Add Transformer internal thread

This thread just starts the player and handles the player callbacks for
now. Sample pipelines are still run on the playback thread.

PiperOrigin-RevId: 491299671

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

@@ -63,6 +63,7 @@ import com.google.android.exoplayer2.video.VideoRendererEventListener;
     void onError(Exception e);
   }
 
+  private final MediaItem mediaItem;
   private final ExoPlayer player;
 
   public ExoPlayerAssetLoader(
@@ -71,8 +72,10 @@ import com.google.android.exoplayer2.video.VideoRendererEventListener;
       boolean removeAudio,
       boolean removeVideo,
       MediaSource.Factory mediaSourceFactory,
+      Looper looper,
       Listener listener,
       Clock clock) {
+    this.mediaItem = mediaItem;
     DefaultTrackSelector trackSelector = new DefaultTrackSelector(context);
     trackSelector.setParameters(
         new DefaultTrackSelector.Parameters.Builder(context)
@@ -92,7 +95,8 @@ import com.google.android.exoplayer2.video.VideoRendererEventListener;
         new ExoPlayer.Builder(context, new RenderersFactoryImpl(removeAudio, removeVideo, listener))
             .setMediaSourceFactory(mediaSourceFactory)
             .setTrackSelector(trackSelector)
-            .setLoadControl(loadControl);
+            .setLoadControl(loadControl)
+            .setLooper(looper);
     if (clock != Clock.DEFAULT) {
       // Transformer.Builder#setClock is also @VisibleForTesting, so if we're using a non-default
       // clock we must be in a test context.
@@ -101,11 +105,11 @@ import com.google.android.exoplayer2.video.VideoRendererEventListener;
     }
 
     player = playerBuilder.build();
-    player.setMediaItem(mediaItem);
     player.addListener(new PlayerListener(listener));
   }
 
   public void start() {
+    player.setMediaItem(mediaItem);
     player.prepare();
   }
 
@@ -113,10 +117,6 @@ import com.google.android.exoplayer2.video.VideoRendererEventListener;
     player.release();
   }
 
-  public Looper getPlaybackLooper() {
-    return player.getPlaybackLooper();
-  }
-
   private static final class RenderersFactoryImpl implements RenderersFactory {
 
     private final TransformerMediaClock mediaClock;

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

@@ -16,12 +16,10 @@
 
 package com.google.android.exoplayer2.transformer;
 
-import static com.google.android.exoplayer2.transformer.TransformerInternal.END_TRANSFORMATION_REASON_CANCELLED;
 import static com.google.android.exoplayer2.util.Assertions.checkState;
 import static java.lang.annotation.ElementType.TYPE_USE;
 
 import android.content.Context;
-import android.os.Handler;
 import android.os.Looper;
 import android.os.ParcelFileDescriptor;
 import androidx.annotation.IntDef;
@@ -43,6 +41,7 @@ import com.google.android.exoplayer2.util.Clock;
 import com.google.android.exoplayer2.util.DebugViewProvider;
 import com.google.android.exoplayer2.util.Effect;
 import com.google.android.exoplayer2.util.FrameProcessor;
+import com.google.android.exoplayer2.util.HandlerWrapper;
 import com.google.android.exoplayer2.util.ListenerSet;
 import com.google.android.exoplayer2.util.MimeTypes;
 import com.google.android.exoplayer2.util.Util;
@@ -732,6 +731,7 @@ public final class Transformer {
             encoderFactory,
             frameProcessorFactory,
             muxerFactory,
+            looper,
             transformerInternalListener,
             fallbackListener,
             debugViewProvider,
@@ -778,11 +778,10 @@ public final class Transformer {
       return;
     }
     try {
-      transformerInternal.release(END_TRANSFORMATION_REASON_CANCELLED);
-    } catch (TransformationException impossible) {
-      throw new IllegalStateException(impossible);
+      transformerInternal.cancel();
+    } finally {
+      transformerInternal = null;
     }
-    transformerInternal = null;
   }
 
   private void verifyApplicationThread() {
@@ -794,18 +793,17 @@ public final class Transformer {
   private final class TransformerInternalListener implements TransformerInternal.Listener {
 
     private final MediaItem mediaItem;
-    private final Handler handler;
+    private final HandlerWrapper handler;
 
     public TransformerInternalListener(MediaItem mediaItem) {
       this.mediaItem = mediaItem;
-      handler = Util.createHandlerForCurrentLooper();
+      handler = clock.createHandler(looper, /* callback= */ null);
     }
 
     @Override
     public void onTransformationCompleted(TransformationResult transformationResult) {
       // TODO(b/213341814): Add event flags for Transformer events.
-      Util.postOrRun(
-          handler,
+      handler.post(
           () -> {
             transformerInternal = null;
             listeners.queueEvent(
@@ -817,8 +815,7 @@ public final class Transformer {
 
     @Override
     public void onTransformationError(TransformationException exception) {
-      Util.postOrRun(
-          handler,
+      handler.post(
           () -> {
             transformerInternal = null;
             listeners.queueEvent(