Fix threading of onFallbackApplied callback

The callback is currently triggered on the ExoPlayer playback thread
instead of the app thread that added the listener.

PiperOrigin-RevId: 492474405
(cherry picked from commit f3fc4fb9)

library/transformer/src/main/java/com/google/android/exoplayer2/transformer/FallbackListener.java

@@ -20,6 +20,7 @@ import static com.google.android.exoplayer2.util.Assertions.checkState;
 
 import com.google.android.exoplayer2.C;
 import com.google.android.exoplayer2.MediaItem;
+import com.google.android.exoplayer2.util.HandlerWrapper;
 import com.google.android.exoplayer2.util.ListenerSet;
 import com.google.android.exoplayer2.util.Util;
 
@@ -32,6 +33,7 @@ import com.google.android.exoplayer2.util.Util;
   private final MediaItem mediaItem;
   private final TransformationRequest originalTransformationRequest;
   private final ListenerSet<Transformer.Listener> transformerListeners;
+  private final HandlerWrapper transformerListenerHandler;
 
   private TransformationRequest fallbackTransformationRequest;
   private int trackCount;
@@ -40,16 +42,20 @@ import com.google.android.exoplayer2.util.Util;
    * Creates a new instance.
    *
    * @param mediaItem The {@link MediaItem} to transform.
-   * @param transformerListeners The {@linkplain Transformer.Listener listeners} to forward events
-   *     to.
+   * @param transformerListeners The {@linkplain Transformer.Listener listeners} to call {@link
+   *     Transformer.Listener#onFallbackApplied} on.
+   * @param transformerListenerHandler The {@link HandlerWrapper} to call {@link
+   *     Transformer.Listener#onFallbackApplied} events on.
    * @param originalTransformationRequest The original {@link TransformationRequest}.
    */
   public FallbackListener(
       MediaItem mediaItem,
       ListenerSet<Transformer.Listener> transformerListeners,
+      HandlerWrapper transformerListenerHandler,
       TransformationRequest originalTransformationRequest) {
     this.mediaItem = mediaItem;
     this.transformerListeners = transformerListeners;
+    this.transformerListenerHandler = transformerListenerHandler;
     this.originalTransformationRequest = originalTransformationRequest;
     this.fallbackTransformationRequest = originalTransformationRequest;
   }
@@ -104,15 +110,19 @@ import com.google.android.exoplayer2.util.Util;
       fallbackRequestBuilder.setEnableRequestSdrToneMapping(
           transformationRequest.enableRequestSdrToneMapping);
     }
-    fallbackTransformationRequest = fallbackRequestBuilder.build();
+    TransformationRequest newFallbackTransformationRequest = fallbackRequestBuilder.build();
+    fallbackTransformationRequest = newFallbackTransformationRequest;
 
     if (trackCount == 0 && !originalTransformationRequest.equals(fallbackTransformationRequest)) {
-      transformerListeners.queueEvent(
-          /* eventFlag= */ C.INDEX_UNSET,
-          listener ->
-              listener.onFallbackApplied(
-                  mediaItem, originalTransformationRequest, fallbackTransformationRequest));
-      transformerListeners.flushEvents();
+      transformerListenerHandler.post(
+          () ->
+              transformerListeners.sendEvent(
+                  /* eventFlag= */ C.INDEX_UNSET,
+                  listener ->
+                      listener.onFallbackApplied(
+                          mediaItem,
+                          originalTransformationRequest,
+                          newFallbackTransformationRequest)));
     }
   }
 }

library/transformer/src/main/java/com/google/android/exoplayer2/transformer/Transformer.java

@@ -727,7 +727,11 @@ public final class Transformer {
             /* asyncErrorListener= */ componentListener);
     this.muxerWrapper = muxerWrapper;
     FallbackListener fallbackListener =
-        new FallbackListener(mediaItem, listeners, transformationRequest);
+        new FallbackListener(
+            mediaItem,
+            listeners,
+            clock.createHandler(looper, /* callback= */ null),
+            transformationRequest);
     exoPlayerAssetLoader.start(
         mediaItem,
         muxerWrapper,

library/transformer/src/test/java/com/google/android/exoplayer2/transformer/FallbackListenerTest.java

@@ -26,10 +26,12 @@ import android.os.Looper;
 import androidx.test.ext.junit.runners.AndroidJUnit4;
 import com.google.android.exoplayer2.MediaItem;
 import com.google.android.exoplayer2.util.Clock;
+import com.google.android.exoplayer2.util.HandlerWrapper;
 import com.google.android.exoplayer2.util.ListenerSet;
 import com.google.android.exoplayer2.util.MimeTypes;
 import org.junit.Test;
 import org.junit.runner.RunWith;
+import org.robolectric.shadows.ShadowLooper;
 
 /** Unit tests for {@link FallbackListener}. */
 @RunWith(AndroidJUnit4.class)
@@ -41,7 +43,8 @@ public class FallbackListenerTest {
   public void onTransformationRequestFinalized_withoutTrackRegistration_throwsException() {
     TransformationRequest transformationRequest = new TransformationRequest.Builder().build();
     FallbackListener fallbackListener =
-        new FallbackListener(PLACEHOLDER_MEDIA_ITEM, createListenerSet(), transformationRequest);
+        new FallbackListener(
+            PLACEHOLDER_MEDIA_ITEM, createListenerSet(), createHandler(), transformationRequest);
 
     assertThrows(
         IllegalStateException.class,
@@ -52,10 +55,12 @@ public class FallbackListenerTest {
   public void onTransformationRequestFinalized_afterTrackRegistration_completesSuccessfully() {
     TransformationRequest transformationRequest = new TransformationRequest.Builder().build();
     FallbackListener fallbackListener =
-        new FallbackListener(PLACEHOLDER_MEDIA_ITEM, createListenerSet(), transformationRequest);
+        new FallbackListener(
+            PLACEHOLDER_MEDIA_ITEM, createListenerSet(), createHandler(), transformationRequest);
 
     fallbackListener.registerTrack();
     fallbackListener.onTransformationRequestFinalized(transformationRequest);
+    ShadowLooper.idleMainLooper();
   }
 
   @Test
@@ -66,10 +71,14 @@ public class FallbackListenerTest {
     Transformer.Listener mockListener = mock(Transformer.Listener.class);
     FallbackListener fallbackListener =
         new FallbackListener(
-            PLACEHOLDER_MEDIA_ITEM, createListenerSet(mockListener), originalRequest);
+            PLACEHOLDER_MEDIA_ITEM,
+            createListenerSet(mockListener),
+            createHandler(),
+            originalRequest);
 
     fallbackListener.registerTrack();
     fallbackListener.onTransformationRequestFinalized(unchangedRequest);
+    ShadowLooper.idleMainLooper();
 
     verify(mockListener, never()).onFallbackApplied(any(), any(), any());
   }
@@ -83,10 +92,14 @@ public class FallbackListenerTest {
     Transformer.Listener mockListener = mock(Transformer.Listener.class);
     FallbackListener fallbackListener =
         new FallbackListener(
-            PLACEHOLDER_MEDIA_ITEM, createListenerSet(mockListener), originalRequest);
+            PLACEHOLDER_MEDIA_ITEM,
+            createListenerSet(mockListener),
+            createHandler(),
+            originalRequest);
 
     fallbackListener.registerTrack();
     fallbackListener.onTransformationRequestFinalized(audioFallbackRequest);
+    ShadowLooper.idleMainLooper();
 
     verify(mockListener)
         .onFallbackApplied(PLACEHOLDER_MEDIA_ITEM, originalRequest, audioFallbackRequest);
@@ -109,12 +122,16 @@ public class FallbackListenerTest {
     Transformer.Listener mockListener = mock(Transformer.Listener.class);
     FallbackListener fallbackListener =
         new FallbackListener(
-            PLACEHOLDER_MEDIA_ITEM, createListenerSet(mockListener), originalRequest);
+            PLACEHOLDER_MEDIA_ITEM,
+            createListenerSet(mockListener),
+            createHandler(),
+            originalRequest);
 
     fallbackListener.registerTrack();
     fallbackListener.registerTrack();
     fallbackListener.onTransformationRequestFinalized(audioFallbackRequest);
     fallbackListener.onTransformationRequestFinalized(videoFallbackRequest);
+    ShadowLooper.idleMainLooper();
 
     verify(mockListener)
         .onFallbackApplied(PLACEHOLDER_MEDIA_ITEM, originalRequest, mergedFallbackRequest);
@@ -130,4 +147,8 @@ public class FallbackListenerTest {
   private static ListenerSet<Transformer.Listener> createListenerSet() {
     return new ListenerSet<>(Looper.myLooper(), Clock.DEFAULT, (listener, flags) -> {});
   }
+
+  private static HandlerWrapper createHandler() {
+    return Clock.DEFAULT.createHandler(Looper.myLooper(), /* callback= */ null);
+  }
 }

library/transformer/src/test/java/com/google/android/exoplayer2/transformer/VideoEncoderWrapperTest.java

@@ -45,6 +45,7 @@ public final class VideoEncoderWrapperTest {
       new FallbackListener(
           MediaItem.fromUri(Uri.EMPTY),
           new ListenerSet<>(Looper.myLooper(), Clock.DEFAULT, (listener, flags) -> {}),
+          Clock.DEFAULT.createHandler(Looper.myLooper(), /* callback= */ null),
           emptyTransformationRequest);
   private final VideoTranscodingSamplePipeline.EncoderWrapper encoderWrapper =
       new VideoTranscodingSamplePipeline.EncoderWrapper(