Update the `emscripten` backend for the removal of the `EventLoop`

For the most part, behaviour should be largely unchanged, however each
individual stream now has its own `set_timeout` callback loop, rather
than using one for processing all streams at once.

Many TODOs remain within the `emscripten` backend. These were left
untouched for the most part in favour of addressing this in a more
web-focused, future PR.

src/host/emscripten/mod.rs

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

src/platform/mod.rs

@@ -435,9 +435,8 @@ mod platform_impl {
     pub use crate::host::emscripten::{
         Device as EmscriptenDevice,
         Devices as EmscriptenDevices,
-        EventLoop as EmscriptenEventLoop,
         Host as EmscriptenHost,
-        StreamId as EmscriptenStreamId,
+        Stream as EmscriptenStream,
         SupportedInputFormats as EmscriptenSupportedInputFormats,
         SupportedOutputFormats as EmscriptenSupportedOutputFormats,
     };