playback: switch audioreactor to callback system

Switch AudioReactor to a callback-based system. This makes updating the
volume much faster, as it's no longer reliant on a 500ms polling
interval.

app/src/main/java/org/oxycblt/auxio/playback/system/AudioReactor.kt

@@ -37,7 +37,10 @@ import kotlin.math.pow
  * Manages the current volume and playback state across ReplayGain and AudioFocus events.
  * @author OxygenCobalt
  */
-class AudioReactor(context: Context) : AudioManager.OnAudioFocusChangeListener, SettingsManager.Callback {
+class AudioReactor(
+    context: Context,
+    private val callback: (Float) -> Unit
+) : AudioManager.OnAudioFocusChangeListener, SettingsManager.Callback {
     private data class Gain(val track: Float, val album: Float)
 
     private val playbackManager = PlaybackStateManager.maybeGetInstance()
@@ -55,12 +58,19 @@ class AudioReactor(context: Context) : AudioManager.OnAudioFocusChangeListener,
         .setOnAudioFocusChangeListener(this)
         .build()
 
-    private var multiplier = 1f
     private var pauseWasTransient = false
+    private var multiplier = 1f
+        set(value) {
+            field = value
+            callback(volume)
+        }
 
-    var volume = 0f
+    private var volume = 0f
         get() = field * multiplier
-        private set
+        set(value) {
+            field = value
+            callback(volume)
+        }
 
     init {
         settingsManager.addCallback(this)

app/src/main/java/org/oxycblt/auxio/playback/system/PlaybackService.kt

@@ -126,9 +126,13 @@ class PlaybackService : Service(), Player.Listener, PlaybackStateManager.Callbac
             false
         )
 
+        audioReactor = AudioReactor(this) { volume ->
+            logD("Updating player volume to $volume")
+            player.volume = volume
+        }
+
         // --- SYSTEM SETUP ---
 
-        audioReactor = AudioReactor(this)
         widgets = WidgetController(this)
 
         // Set up the media button callbacks
@@ -402,19 +406,16 @@ class PlaybackService : Service(), Player.Listener, PlaybackStateManager.Callbac
      * Start polling the position on a coroutine.
      */
     private fun startPolling() {
-        data class Poll(val pos: Long, val multiplier: Float)
-
         val pollFlow = flow {
             while (true) {
-                emit(Poll(player.currentPosition, audioReactor.volume))
+                emit(player.currentPosition)
                 delay(POS_POLL_INTERVAL)
             }
         }.conflate()
 
         serviceScope.launch {
-            pollFlow.takeWhile { player.isPlaying }.collect { poll ->
-                playbackManager.setPosition(poll.pos)
-                player.volume = audioReactor.volume
+            pollFlow.takeWhile { player.isPlaying }.collect { pos ->
+                playbackManager.setPosition(pos)
             }
         }
     }