code: public void addListener(AnimatorListener listener) {
          if (mListeners == null) {
              mListeners = new ArrayList<AnimatorListener>();
          }
          mListeners.add(listener);
      }
docs: Adds a listener to the set of listeners that are sent events through the life of an
  animation, such as start, repeat, and end.
  
  @param listener the listener to be added to the current set of listeners for this animation.