开发日记：按两次退出

onWillPop: () async {
        if (dateTime == null ||
            DateTime.now().difference(dateTime!) > Duration(seconds: 2)) {
          dateTime = DateTime.now();
          fluttertoast(msg: '再按一次退出', toast: ToastGravity.CENTER);
          return false;
        }
        return true;
      },child：Widget();