onLongTapDown property
getter/setter pair
      A pointer that might cause a tap is still in contact with the screen at a particular location after longTapDelay.
Implementation
GestureMultiTapDownCallback? onLongTapDown;A pointer that might cause a tap is still in contact with the screen at a particular location after longTapDelay.
GestureMultiTapDownCallback? onLongTapDown;