private void startGesture() {
    if (!mGestureInProgress) {
      if (mListener != null) {
        mListener.onGestureBegin(this);
      }
      mGestureInProgress = true;
    }
  }
Starts a new gesture and calls the listener just before starting it.
samples/gestures/src/main/java/com/facebook/samples/gestures/MultiPointerGestureDetector.java