cpal/src/host/coreaudio/mod.rs

extern crate core_foundation_sys;
extern crate coreaudio;

use self::core_foundation_sys::string::{CFStringGetCStringPtr, CFStringRef};
use self::coreaudio::audio_unit::render_callback::{self, data};
use self::coreaudio::audio_unit::{AudioUnit, Element, Scope};
use self::coreaudio::sys::{
    kAudioDevicePropertyAvailableNominalSampleRates, kAudioDevicePropertyDeviceNameCFString,
    kAudioDevicePropertyNominalSampleRate, kAudioDevicePropertyScopeOutput,
    kAudioDevicePropertyStreamConfiguration, kAudioDevicePropertyStreamFormat,
    kAudioFormatFlagIsFloat, kAudioFormatFlagIsPacked, kAudioFormatLinearPCM,
    kAudioObjectPropertyElementMaster, kAudioObjectPropertyScopeGlobal,
    kAudioObjectPropertyScopeInput, kAudioObjectPropertyScopeOutput,
    kAudioOutputUnitProperty_CurrentDevice, kAudioOutputUnitProperty_EnableIO,
    kAudioUnitProperty_StreamFormat, kCFStringEncodingUTF8, AudioBuffer, AudioBufferList,
    AudioDeviceID, AudioObjectAddPropertyListener, AudioObjectGetPropertyData,
    AudioObjectGetPropertyDataSize, AudioObjectID, AudioObjectPropertyAddress,
    AudioObjectPropertyScope, AudioObjectRemovePropertyListener, AudioObjectSetPropertyData,
    AudioStreamBasicDescription, AudioValueRange, OSStatus,
};
use crate::traits::{DeviceTrait, HostTrait, StreamTrait};
use crate::{
    BackendSpecificError, BuildStreamError, ChannelCount, Data, DefaultFormatError,
    DeviceNameError, DevicesError, Format, PauseStreamError, PlayStreamError, SampleFormat,
    SampleRate, StreamError, SupportedFormat, SupportedFormatsError,
};
use std::cell::RefCell;
use std::ffi::CStr;
use std::fmt;
use std::mem;
use std::os::raw::c_char;
use std::ptr::null;
use std::slice;
use std::thread;
use std::time::Duration;

mod enumerate;

pub use self::enumerate::{
    default_input_device, default_output_device, Devices, SupportedInputFormats,
    SupportedOutputFormats,
};

/// 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;

    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()
    }
}

impl DeviceTrait for Device {
    type SupportedInputFormats = SupportedInputFormats;
    type SupportedOutputFormats = SupportedOutputFormats;
    type Stream = Stream;

    fn name(&self) -> Result<String, DeviceNameError> {
        Device::name(self)
    }

    fn supported_input_formats(
        &self,
    ) -> Result<Self::SupportedInputFormats, SupportedFormatsError> {
        Device::supported_input_formats(self)
    }

    fn supported_output_formats(
        &self,
    ) -> Result<Self::SupportedOutputFormats, SupportedFormatsError> {
        Device::supported_output_formats(self)
    }

    fn default_input_format(&self) -> Result<Format, DefaultFormatError> {
        Device::default_input_format(self)
    }

    fn default_output_format(&self) -> Result<Format, DefaultFormatError> {
        Device::default_output_format(self)
    }

    fn build_input_stream_raw<D, E>(
        &self,
        format: &Format,
        data_callback: D,
        error_callback: E,
    ) -> Result<Self::Stream, BuildStreamError>
    where
        D: FnMut(&Data) + Send + 'static,
        E: FnMut(StreamError) + Send + 'static,
    {
        Device::build_input_stream_raw(self, format, data_callback, error_callback)
    }

    fn build_output_stream_raw<D, E>(
        &self,
        format: &Format,
        data_callback: D,
        error_callback: E,
    ) -> Result<Self::Stream, BuildStreamError>
    where
        D: FnMut(&mut Data) + Send + 'static,
        E: FnMut(StreamError) + Send + 'static,
    {
        Device::build_output_stream_raw(self, format, data_callback, error_callback)
    }
}

#[derive(Clone, PartialEq, Eq)]
pub struct Device {
    audio_device_id: AudioDeviceID,
}

impl Device {
    fn name(&self) -> Result<String, DeviceNameError> {
        let property_address = AudioObjectPropertyAddress {
            mSelector: kAudioDevicePropertyDeviceNameCFString,
            mScope: kAudioDevicePropertyScopeOutput,
            mElement: kAudioObjectPropertyElementMaster,
        };
        let device_name: CFStringRef = null();
        let data_size = mem::size_of::<CFStringRef>();
        let c_str = unsafe {
            let status = AudioObjectGetPropertyData(
                self.audio_device_id,
                &property_address as *const _,
                0,
                null(),
                &data_size as *const _ as *mut _,
                &device_name as *const _ as *mut _,
            );
            check_os_status(status)?;

            let c_string: *const c_char = CFStringGetCStringPtr(device_name, kCFStringEncodingUTF8);
            if c_string == null() {
                let description = "core foundation unexpectedly returned null string".to_string();
                let err = BackendSpecificError { description };
                return Err(err.into());
            }
            CStr::from_ptr(c_string as *mut _)
        };
        Ok(c_str.to_string_lossy().into_owned())
    }

    // Logic re-used between `supported_input_formats` and `supported_output_formats`.
    fn supported_formats(
        &self,
        scope: AudioObjectPropertyScope,
    ) -> Result<SupportedOutputFormats, SupportedFormatsError> {
        let mut property_address = AudioObjectPropertyAddress {
            mSelector: kAudioDevicePropertyStreamConfiguration,
            mScope: scope,
            mElement: kAudioObjectPropertyElementMaster,
        };

        unsafe {
            // Retrieve the devices audio buffer list.
            let data_size = 0u32;
            let status = AudioObjectGetPropertyDataSize(
                self.audio_device_id,
                &property_address as *const _,
                0,
                null(),
                &data_size as *const _ as *mut _,
            );
            check_os_status(status)?;

            let mut audio_buffer_list: Vec<u8> = vec![];
            audio_buffer_list.reserve_exact(data_size as usize);
            let status = AudioObjectGetPropertyData(
                self.audio_device_id,
                &property_address as *const _,
                0,
                null(),
                &data_size as *const _ as *mut _,
                audio_buffer_list.as_mut_ptr() as *mut _,
            );
            check_os_status(status)?;

            let audio_buffer_list = audio_buffer_list.as_mut_ptr() as *mut AudioBufferList;

            // If there's no buffers, skip.
            if (*audio_buffer_list).mNumberBuffers == 0 {
                return Ok(vec![].into_iter());
            }

            // Count the number of channels as the sum of all channels in all output buffers.
            let n_buffers = (*audio_buffer_list).mNumberBuffers as usize;
            let first: *const AudioBuffer = (*audio_buffer_list).mBuffers.as_ptr();
            let buffers: &'static [AudioBuffer] = slice::from_raw_parts(first, n_buffers);
            let mut n_channels = 0;
            for buffer in buffers {
                n_channels += buffer.mNumberChannels as usize;
            }

            // AFAIK the sample format should always be f32 on macos and i16 on iOS? Feel free to
            // fix this if more pcm formats are supported.
            let sample_format = if cfg!(target_os = "ios") {
                SampleFormat::I16
            } else {
                SampleFormat::F32
            };

            // Get available sample rate ranges.
            property_address.mSelector = kAudioDevicePropertyAvailableNominalSampleRates;
            let data_size = 0u32;
            let status = AudioObjectGetPropertyDataSize(
                self.audio_device_id,
                &property_address as *const _,
                0,
                null(),
                &data_size as *const _ as *mut _,
            );
            check_os_status(status)?;

            let n_ranges = data_size as usize / mem::size_of::<AudioValueRange>();
            let mut ranges: Vec<u8> = vec![];
            ranges.reserve_exact(data_size as usize);
            let status = AudioObjectGetPropertyData(
                self.audio_device_id,
                &property_address as *const _,
                0,
                null(),
                &data_size as *const _ as *mut _,
                ranges.as_mut_ptr() as *mut _,
            );
            check_os_status(status)?;

            let ranges: *mut AudioValueRange = ranges.as_mut_ptr() as *mut _;
            let ranges: &'static [AudioValueRange] = slice::from_raw_parts(ranges, n_ranges);

            // Collect the supported formats for the device.
            let mut fmts = vec![];
            for range in ranges {
                let fmt = SupportedFormat {
                    channels: n_channels as ChannelCount,
                    min_sample_rate: SampleRate(range.mMinimum as _),
                    max_sample_rate: SampleRate(range.mMaximum as _),
                    data_type: sample_format,
                };
                fmts.push(fmt);
            }

            Ok(fmts.into_iter())
        }
    }

    fn supported_input_formats(&self) -> Result<SupportedOutputFormats, SupportedFormatsError> {
        self.supported_formats(kAudioObjectPropertyScopeInput)
    }

    fn supported_output_formats(&self) -> Result<SupportedOutputFormats, SupportedFormatsError> {
        self.supported_formats(kAudioObjectPropertyScopeOutput)
    }

    fn default_format(
        &self,
        scope: AudioObjectPropertyScope,
    ) -> Result<Format, DefaultFormatError> {
        fn default_format_error_from_os_status(status: OSStatus) -> Result<(), DefaultFormatError> {
            let err = match coreaudio::Error::from_os_status(status) {
                Err(err) => err,
                Ok(_) => return Ok(()),
            };
            match err {
                coreaudio::Error::AudioUnit(
                    coreaudio::error::AudioUnitError::FormatNotSupported,
                )
                | coreaudio::Error::AudioCodec(_)
                | coreaudio::Error::AudioFormat(_) => {
                    Err(DefaultFormatError::StreamTypeNotSupported)
                }
                coreaudio::Error::AudioUnit(coreaudio::error::AudioUnitError::NoConnection) => {
                    Err(DefaultFormatError::DeviceNotAvailable)
                }
                err => {
                    let description = format!("{}", std::error::Error::description(&err));
                    let err = BackendSpecificError { description };
                    Err(err.into())
                }
            }
        }

        let property_address = AudioObjectPropertyAddress {
            mSelector: kAudioDevicePropertyStreamFormat,
            mScope: scope,
            mElement: kAudioObjectPropertyElementMaster,
        };

        unsafe {
            let asbd: AudioStreamBasicDescription = mem::zeroed();
            let data_size = mem::size_of::<AudioStreamBasicDescription>() as u32;
            let status = AudioObjectGetPropertyData(
                self.audio_device_id,
                &property_address as *const _,
                0,
                null(),
                &data_size as *const _ as *mut _,
                &asbd as *const _ as *mut _,
            );
            default_format_error_from_os_status(status)?;

            let sample_format = {
                let audio_format = coreaudio::audio_unit::AudioFormat::from_format_and_flag(
                    asbd.mFormatID,
                    Some(asbd.mFormatFlags),
                );
                let flags = match audio_format {
                    Some(coreaudio::audio_unit::AudioFormat::LinearPCM(flags)) => flags,
                    _ => return Err(DefaultFormatError::StreamTypeNotSupported),
                };
                let maybe_sample_format =
                    coreaudio::audio_unit::SampleFormat::from_flags_and_bytes_per_frame(
                        flags,
                        asbd.mBytesPerFrame,
                    );
                match maybe_sample_format {
                    Some(coreaudio::audio_unit::SampleFormat::F32) => SampleFormat::F32,
                    Some(coreaudio::audio_unit::SampleFormat::I16) => SampleFormat::I16,
                    _ => return Err(DefaultFormatError::StreamTypeNotSupported),
                }
            };

            let format = Format {
                sample_rate: SampleRate(asbd.mSampleRate as _),
                channels: asbd.mChannelsPerFrame as _,
                data_type: sample_format,
            };
            Ok(format)
        }
    }

    fn default_input_format(&self) -> Result<Format, DefaultFormatError> {
        self.default_format(kAudioObjectPropertyScopeInput)
    }

    fn default_output_format(&self) -> Result<Format, DefaultFormatError> {
        self.default_format(kAudioObjectPropertyScopeOutput)
    }
}

impl fmt::Debug for Device {
    fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
        f.debug_struct("Device")
            .field("audio_device_id", &self.audio_device_id)
            .field("name", &self.name())
            .finish()
    }
}

struct StreamInner {
    playing: bool,
    audio_unit: AudioUnit,
    // Track the device with which the audio unit was spawned.
    //
    // We must do this so that we can avoid changing the device sample rate if there is already
    // a stream associated with the device.
    device_id: AudioDeviceID,
}

// TODO need stronger error identification
impl From<coreaudio::Error> for BuildStreamError {
    fn from(err: coreaudio::Error) -> BuildStreamError {
        match err {
            coreaudio::Error::RenderCallbackBufferFormatDoesNotMatchAudioUnitStreamFormat
            | coreaudio::Error::NoKnownSubtype
            | coreaudio::Error::AudioUnit(coreaudio::error::AudioUnitError::FormatNotSupported)
            | coreaudio::Error::AudioCodec(_)
            | coreaudio::Error::AudioFormat(_) => BuildStreamError::FormatNotSupported,
            _ => BuildStreamError::DeviceNotAvailable,
        }
    }
}

// Create a coreaudio AudioStreamBasicDescription from a CPAL Format.
fn asbd_from_format(format: &Format) -> AudioStreamBasicDescription {
    let n_channels = format.channels as usize;
    let sample_rate = format.sample_rate.0;
    let bytes_per_channel = format.data_type.sample_size();
    let bits_per_channel = bytes_per_channel * 8;
    let bytes_per_frame = n_channels * bytes_per_channel;
    let frames_per_packet = 1;
    let bytes_per_packet = frames_per_packet * bytes_per_frame;
    let sample_format = format.data_type;
    let format_flags = match sample_format {
        SampleFormat::F32 => (kAudioFormatFlagIsFloat | kAudioFormatFlagIsPacked) as u32,
        _ => kAudioFormatFlagIsPacked as u32,
    };
    let asbd = AudioStreamBasicDescription {
        mBitsPerChannel: bits_per_channel as _,
        mBytesPerFrame: bytes_per_frame as _,
        mChannelsPerFrame: n_channels as _,
        mBytesPerPacket: bytes_per_packet as _,
        mFramesPerPacket: frames_per_packet as _,
        mFormatFlags: format_flags,
        mFormatID: kAudioFormatLinearPCM,
        mSampleRate: sample_rate as _,
        ..Default::default()
    };
    asbd
}

fn audio_unit_from_device(device: &Device, input: bool) -> Result<AudioUnit, coreaudio::Error> {
    let mut audio_unit = {
        let au_type = if cfg!(target_os = "ios") {
            // The HalOutput unit isn't available in iOS unfortunately.
            // RemoteIO is a sensible replacement.
            // See https://goo.gl/CWwRTx
            coreaudio::audio_unit::IOType::RemoteIO
        } else {
            coreaudio::audio_unit::IOType::HalOutput
        };
        AudioUnit::new(au_type)?
    };

    if input {
        // Enable input processing.
        let enable_input = 1u32;
        audio_unit.set_property(
            kAudioOutputUnitProperty_EnableIO,
            Scope::Input,
            Element::Input,
            Some(&enable_input),
        )?;

        // Disable output processing.
        let disable_output = 0u32;
        audio_unit.set_property(
            kAudioOutputUnitProperty_EnableIO,
            Scope::Output,
            Element::Output,
            Some(&disable_output),
        )?;
    }

    audio_unit.set_property(
        kAudioOutputUnitProperty_CurrentDevice,
        Scope::Global,
        Element::Output,
        Some(&device.audio_device_id),
    )?;

    Ok(audio_unit)
}

impl Device {
    fn build_input_stream_raw<D, E>(
        &self,
        format: &Format,
        mut data_callback: D,
        _error_callback: E,
    ) -> Result<Stream, BuildStreamError>
    where
        D: FnMut(&Data) + Send + 'static,
        E: FnMut(StreamError) + Send + 'static,
    {
        // The scope and element for working with a device's input stream.
        let scope = Scope::Output;
        let element = Element::Input;

        // Check whether or not we need to change the device sample rate to suit the one specified for the stream.
        unsafe {
            // Get the current sample rate.
            let mut property_address = AudioObjectPropertyAddress {
                mSelector: kAudioDevicePropertyNominalSampleRate,
                mScope: kAudioObjectPropertyScopeGlobal,
                mElement: kAudioObjectPropertyElementMaster,
            };
            let sample_rate: f64 = 0.0;
            let data_size = mem::size_of::<f64>() as u32;
            let status = AudioObjectGetPropertyData(
                self.audio_device_id,
                &property_address as *const _,
                0,
                null(),
                &data_size as *const _ as *mut _,
                &sample_rate as *const _ as *mut _,
            );
            coreaudio::Error::from_os_status(status)?;

            // If the requested sample rate is different to the device sample rate, update the device.
            if sample_rate as u32 != format.sample_rate.0 {
                // Get available sample rate ranges.
                property_address.mSelector = kAudioDevicePropertyAvailableNominalSampleRates;
                let data_size = 0u32;
                let status = AudioObjectGetPropertyDataSize(
                    self.audio_device_id,
                    &property_address as *const _,
                    0,
                    null(),
                    &data_size as *const _ as *mut _,
                );
                coreaudio::Error::from_os_status(status)?;
                let n_ranges = data_size as usize / mem::size_of::<AudioValueRange>();
                let mut ranges: Vec<u8> = vec![];
                ranges.reserve_exact(data_size as usize);
                let status = AudioObjectGetPropertyData(
                    self.audio_device_id,
                    &property_address as *const _,
                    0,
                    null(),
                    &data_size as *const _ as *mut _,
                    ranges.as_mut_ptr() as *mut _,
                );
                coreaudio::Error::from_os_status(status)?;
                let ranges: *mut AudioValueRange = ranges.as_mut_ptr() as *mut _;
                let ranges: &'static [AudioValueRange] = slice::from_raw_parts(ranges, n_ranges);

                // Now that we have the available ranges, pick the one matching the desired rate.
                let sample_rate = format.sample_rate.0;
                let maybe_index = ranges.iter().position(|r| {
                    r.mMinimum as u32 == sample_rate && r.mMaximum as u32 == sample_rate
                });
                let range_index = match maybe_index {
                    None => return Err(BuildStreamError::FormatNotSupported),
                    Some(i) => i,
                };

                // Update the property selector to specify the nominal sample rate.
                property_address.mSelector = kAudioDevicePropertyNominalSampleRate;

                // Setting the sample rate of a device is an asynchronous process in coreaudio.
                //
                // Thus we are required to set a `listener` so that we may be notified when the
                // change occurs.
                unsafe extern "C" fn rate_listener(
                    device_id: AudioObjectID,
                    _n_addresses: u32,
                    _properties: *const AudioObjectPropertyAddress,
                    rate_ptr: *mut ::std::os::raw::c_void,
                ) -> OSStatus {
                    let rate_ptr: *const f64 = rate_ptr as *const _;
                    let data_size = mem::size_of::<f64>();
                    let property_address = AudioObjectPropertyAddress {
                        mSelector: kAudioDevicePropertyNominalSampleRate,
                        mScope: kAudioObjectPropertyScopeGlobal,
                        mElement: kAudioObjectPropertyElementMaster,
                    };
                    AudioObjectGetPropertyData(
                        device_id,
                        &property_address as *const _,
                        0,
                        null(),
                        &data_size as *const _ as *mut _,
                        rate_ptr as *const _ as *mut _,
                    )
                }

                // Add our sample rate change listener callback.
                let reported_rate: f64 = 0.0;
                let status = AudioObjectAddPropertyListener(
                    self.audio_device_id,
                    &property_address as *const _,
                    Some(rate_listener),
                    &reported_rate as *const _ as *mut _,
                );
                coreaudio::Error::from_os_status(status)?;

                // Finally, set the sample rate.
                let sample_rate = sample_rate as f64;
                let status = AudioObjectSetPropertyData(
                    self.audio_device_id,
                    &property_address as *const _,
                    0,
                    null(),
                    data_size,
                    &ranges[range_index] as *const _ as *const _,
                );
                coreaudio::Error::from_os_status(status)?;

                // Wait for the reported_rate to change.
                //
                // This should not take longer than a few ms, but we timeout after 1 sec just in case.
                let timer = ::std::time::Instant::now();
                while sample_rate != reported_rate {
                    if timer.elapsed() > Duration::from_secs(1) {
                        let description =
                            "timeout waiting for sample rate update for device".into();
                        let err = BackendSpecificError { description };
                        return Err(err.into());
                    }
                    thread::sleep(Duration::from_millis(5));
                }

                // Remove the `rate_listener` callback.
                let status = AudioObjectRemovePropertyListener(
                    self.audio_device_id,
                    &property_address as *const _,
                    Some(rate_listener),
                    &reported_rate as *const _ as *mut _,
                );
                coreaudio::Error::from_os_status(status)?;
            }
        }

        let mut audio_unit = audio_unit_from_device(self, true)?;

        // Set the stream in interleaved mode.
        let asbd = asbd_from_format(format);
        audio_unit.set_property(kAudioUnitProperty_StreamFormat, scope, element, Some(&asbd))?;

        // Register the callback that is being called by coreaudio whenever it needs data to be
        // fed to the audio buffer.
        let sample_format = format.data_type;
        let bytes_per_channel = sample_format.sample_size();
        type Args = render_callback::Args<data::Raw>;
        audio_unit.set_input_callback(move |args: Args| unsafe {
            let ptr = (*args.data.data).mBuffers.as_ptr() as *const AudioBuffer;
            let len = (*args.data.data).mNumberBuffers as usize;
            let buffers: &[AudioBuffer] = slice::from_raw_parts(ptr, len);

            // TODO: Perhaps loop over all buffers instead?
            let AudioBuffer {
                mNumberChannels: _num_channels,
                mDataByteSize: data_byte_size,
                mData: data,
            } = buffers[0];

            let data = data as *mut ();
            let len = (data_byte_size as usize / bytes_per_channel) as usize;
            let data = Data::from_parts(data, len, sample_format);
            data_callback(&data);
            Ok(())
        })?;

        audio_unit.start()?;

        Ok(Stream::new(StreamInner {
            playing: true,
            audio_unit,
            device_id: self.audio_device_id,
        }))
    }

    fn build_output_stream_raw<D, E>(
        &self,
        format: &Format,
        mut data_callback: D,
        _error_callback: E,
    ) -> Result<Stream, BuildStreamError>
    where
        D: FnMut(&mut Data) + Send + 'static,
        E: FnMut(StreamError) + Send + 'static,
    {
        let mut audio_unit = audio_unit_from_device(self, false)?;

        // The scope and element for working with a device's output stream.
        let scope = Scope::Input;
        let element = Element::Output;

        // Set the stream in interleaved mode.
        let asbd = asbd_from_format(format);
        audio_unit.set_property(kAudioUnitProperty_StreamFormat, scope, element, Some(&asbd))?;

        // Register the callback that is being called by coreaudio whenever it needs data to be
        // fed to the audio buffer.
        let sample_format = format.data_type;
        let bytes_per_channel = sample_format.sample_size();
        type Args = render_callback::Args<data::Raw>;
        audio_unit.set_render_callback(move |args: Args| unsafe {
            // If `run()` is currently running, then a callback will be available from this list.
            // Otherwise, we just fill the buffer with zeroes and return.

            let AudioBuffer {
                mNumberChannels: _num_channels,
                mDataByteSize: data_byte_size,
                mData: data,
            } = (*args.data.data).mBuffers[0];

            let data = data as *mut ();
            let len = (data_byte_size as usize / bytes_per_channel) as usize;
            let mut data = Data::from_parts(data, len, sample_format);
            data_callback(&mut data);
            Ok(())
        })?;

        audio_unit.start()?;

        Ok(Stream::new(StreamInner {
            playing: true,
            audio_unit,
            device_id: self.audio_device_id,
        }))
    }
}

pub struct Stream {
    inner: RefCell<StreamInner>,
}

impl Stream {
    fn new(inner: StreamInner) -> Self {
        Self {
            inner: RefCell::new(inner),
        }
    }
}

impl StreamTrait for Stream {
    fn play(&self) -> Result<(), PlayStreamError> {
        let mut stream = self.inner.borrow_mut();

        if !stream.playing {
            if let Err(e) = stream.audio_unit.start() {
                let description = format!("{}", std::error::Error::description(&e));
                let err = BackendSpecificError { description };
                return Err(err.into());
            }
            stream.playing = true;
        }
        Ok(())
    }

    fn pause(&self) -> Result<(), PauseStreamError> {
        let mut stream = self.inner.borrow_mut();

        if stream.playing {
            if let Err(e) = stream.audio_unit.stop() {
                let description = format!("{}", std::error::Error::description(&e));
                let err = BackendSpecificError { description };
                return Err(err.into());
            }

            stream.playing = false;
        }
        Ok(())
    }
}

fn check_os_status(os_status: OSStatus) -> Result<(), BackendSpecificError> {
    match coreaudio::Error::from_os_status(os_status) {
        Ok(()) => Ok(()),
        Err(err) => {
            let description = std::error::Error::description(&err).to_string();
            Err(BackendSpecificError { description })
        }
    }
}