code: private void setupStartEndListeners() {
          TransitionSetListener listener = new TransitionSetListener(this);
          for (Transition childTransition : mTransitions) {
              childTransition.addListener(listener);
          }
          mCurrentListeners = mTransitions.size();
      }
docs: Sets up listeners for each of the child transitions. This is used to
  determine when this transition set is finished (all child transitions
  must finish first).