Add EventListener.onPlaybackSuppressionReasonChanged

Adding this callback makes sense for completeness (we have similar callbacks
for all other playback state properties), and also to detect audio focus loss
while buffering which would currently trigger no callback because isPlaying
is still false.

Issue:#6203
PiperOrigin-RevId: 271347351

RELEASENOTES.md

@@ -66,6 +66,9 @@
   ([#6161](https://github.com/google/ExoPlayer/issues/6161)).
 * Add demo app to show how to use the Android 10 `SurfaceControl` API with
   ExoPlayer ([#677](https://github.com/google/ExoPlayer/issues/677)).
+* Add `Player.onPlaybackSuppressionReasonChanged` to allow listeners to
+  detect playbacks suppressions (e.g. audio focus loss) directly
+  ([#6203](https://github.com/google/ExoPlayer/issues/6203)).
 
 ### 2.10.5 (2019-09-20) ###
 

library/core/src/main/java/com/google/android/exoplayer2/ExoPlayerImpl.java

@@ -264,17 +264,21 @@ public void setPlayWhenReady(
       internalPlayer.setPlayWhenReady(internalPlayWhenReady);
     }
     boolean playWhenReadyChanged = this.playWhenReady != playWhenReady;
+    boolean suppressionReasonChanged = this.playbackSuppressionReason != playbackSuppressionReason;
     this.playWhenReady = playWhenReady;
     this.playbackSuppressionReason = playbackSuppressionReason;
     boolean isPlaying = isPlaying();
     boolean isPlayingChanged = oldIsPlaying != isPlaying;
-    if (playWhenReadyChanged || isPlayingChanged) {
+    if (playWhenReadyChanged || suppressionReasonChanged || isPlayingChanged) {
       int playbackState = playbackInfo.playbackState;
       notifyListeners(
           listener -> {
             if (playWhenReadyChanged) {
               listener.onPlayerStateChanged(playWhenReady, playbackState);
             }
+            if (suppressionReasonChanged) {
+              listener.onPlaybackSuppressionReasonChanged(playbackSuppressionReason);
+            }
             if (isPlayingChanged) {
               listener.onIsPlayingChanged(isPlaying);
             }

library/core/src/main/java/com/google/android/exoplayer2/Player.java

@@ -392,6 +392,14 @@ default void onLoadingChanged(boolean isLoading) {}
      */
     default void onPlayerStateChanged(boolean playWhenReady, @State int playbackState) {}
 
+    /**
+     * Called when the value returned from {@link #getPlaybackSuppressionReason()} changes.
+     *
+     * @param playbackSuppressionReason The current {@link PlaybackSuppressionReason}.
+     */
+    default void onPlaybackSuppressionReasonChanged(
+        @PlaybackSuppressionReason int playbackSuppressionReason) {}
+
     /**
      * Called when the value of {@link #isPlaying()} changes.
      *