code: public void addUpdateListener(AnimatorUpdateListener listener) {
          if (mUpdateListeners == null) {
              mUpdateListeners = new ArrayList<AnimatorUpdateListener>();
          }
          mUpdateListeners.add(listener);
      }
docs: Adds a listener to the set of listeners that are sent update events through the life of
  an animation. This method is called on all listeners for every frame of the animation,
  after the values for the animation have been calculated.
  
  @param listener the listener to be added to the current set of listeners for this animation.