question: public void addUpdateListener ( AnimatorUpdateListener listener ) { if ( mUpdateListeners == null ) { mUpdateListeners = new ArrayList < AnimatorUpdateListener > ( ) ; } mUpdateListeners . add ( listener ) ; }
target:  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.