isPointerAllowed method
- PointerDownEvent event
override
    Checks whether or not a pointer is allowed to be tracked by this recognizer.
Implementation
@override
bool isPointerAllowed(PointerDownEvent event) {
  if (onSerialTapDown == null && onSerialTapCancel == null && onSerialTapUp == null) {
    return false;
  }
  return super.isPointerAllowed(event);
}