[WIP] Introduce a Host API

This is an implementation of the API described at RustAudio#204. Please see that
issue for more details on the motivation.

-----

A **Host** provides access to the available audio devices on the system.
Some platforms have more than one host available, e.g.
wasapi/asio/dsound on windows, alsa/pulse/jack on linux and so on. As a
result, some audio devices are only available on certain hosts, while
others are only available on other hosts. Every platform supported by
CPAL has at least one **DefaultHost** that is guaranteed to be available
(alsa, wasapi and coreaudio). Currently, the default hosts are the only
hosts supported by CPAL, however this will change as of landing RustAudio#221 (cc
@freesig). These changes should also accommodate support for other hosts
such as jack RustAudio#250 (cc @derekdreery) and pulseaudio (cc @knappador) RustAudio#259.

This introduces a suite of traits allowing for both compile time and
runtime dispatch of different hosts and their uniquely associated device
and event loop types.

A new private **host** module has been added containing the individual
host implementations, each in their own submodule gated to the platforms
on which they are available.

A new **platform** module has been added containing platform-specific
items, including a dynamically dispatched host type that allows for
easily switching between hosts at runtime.

The **ALL_HOSTS** slice contains a **HostId** for each host supported on
the current platform. The **available_hosts** function produces a
**HostId** for each host that is currently *available* on the platform.
The **host_from_id** function allows for initialising a host from its
associated ID, failing with a **HostUnavailable** error. The
**default_host** function returns the default host and should never
fail.

Please see the examples for a demonstration of the change in usage. For
the most part, things look the same at the surface level, however the
role of device enumeration and creating the event loop have been moved
from global functions to host methods. The enumerate.rs example has been
updated to enumerate all devices for each host, not just the default.

**TODO**

- [x] Add the new **Host** API
- [x] Update examples for the new API.
- [x] ALSA host
- [ ] WASAPI host
- [ ] CoreAudio host
- [ ] Emscripten host **Follow-up PR**
- [ ] ASIO host RustAudio#221

cc @ishitatsuyuki more to review for you if you're interested, but it
might be easier after RustAudio#288 lands and this gets rebased.

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());
-    let default_out = cpal::default_output_device().map(|e| e.name().unwrap());
-    println!("Default Input Device:\n  {:?}", default_in);
-    println!("Default Output Device:\n  {:?}", default_out);
+    println!("Supported hosts:\n  {:?}", cpal::ALL_HOSTS);
+    let available_hosts = cpal::available_hosts();
+    println!("Available hosts:\n  {:?}", available_hosts);
 
-    let devices = cpal::devices()?;
-    println!("Devices: ");
-    for (device_index, device) in devices.enumerate() {
-        println!("{}. \"{}\"", device_index + 1, device.name()?);
+    for host_id in available_hosts {
+        println!("{:?}", host_id);
+        let host = cpal::host_from_id(host_id)?;
+        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
-        if let Ok(fmt) = device.default_input_format() {
-            println!("  Default input stream format:\n    {:?}", fmt);
-        }
-        let mut input_formats = match device.supported_input_formats() {
-            Ok(f) => f.peekable(),
-            Err(e) => {
-                println!("Error: {:?}", e);
-                continue;
-            },
-        };
-        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);
+        let devices = host.devices()?;
+        println!("  Devices: ");
+        for (device_index, device) in devices.enumerate() {
+            println!("  {}. \"{}\"", device_index + 1, device.name()?);
+
+            // Input formats
+            if let Ok(fmt) = device.default_input_format() {
+                println!("    Default input stream format:\n      {:?}", fmt);
+            }
+            let mut input_formats = match device.supported_input_formats() {
+                Ok(f) => f.peekable(),
+                Err(e) => {
+                    println!("Error: {:?}", e);
+                    continue;
+                },
+            };
+            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
-        if let Ok(fmt) = device.default_output_format() {
-            println!("  Default output stream format:\n    {:?}", fmt);
-        }
-        let mut output_formats = match device.supported_output_formats() {
-            Ok(f) => f.peekable(),
-            Err(e) => {
-                println!("Error: {:?}", e);
-                continue;
-            },
-        };
-        if output_formats.peek().is_some() {
-            println!("  All supported output stream formats:");
-            for (format_index, format) in output_formats.enumerate() {
-                println!("    {}.{}. {:?}", device_index + 1, format_index + 1, format);
+            // Output formats
+            if let Ok(fmt) = device.default_output_format() {
+                println!("    Default output stream format:\n      {:?}", fmt);
+            }
+            let mut output_formats = match device.supported_output_formats() {
+                Ok(f) => f.peekable(),
+                Err(e) => {
+                    println!("Error: {:?}", e);
+                    continue;
+                },
+            };
+            if output_formats.peek().is_some() {
+                println!("    All supported output stream formats:");
+                for (format_index, format) in output_formats.enumerate() {
+                    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 output_device = cpal::default_output_device().expect("failed to get default output device");
+    let input_device = host.default_input_device().expect("failed to get default input device");
+    let output_device = host.default_output_device().expect("failed to get default output device");
     println!("Using default input device: \"{}\"", input_device.name()?);
     println!("Using default output device: \"{}\"", output_device.name()?);
 

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/alsa/mod.rs → 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 StreamError;
 use StreamEvent;
+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, StreamEvent) + 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, StreamEvent)
     {
         self.run_inner(&mut callback)
@@ -648,7 +756,7 @@ impl EventLoop {
         panic!("`cpal::EventLoop::run` API currently disallows returning");
     }
 
-    pub fn build_input_stream(
+    fn build_input_stream(
         &self,
         device: &Device,
         format: &Format,
@@ -727,7 +835,7 @@ impl EventLoop {
         }
     }
 
-    pub fn build_output_stream(
+    fn build_output_stream(
         &self,
         device: &Device,
         format: &Format,
@@ -808,18 +916,18 @@ 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/mod.rs

@@ -0,0 +1,10 @@
+#[cfg(any(target_os = "linux", target_os = "freebsd"))]
+pub(crate) mod alsa;
+#[cfg(any(target_os = "macos", target_os = "ios"))]
+mod coreaudio;
+//mod dynamic;
+#[cfg(target_os = "emscripten")]
+mod emscripten;
+mod null;
+#[cfg(windows)]
+mod wasapi;