@Nullable
  public CloseableReference<CloseableImage> get(int frameIndex) {
    return mBackingCache.get(keyFor(frameIndex));
  }
Gets the image for the given frame index.

<p>It is the caller's responsibility to close the returned reference once not needed anymore.
animated-base/src/main/java/com/facebook/imagepipeline/animated/impl/AnimatedFrameCache.java