Device Dis-/Connection Notifications

[LastExceed]

Summary of a discussion on Discord

Description

@edwloef
API idea: some way to get notified for audio device connects/disconnects? E.g. Discord shows a popup when a new audio device becomes available

Prior Art

@roderickvd:
Right now, ErrorKind has DeviceChanged and DeviceNotAvailable (which includes disconnections). On hosts that support it, DeviceChanged is emitted when a default device is opened and is rerouted to another device.
@edwloef:
Yeah, but for that you need to have a stream open on the default device.

Host Support

Host	Support	Docs
AAudio	❔
ALSA	❔
ASIO	❌¹	N/A
AudioWorklet	❔
CoreAudio	✔️	link
JACK	✔️	link
PipeWire	✔️	link
PulseAudio	✔️	link
WASAPI	✔️	link
WebAudio	❔

¹Of course, this feature could be emulated for any host by periodically polling the device list

Design Proposals

@LastExceed
maybe a callback? or a message channel ?
@edwloef
I'd say a callback + a handle, similar to an audio stream.
@ mikedorf
It would be nice to have a futures::Stream at the low level but I understand that not everyone is using async and the APIs may force you to wire a callback anyway
@roderickvd
It would probably have to live on Host and get registered, like register_notification with a device ID indeed plus a callback, returning a notification handle that you could drop or unregister. And for the callback to handle a notification kind and payload.

[edwloef]

For ALSA you could presumably listen to udev events to get notified when devices are connected or disconnected.

[Marco-Farruggio]

For WASAPI, there's an IMMNotificationClient which i register to a IMMDeviceEnumerator, and set an atomic flag whenever it changes to repoll devices, (I believe a delta-change system could also be setup to avoid a full re-poll, but I only do a crude repoll on any change). porting this cross platform could be interesting as im pretty sure some backends dont have an alternative (such as ASIO). either a channel or a callback registered with cpal most likely for the notifications itself

[Marco-Farruggio]

note on AAudio support, it itself doesnt expose a device-added/removed notification callback, but you can register a audio device-added/removed through AudioManager.registerAudioDeviceCallback() with an AudioDeviceCallback which does indeed provide onAudioDevicesAddeed() and onAudioDeviceRemoved() (this is from API 23 (Android 6.0) onwards, which is well below cpals min of API 26 (Android 8.0)

[Marco-Farruggio]

Similar with Web Audio (and AudioWorklet), it doesnt expose device notifications on AudioContext itself, but their provided by the web platform via navigator.mediaDevices.ondevicechange / addEventListener("devicechange", ...)

so i think ever platform in some way or other this could be implemented, other than ASIO, but if repolling on asio (assuming its viable) meant we could make a full rich cross-platform api I think its worth doing

[Marco-Farruggio]

Host Support (Updated)

Host	Support	Docs
AAudio	✔️	Android AudioDeviceCallback
ALSA	✔️	libudev udev_monitor
ASIO	❌	N/A
AudioWorklet	✔️	MediaDevices devicechange event
CoreAudio	✅	AudioObjectPropertyListenerBlock
JACK	✅	JACK Client Callbacks
PipeWire	✅	PipeWire Registry
PulseAudio	✅	PulseAudio subscribe API
WASAPI	✅	WASAPI Device Events
WebAudio	✔️	MediaDevices devicechange event (same as AudioWorklet)

✅ => Host specific support
✔️ => Supported (via the browser/OS), but not directly
❌ => No support, (could be emulated via polling)

Note: I'm not very familiar with Linux's sound or device system atall, I've never done any audio work with it, libudev is from the comment above by @edwloef and a rough google search), this is my current research ive just done so please do correct me if ive misunderstood anything :)

[roderickvd]

We'd somehow have to find a way to deduplicate notifications firing on the "overall" notification handler, and ones specific to streams and their error callbacks.

[LastExceed]

Why? If you subscribe to multiple overlapping event feeds, then it is perfectly reasonable to expect multiple events in the overlapped area

[Marco-Farruggio]

I think if i was playing a stream and suddenly a device was removed, i would expect the global Notification::DeviceRemoved (or similar naming), and also for the callback itself to alert me, infact I think I'd prefer that to any deduplication (if this is what you meant)