如何编写一个双后退按钮以使用Flutter退出应用程序

这是我的代码的示例（我使用“ fluttertoast”显示吐司消息，您可以使用小吃栏或警报或其他方式）

DateTime currentBackPressTime;@overrideWidget build(BuildContext context) {  return Scaffold(    ...    body: WillPopScope(child: getBody(), onWillPop: onWillPop),  );}Future<bool> onWillPop() {    DateTime now = DateTime.now();    if (currentBackPressTime == null ||         now.difference(currentBackPressTime) > Duration(seconds: 2)) {      currentBackPressTime = now;      Fluttertoast.showToast(msg: exit_warning);      return Future.value(false);    }    return Future.value(true);  }