Update to audioplayers v4

Author: DoumanAsh

README.md

@@ -42,6 +42,7 @@ If you encounter issues running flutter-pi on any of the supported platforms lis
 2.2 [Building the App](#building-the-app-new-method-linux-only)  
 2.3 [Running your App with flutter-pi](#running-your-app-with-flutter-pi)  
 2.4 [gstreamer video player](#gstreamer-video-player)  
+2.5 [audioplayers](#audioplayers)
 3. **[Performance](#-performance)**  
 3.1 [Graphics Performance](#graphics-performance)  
 3.2 [Touchscreen latency](#touchscreen-latency)
@@ -414,6 +415,18 @@ To use the gstreamer video player, just rebuild flutter-pi (delete your build fo
 
 And then, just use the stuff in the official [video_player](https://pub.dev/packages/video_player) package. (`VideoPlayer`, `VideoPlayerController`, etc, there's nothing specific you need to do on the dart-side)
 
+### audioplayers
+As of current moment flutter-pi implements plugin for `audioplayers: ^4.0.0`.
+There are several things you need to keep in mind:
+- As flutter-pi is intended for use on constrained systems like raspberry pi, you should avoid creating multiple temporary instances and instead prefer to use one global instance of `AudioPlayer`. There is limit you can easily hit if you're going to spam multiple instances of `AudioPlayer`
+- Plugin was tested to work with ALSA and `pulseaudio` might prevent the plugin from playing audio correctly:
+    - Hence please make sure you delete `pulseaudio` package from your system.
+    - Make sure you have `gstreamer1.0-alsa` package installed in addition to packages needed for gstreamer video player.
+    - Make sure you can list audio devices using command: `aplay -L`
+        - If there is error, please investigate why and fix it before using audio
+        - One of the common reasons is outdated ALSA config in which case you should delete existing config and replace it with up to date one
+- Finally, if you want to verify your audio setup is good, you can use `gst-launch` command to invoke `playbin` on audio file directly.
+
 ## 📊 Performance
 ### Graphics Performance
 Graphics performance is actually pretty good. With most of the apps inside the `flutter SDK -> examples -> catalog` directory I get smooth 50-60fps on the Pi 4 2GB and Pi 3 A+.

src/plugins/audioplayers.h

@@ -30,10 +30,21 @@ void audio_player_set_volume(struct audio_player *self, double volume);
 
 void audio_player_set_playback_rate(struct audio_player *self, double rate);
 
+void audio_player_set_balance(struct audio_player *self, double balance);
+
 void audio_player_set_position(struct audio_player *self, int64_t position);
 
 void audio_player_set_source_url(struct audio_player *self, char *url);
 
 bool audio_player_is_id(struct audio_player *self, char *id);
 
+const char* audio_player_subscribe_channel_name(const struct audio_player *self);
+
+///Asks to subscribe to channel events
+///
+///`value` - Indicates whether to subscribe or unsubscribe
+///
+///Returns `true` if player uses `channel`, otherwise returns `false
+bool audio_player_set_subscription_status(struct audio_player *self, const char *channel, bool value);
+
 #endif  // AUDIOPLAYERS_H_

src/plugins/audioplayers/README.md

@@ -0,0 +1,17 @@
+## audioplayers plugin
+
+### Requirements
+
+- `audioplayers` version `^4.0.0`
+- Working gstreamer installation, including corresponding audio plugin (e.g. `gstreamer1.0-alsa`)
+
+### Troubleshooting
+
+- Check that you can list ALSA devices via command `aplay -L`;
+- Check that you can launch `playbin` on any audio file via `gst-launch`;
+- Make sure `pulseaudio` is deleted
+
+### pulseaudio
+
+Please note that plugin was not tested with `pulseaudio` and it is up to you to make gstreamer work via it.
+As `pulseaudio` takes full control over audio devices, `ALSA` will no longer function correctly with `pulseaudio` installed