docs: protected void start() {
          if (mNumInstances == 0) {
              if (mListeners != null && mListeners.size() > 0) {
                  ArrayList<TransitionListener> tmpListeners =
                          (ArrayList<TransitionListener>) mListeners.clone();
                  int numListeners = tmpListeners.size();
                  for (int i = 0; i < numListeners; ++i) {
                      tmpListeners.get(i).onTransitionStart(this);
                  }
              }
              mEnded = false;
          }
          mNumInstances++;
      }