[Example] Pass data to backgroundAudioPlayerTask

[akdasa]

Good getlocaltime()!

I'm struggling to pass any data to _backgroundAudioPlayerTask function (according to this example https://github.com/ryanheise/audio_service/blob/master/example/lib/main.dart). I want to pass URL to play.

var res = AudioService.start(
  backgroundTask: () async {
    // I modified CustomAudioPlayer also
    CustomAudioPlayer player = CustomAudioPlayer("HERE IS MY URL");
    AudioServiceBackground.run(
      onStart: player.run,
      onPlay: player.play,
      onPause: player.pause,
      onStop: player.stop,
      onClick: (MediaButton button) => player.playPause(),
    );
  },
  // omited
);

But it doesn't work. It returns instance of Future<bool> which resolves to false:

I/flutter (19613): Instance of 'Future<bool>'
I/flutter (19613): false

I also was trying to do something like this:

Function play(String url) {
  void handler() async {
    CustomAudioPlayer player = CustomAudioPlayer(url);
    AudioServiceBackground.run(
      onStart: player.run,
      onPlay: player.play,
      onPause: player.pause,
      onStop: player.stop,
      onClick: (MediaButton button) => player.playPause(),
    );
  }
  return handler;
}

Error:

E/flutter (20407): [ERROR:flutter/shell/common/shell.cc(184)] Dart Error: Unhandled exception:
E/flutter (20407): NoSuchMethodError: No top-level getter 'handler' declared.
E/flutter (20407): Receiver: top-level
E/flutter (20407): Tried calling: handler
E/flutter (20407): #0      NoSuchMethodError._throwNew (dart:core/runtime/liberrors_patch.dart:212:5)
E/flutter (20407): [ERROR:flutter/shell/common/engine.cc(178)] Could not run the isolate.
E/flutter (20407): [ERROR:flutter/shell/common/engine.cc(119)] Engine not prepare and launch isolate.
E/flutter (20407): [ERROR:flutter/shell/platform/android/android_shell_holder.cc(167)] Could not launch engine in configuration.

Can you help me please, what I'm doing wrong?

p.s: I'm new to dart and flutter, so maybe I'm doing something really stupid :)

[ryanheise]

The error suggests that you need to use top-level functions. Although it's complaining about handle, I think the problem actually begins with the backgroundTask parameter which the documentation of this plugin states must be a top-level function. See the included example app for how to define this as a top-level function.

So instead of:

backgroundTask: () { .... }

Try this:

backgroundTask: _myBackgroundTask

...

// This must be at the "top level"
// i.e. not nested within any other function or class
void _myBackgroundTask() {
}

[ryanheise]

To address the other part of your question, you can't directly pass any arguments to the background task function, so your background task needs to internally know what it wants to play. However, most apps typically store their state using some form of persistence (shared preferences[1] or a database[2]), so if the current URL is stored in a shared preference, for example, your background task will be able to load that shared preference and fetch the URL.

Future release may provide some alternative options, but this is the way to do it for now.

[1] https://pub.dartlang.org/packages/shared_preferences
[2] https://pub.dartlang.org/packages/sqflite

[ryanheise]

I've just published 0.0.10 with the ability to await AudioService.start which completes when the background task is ready to receive messages. This may give you another option to pass data to the background task on start. For example, on the client side:

bool success = await AudioService.start(...);
if (success) {
  AudioService.customAction('url', url);
}

And in the background task:

Completer urlCompleter = Completer();
AudioServiceBackground.run(
   ...
  onCustomAction: (String name, dynamic arguments) {
    switch (name) {
      case "url":
        String url = arguments;
        ...
        break;
    },
  }
);

[hacker1024]

I need to pass multiple objects into my audioTask. I tried using top-level variables, but they're null in the isolate.
To my understanding, I can only pass primitive types in AudioService.customAction.

How can I do this?

[ryanheise]

The main isolate in which your UI runs and the background isolate that is spawned by the audio service do not share any memory, so top-level variables will not be the same between isolates. You have to use one of the various message passing facilities and typically that involves encoding your data into primitives or lists and maps consisting of primitives. If you want to pass whole objects without encoding them, you may try send ports and receive ports via the IsolateNameServer API.

Using the customAction approach, you just need to send your data as a data structure consisting of only lists, maps and primitives. If you already have the data stored in persistence, then you can simplify the amount of data you need to communicate by just sending the keys and on the receiving end use those keys to lookup the full records in your persistence container.

In many cases, you won't need to pass anything since everything will be in persistence. For example, an MP3 player with a playlist and a current track will surely persist this data so that the playlist and current track is remembered on the next startup, and so when the background task starts within its own isolate, it can simply pull this data out of persistence and play it on its own.

[hacker1024]

If you want to pass whole objects without encoding them, you may try send ports and receive ports via the IsolateNameServer API.

This works beautifully, thanks.
It's work noting that this method won't work with dart2js, so my app won't run on Flutter for web when it's available.

[ryanheise]

Great! I'll close this issue.

[akdasa]

@ryanheise, thanks a lot.

[Mahfoud047]

@ryanheise please can you give as a complete example of using customAction, I tried the code bellow but the player stops playing audio abruptly after few seconds.

void _backgroundAudioPlayerTask() async {
  String url = '';

  CustomAudioPlayer player = CustomAudioPlayer(url); // changed the code inside CustomAudioPlayer
  AudioServiceBackground.run(
    onCustomAction: (String name, dynamic arguments) {
      switch (name) {
        case "url":
          url = arguments;
          break;
      }
    },
    onStart: player.run,
    onPlay: player.play,
    onPause: player.pause,
    onStop: player.stop,
    onClick: (MediaButton button) => player.playPause(),
  );
}

Otherwise how can I use methods like AudioService.addQueueItem() and AudioService.currentMediaItem to pass the MediaItem to the backgroundAudioPlayerTask and play audio from url which is the id of the MediaItem. And in this case I will not need the customAction.

Please give us a complete working example.