Support bi-directional custom actions

[ryanheise]

Note to self: Implement bi-directional custom actions. This essentially means adding a mechanism to send arbitrary messages from the background task to the UI.

[YaarPatandarAA]

Any Updates on this?
Any workarounds until this is completed?

[ryanheise]

This is definitely high on my priority list. I plan to take a crack at it on the weekend.

There have always been workarounds using Dart's standard APIs to send arbitrary messages between isolates using send/receive ports (you may find mentions of the approach in past issues).

[lkho]

Hi everyone,

Is it possible to support partially bi-directional actions first (by changing the void interface functions to return Future and returning the Future through the method channel)? Since currently it is not even possible to query the background for a simple value from the UI, or wait for the action to complete (awaiting from the UI does not wait for the background async function).

[ryanheise]

Hi @lkho

You can't send futures through a method channel. I am wondering what API would be most appropriate for this. Currently all communication from the background task to the UI is via streams, so my current thinking is to make a custom event stream.

By the way, I have added an FAQ entry in the wiki for how to use the standard isolate APIs to send messages if you need something immediately.

[lkho]

I mean by changing this line to return the result (and if the result is a Future)

audio_service/lib/audio_service.dart

Lines 852 to 853 in 19712fa

	task.onCustomAction(
	call.method.substring(_CUSTOM_PREFIX.length), call.arguments);

i.e. return task.onCustomAction(

then we can call await AudioService.customAction('foo') from the UI and receive the result from the background.

Similarly for all other onXXX methods, currently the result is not returned, thus when the UI calls await AudioService.xXX(), the await does NOT actually wait for the completion of the method in the background task, but only waits for the MethodChannel to execute the callback on

audio_service/lib/audio_service.dart

Line 762 in 19712fa

_backgroundChannel.setMethodCallHandler((MethodCall call) async {

I know this is not a perfect bi-directional communication, but at least the background task can passively react to some action and return some info.

[ryanheise]

I see, that could be useful as well. Maybe would integrate both ideas into the API going forward.