code: @Nullable
    public CloseableReference<CloseableImage> cache(
        int frameIndex,
        CloseableReference<CloseableImage> imageRef) {
      return mBackingCache.cache(keyFor(frameIndex), imageRef, mEntryStateObserver);
    }
docs: Caches the image for the given frame index.
  
  <p> Important: the client should use the returned reference instead of the original one.
  It is the caller's responsibility to close the returned reference once not needed anymore.
  
  @return the new reference to be used, null if the value cannot be cached