-    pub fn supported_input_formats(&self) -> Result<SupportedInputFormats, SupportedFormatsError> {
-        unimplemented!()
-    }
-
-    #[inline]
-    pub fn supported_output_formats(&self) -> Result<SupportedOutputFormats, SupportedFormatsError> {
-        unimplemented!()
-    }
-
-    #[inline]
-    pub fn default_input_format(&self) -> Result<Format, DefaultFormatError> {
-        unimplemented!()
-    }
-
-    #[inline]
-    pub fn default_output_format(&self) -> Result<Format, DefaultFormatError> {
-        unimplemented!()
-    }
-
-    #[inline]
-    pub fn name(&self) -> Result<String, DeviceNameError> {
-        Ok("null".to_owned())
-    }
-}
-
-pub struct SupportedInputFormats;
-pub struct SupportedOutputFormats;
-
-impl Iterator for SupportedInputFormats {
-    type Item = SupportedFormat;
-
-    #[inline]
-    fn next(&mut self) -> Option<SupportedFormat> {
-        None
-    }
-}
-
-impl Iterator for SupportedOutputFormats {
-    type Item = SupportedFormat;
-
-    #[inline]
-    fn next(&mut self) -> Option<SupportedFormat> {
-        None
-    }
-}
-
-pub struct InputBuffer<'a, T: 'a> {
-    marker: PhantomData<&'a T>,
-}
-
-pub struct OutputBuffer<'a, T: 'a> {
-    marker: PhantomData<&'a mut T>,
-}

src/platform/mod.rs

@@ -0,0 +1,560 @@
+//! Platform-specific items.
+//!
+//! This module also contains the implementation of the platform's dynamically dispatched `Host`
+//! type and its associated `EventLoop`, `Device`, `StreamId` and other associated types. These
+//! types are useful in the case that users require switching between audio host APIs at runtime.
+
+#[doc(inline)]
+pub use self::platform_impl::*;
+
+// A macro to assist with implementing a platform's dynamically dispatched `Host` type.
+//
+// These dynamically dispatched types are necessary to allow for users to switch between hosts at
+// runtime.
+//
+// For example the invocation `impl_platform_host(Wasapi wasapi, Asio asio)`, this macro should
+// expand to:
+//
+// ```
+// pub enum HostId {
+//     Wasapi,
+//     Asio,
+// }
+//
+// pub enum Host {
+//     Wasapi(crate::host::wasapi::Host),
+//     Asio(crate::host::asio::Host),
+// }
+// ```
+//
+// And so on for Device, Devices, EventLoop, Host, StreamId, SupportedInputFormats,
+// SupportedOutputFormats and all their necessary trait implementations.
+// ```
+macro_rules! impl_platform_host {
+    ($($HostVariant:ident $host_mod:ident),*) => {
+        /// All hosts supported by CPAL on this platform.
+        pub const ALL_HOSTS: &'static [HostId] = &[
+            $(
+                HostId::$HostVariant,
+            )*
+        ];
+
+        /// The platform's dynamically dispatched **Host** type.
+        ///
+        /// An instance of this **Host** type may represent one of any of the **Host**s available
+        /// on the platform.
+        ///
+        /// Use this type if you require switching between available hosts at runtime.
+        ///
+        /// This type may be constructed via the **host_from_id** function. **HostId**s may
+        /// be acquired via the **ALL_HOSTS** const and the **available_hosts** function.
+        pub struct Host(HostInner);
+
+        /// The **Device** implementation associated with the platform's dynamically dispatched
+        /// **Host** type.
+        pub struct Device(DeviceInner);
+
+        /// The **Devices** iterator associated with the platform's dynamically dispatched **Host**
+        /// type.
+        pub struct Devices(DevicesInner);
+
+        /// The **EventLoop** 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, PartialEq)]
+        pub struct StreamId(StreamIdInner);
+
+        /// The **SupportedInputFormats** iterator associated with the platform's dynamically
+        /// dispatched **Host** type.
+        pub struct SupportedInputFormats(SupportedInputFormatsInner);
+
+        /// The **SupportedOutputFormats** iterator associated with the platform's dynamically
+        /// dispatched **Host** type.
+        pub struct SupportedOutputFormats(SupportedOutputFormatsInner);
+
+        /// Unique identifier for available hosts on the platform.
+        #[derive(Copy, Clone, Debug, Eq, Hash, PartialEq)]
+        pub enum HostId {
+            $(
+                $HostVariant,
+            )*
+        }
+
+        enum DeviceInner {
+            $(
+                $HostVariant(crate::host::$host_mod::Device),
+            )*
+        }
+
+        enum DevicesInner {
+            $(
+                $HostVariant(crate::host::$host_mod::Devices),
+            )*
+        }
+
+        enum EventLoopInner {
+            $(
+                $HostVariant(crate::host::$host_mod::EventLoop),
+            )*
+        }
+
+        enum HostInner {
+            $(
+                $HostVariant(crate::host::$host_mod::Host),
+            )*
+        }
+
+        #[derive(Clone, Debug, Eq, PartialEq)]
+        enum StreamIdInner {
+            $(
+                $HostVariant(crate::host::$host_mod::StreamId),
+            )*
+        }
+
+        enum SupportedInputFormatsInner {
+            $(
+                $HostVariant(crate::host::$host_mod::SupportedInputFormats),
+            )*
+        }
+
+        enum SupportedOutputFormatsInner {
+            $(
+                $HostVariant(crate::host::$host_mod::SupportedOutputFormats),
+            )*
+        }
+
+        impl Host {
+            /// The unique identifier associated with this host.
+            pub fn id(&self) -> HostId {
+                match self.0 {
+                    $(
+                        HostInner::$HostVariant(_) => HostId::$HostVariant,
+                    )*
+                }
+            }
+        }
+
+        impl Iterator for Devices {
+            type Item = Device;
+
+            fn next(&mut self) -> Option<Self::Item> {
+                match self.0 {
+                    $(
+                        DevicesInner::$HostVariant(ref mut d) => {
+                            d.next().map(DeviceInner::$HostVariant).map(Device)
+                        }
+                    )*
+                }
+            }
+
+            fn size_hint(&self) -> (usize, Option<usize>) {
+                match self.0 {
+                    $(
+                        DevicesInner::$HostVariant(ref d) => d.size_hint(),
+                    )*
+                }
+            }
+        }
+
+        impl Iterator for SupportedInputFormats {
+            type Item = crate::SupportedFormat;
+
+            fn next(&mut self) -> Option<Self::Item> {
+                match self.0 {
+                    $(
+                        SupportedInputFormatsInner::$HostVariant(ref mut s) => s.next(),
+                    )*
+                }
+            }
+
+            fn size_hint(&self) -> (usize, Option<usize>) {
+                match self.0 {
+                    $(
+                        SupportedInputFormatsInner::$HostVariant(ref d) => d.size_hint(),
+                    )*
+                }
+            }
+        }
+
+        impl Iterator for SupportedOutputFormats {
+            type Item = crate::SupportedFormat;
+
+            fn next(&mut self) -> Option<Self::Item> {
+                match self.0 {
+                    $(
+                        SupportedOutputFormatsInner::$HostVariant(ref mut s) => s.next(),
+                    )*
+                }
+            }
+
+            fn size_hint(&self) -> (usize, Option<usize>) {
+                match self.0 {
+                    $(
+                        SupportedOutputFormatsInner::$HostVariant(ref d) => d.size_hint(),
+                    )*
+                }
+            }
+        }
+
+        impl crate::Device for Device {
+            type SupportedInputFormats = SupportedInputFormats;
+            type SupportedOutputFormats = SupportedOutputFormats;
+
+            fn name(&self) -> Result<String, crate::DeviceNameError> {
+                match self.0 {
+                    $(
+                        DeviceInner::$HostVariant(ref d) => d.name(),
+                    )*
+                }
+            }
+
+            fn supported_input_formats(&self) -> Result<Self::SupportedInputFormats, crate::SupportedFormatsError> {
+                match self.0 {
+                    $(
+                        DeviceInner::$HostVariant(ref d) => {
+                            d.supported_input_formats()
+                                .map(SupportedInputFormatsInner::$HostVariant)
+                                .map(SupportedInputFormats)
+                        }
+                    )*
+                }
+            }
+
+            fn supported_output_formats(&self) -> Result<Self::SupportedOutputFormats, crate::SupportedFormatsError> {
+                match self.0 {
+                    $(
+                        DeviceInner::$HostVariant(ref d) => {
+                            d.supported_output_formats()
+                                .map(SupportedOutputFormatsInner::$HostVariant)
+                                .map(SupportedOutputFormats)
+                        }
+                    )*
+                }
+            }
+
+            fn default_input_format(&self) -> Result<crate::Format, crate::DefaultFormatError> {
+                match self.0 {
+                    $(
+                        DeviceInner::$HostVariant(ref d) => d.default_input_format(),
+                    )*
+                }
+            }
+
+            fn default_output_format(&self) -> Result<crate::Format, crate::DefaultFormatError> {
+                match self.0 {
+                    $(
+                        DeviceInner::$HostVariant(ref d) => d.default_output_format(),
+                    )*
+                }
+            }
+        }
+
+        impl crate::EventLoop for EventLoop {
+            type StreamId = StreamId;
+            type Device = Device;
+
+            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)
+                        }
+                    )*
+                }
+            }
+
+            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)
+                        }
+                    )*
+                }
+            }
+
+            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())
+                        }
+                    )*
+                }
+            }
+
+            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())
+                        }
+                    )*
+                }
+            }
+
+            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())
+                        }
+                    )*
+                }
+            }
+
+            fn run<F>(&self, mut callback: F) -> !
+            where
+                F: FnMut(Self::StreamId, crate::StreamDataResult) + Send
+            {
+                match self.0 {
+                    $(
+                        EventLoopInner::$HostVariant(ref e) => {
+                            e.run(|id, result| {
+                                let result = result;
+                                callback(StreamId(StreamIdInner::$HostVariant(id)), result);
+                            });
+                        },
+                    )*
+                }
+            }
+        }
+
+        impl crate::Host for Host {
+            type Devices = Devices;
+            type Device = Device;
+            type EventLoop = EventLoop;
+
+            fn is_available() -> bool {
+                $( crate::host::$host_mod::Host::is_available() ||)* false
+            }
+
+            fn devices(&self) -> Result<Self::Devices, crate::DevicesError> {
+                match self.0 {
+                    $(
+                        HostInner::$HostVariant(ref h) => {
+                            h.devices().map(DevicesInner::$HostVariant).map(Devices)
+                        }
+                    )*
+                }
+            }
+
+            fn default_input_device(&self) -> Option<Self::Device> {
+                match self.0 {
+                    $(
+                        HostInner::$HostVariant(ref h) => {
+                            h.default_input_device().map(DeviceInner::$HostVariant).map(Device)
+                        }
+                    )*
+                }
+            }
+
+            fn default_output_device(&self) -> Option<Self::Device> {
+                match self.0 {
+                    $(
+                        HostInner::$HostVariant(ref h) => {
+                            h.default_output_device().map(DeviceInner::$HostVariant).map(Device)
+                        }
+                    )*
+                }
+            }
+
+            fn event_loop(&self) -> Self::EventLoop {
+                match self.0 {
+                    $(
+                        HostInner::$HostVariant(ref h) => {
+                            EventLoop(EventLoopInner::$HostVariant(h.event_loop()))
+                        }
+                    )*
+                }
+            }
+        }
+
+        impl crate::StreamId for StreamId {}
+
+        $(
+            impl From<crate::host::$host_mod::Device> for Device {
+                fn from(h: crate::host::$host_mod::Device) -> Self {
+                    Device(DeviceInner::$HostVariant(h))
+                }
+            }
+
+            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))
+                }
+            }
+
+            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 {
+                fn from(h: crate::host::$host_mod::StreamId) -> Self {
+                    StreamId(StreamIdInner::$HostVariant(h))
+                }
+            }
+        )*
+
+        /// Produces a list of hosts that are currently available on the system.
+        pub fn available_hosts() -> Vec<HostId> {
+            let mut host_ids = vec![];
+            $(
+                if <crate::host::$host_mod::Host as crate::Host>::is_available() {
+                    host_ids.push(HostId::$HostVariant);
+                }
+            )*
+            host_ids
+        }
+
+        /// Given a unique host identifier, initialise and produce the host if it is available.
+        pub fn host_from_id(id: HostId) -> Result<Host, crate::HostUnavailable> {
+            match id {
+                $(
+                    HostId::$HostVariant => {
+                        crate::host::$host_mod::Host::new()
+                            .map(HostInner::$HostVariant)
+                            .map(Host)
+                    }
+                )*
+            }
+        }
+    };
+}
+
+// TODO: Add pulseaudio and jack here eventually.
+#[cfg(any(target_os = "linux", target_os = "freebsd"))]
+mod platform_impl {
+    pub use crate::host::alsa::{
+        Device as AlsaDevice,
+        Devices as AlsaDevices,
+        EventLoop as AlsaEventLoop,
+        Host as AlsaHost,
+        StreamId as AlsaStreamId,
+        SupportedInputFormats as AlsaSupportedInputFormats,
+        SupportedOutputFormats as AlsaSupportedOutputFormats,
+    };
+
+    impl_platform_host!(Alsa alsa);
+
+    /// The default host for the current compilation target platform.
+    pub fn default_host() -> Host {
+        AlsaHost::new()
+            .expect("the default host should always be available")
+            .into()
+    }
+}
+
+
+#[cfg(any(target_os = "macos", target_os = "ios"))]
+mod platform_impl {
+    pub use crate::host::coreaudio::{
+        Device as CoreAudioDevice,
+        Devices as CoreAudioDevices,
+        EventLoop as CoreAudioEventLoop,
+        Host as CoreAudioHost,
+        StreamId as CoreAudioStreamId,
+        SupportedInputFormats as CoreAudioSupportedInputFormats,
+        SupportedOutputFormats as CoreAudioSupportedOutputFormats,
+    };
+
+    impl_platform_host!(CoreAudio coreaudio);
+
+    /// The default host for the current compilation target platform.
+    pub fn default_host() -> Host {
+        CoreAudioHost::new()
+            .expect("the default host should always be available")
+            .into()
+    }
+}
+
+#[cfg(target_os = "emscripten")]
+mod platform_impl {
+    pub use crate::host::emscripten::{
+        Device as EmscriptenDevice,
+        Devices as EmscriptenDevices,
+        EventLoop as EmscriptenEventLoop,
+        Host as EmscriptenHost,
+        StreamId as EmscriptenStreamId,
+        SupportedInputFormats as EmscriptenSupportedInputFormats,
+        SupportedOutputFormats as EmscriptenSupportedOutputFormats,
+    };
+
+    impl_platform_host!(Emscripten emscripten);
+
+    /// The default host for the current compilation target platform.
+    pub fn default_host() -> Host {
+        EmscriptenHost::new()
+            .expect("the default host should always be available")
+            .into()
+    }
+}
+
+// TODO: Add `Asio asio` once #221 lands.
+#[cfg(windows)]
+mod platform_impl {
+    pub use crate::host::wasapi::{
+        Device as WasapiDevice,
+        Devices as WasapiDevices,
+        EventLoop as WasapiEventLoop,
+        Host as WasapiHost,
+        StreamId as WasapiStreamId,
+        SupportedInputFormats as WasapiSupportedInputFormats,
+        SupportedOutputFormats as WasapiSupportedOutputFormats,
+    };
+
+    impl_platform_host!(Wasapi wasapi);
+
+    /// The default host for the current compilation target platform.
+    pub fn default_host() -> Host {
+        WasapiHost::new()
+            .expect("the default host should always be available")
+            .into()
+    }
+}
+
+#[cfg(not(any(windows, target_os = "linux", target_os = "freebsd", target_os = "macos",
+              target_os = "ios", target_os = "emscripten")))]
+mod platform_impl {
+    pub use crate::host::null::{
+        Device as NullDevice,
+        Devices as NullDevices,
+        EventLoop as NullEventLoop,
+        Host as NullHost,
+        StreamId as NullStreamId,
+        SupportedInputFormats as NullSupportedInputFormats,
+        SupportedOutputFormats as NullSupportedOutputFormats,
+    };
+
+    impl_platform_host!(Null null);
+
+    /// The default host for the current compilation target platform.
+    pub fn default_host() -> Host {
+        NullHost::new()
+            .expect("the default host should always be available")
+            .into()
+    }
+}

src/wasapi/mod.rs

@@ -1,30 +0,0 @@
-extern crate winapi;
-
-use BackendSpecificError;
-use self::winapi::um::winnt::HRESULT;
-use std::io::Error as IoError;
-pub use self::device::{Device, Devices, SupportedInputFormats, SupportedOutputFormats, default_input_device, default_output_device};
-pub use self::stream::{EventLoop, StreamId};
-
-mod com;
-mod device;
-mod stream;
-
-#[inline]
-fn check_result(result: HRESULT) -> Result<(), IoError> {
-    if result < 0 {
-        Err(IoError::from_raw_os_error(result))
-    } else {
-        Ok(())
-    }
-}
-
-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 });
-        }
-    }
-}