public void fadeToLayer(int index) {
    mTransitionState = TRANSITION_STARTING;
    Arrays.fill(mIsLayerOn, false);
    mIsLayerOn[index] = true;
    invalidateSelf();
  }
Starts fading to the specified layer.

@param index the index of the layer to fade to
drawee/src/main/java/com/facebook/drawee/drawable/FadeDrawable.java