Add test action to wait for an isLoading change.

This allows to wait until loading started or finished.

-------------
Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=215704424

testutils/src/main/java/com/google/android/exoplayer2/testutil/Action.java

@@ -687,6 +687,56 @@ public abstract class Action {
   }
 
   /**
+   * Waits for a specified loading state, returning either immediately or after a call to {@link
+   * Player.EventListener#onLoadingChanged(boolean)}.
+   */
+  public static final class WaitForIsLoading extends Action {
+
+    private final boolean targetIsLoading;
+
+    /**
+     * @param tag A tag to use for logging.
+     * @param targetIsLoading The loading state to wait for.
+     */
+    public WaitForIsLoading(String tag, boolean targetIsLoading) {
+      super(tag, "WaitForIsLoading");
+      this.targetIsLoading = targetIsLoading;
+    }
+
+    @Override
+    protected void doActionAndScheduleNextImpl(
+        final SimpleExoPlayer player,
+        final DefaultTrackSelector trackSelector,
+        final Surface surface,
+        final HandlerWrapper handler,
+        final ActionNode nextAction) {
+      if (nextAction == null) {
+        return;
+      }
+      if (targetIsLoading == player.isLoading()) {
+        nextAction.schedule(player, trackSelector, surface, handler);
+      } else {
+        player.addListener(
+            new Player.EventListener() {
+              @Override
+              public void onLoadingChanged(boolean isLoading) {
+                if (targetIsLoading == isLoading) {
+                  player.removeListener(this);
+                  nextAction.schedule(player, trackSelector, surface, handler);
+                }
+              }
+            });
+      }
+    }
+
+    @Override
+    protected void doActionImpl(
+        SimpleExoPlayer player, DefaultTrackSelector trackSelector, Surface surface) {
+      // Not triggered.
+    }
+  }
+
+  /**
    * Waits for {@link Player.EventListener#onSeekProcessed()}.
    */
   public static final class WaitForSeekProcessed extends Action {

testutils/src/main/java/com/google/android/exoplayer2/testutil/ActionSchedule.java

@@ -41,6 +41,7 @@ import com.google.android.exoplayer2.testutil.Action.SetShuffleModeEnabled;
 import com.google.android.exoplayer2.testutil.Action.SetVideoSurface;
 import com.google.android.exoplayer2.testutil.Action.Stop;
 import com.google.android.exoplayer2.testutil.Action.ThrowPlaybackException;
+import com.google.android.exoplayer2.testutil.Action.WaitForIsLoading;
 import com.google.android.exoplayer2.testutil.Action.WaitForPlaybackState;
 import com.google.android.exoplayer2.testutil.Action.WaitForPositionDiscontinuity;
 import com.google.android.exoplayer2.testutil.Action.WaitForSeekProcessed;
@@ -415,6 +416,16 @@ public final class ActionSchedule {
     }
 
     /**
+     * Schedules a delay until {@code player.isLoading()} changes to the specified value.
+     *
+     * @param targetIsLoading The target value of {@code player.isLoading()}.
+     * @return The builder, for convenience.
+     */
+    public Builder waitForIsLoading(boolean targetIsLoading) {
+      return apply(new WaitForIsLoading(tag, targetIsLoading));
+    }
+
+    /**
      * Schedules a {@link Runnable} to be executed.
      *
      * @return The builder, for convenience.