Callback interface used by {@link Layout#layout(int, AnimationCallback)} to provide updates on animation progress.

abstract class LayoutAnimationCallback {

 /**
  * Called immediately after the animation is complete, and the entire layout
  * is in its final state.
  */
 void onAnimationComplete();

 /**
  * Called at each step of the animation, for each layer being laid out.
  *
  * @param layer the layer being laid out
  */
 void onLayout(Layer layer, double progress);
}