class HomePage extends StatelessWidget {
  /// 单击提示退出
  Future<bool> _dialogExitApp(BuildContext context) {
    return showDialog(
        context: context,
        builder: (context) => new AlertDialog(
              content: new Text("是否退出"),
              actions: <Widget>[
                new FlatButton(onPressed: () => Navigator.of(context).pop(false), child:  new Text("取消")),
                new FlatButton(
                    onPressed: () {
                      Navigator.of(context).pop(true);
                    },
                    child: new Text("确定"))
              ],
            ));
  }

  // This widget is the root of your application.
  @override
  Widget build(BuildContext context) {
    return WillPopScope(
      onWillPop: () {
        ///如果返回 return new Future.value(false); popped 就不会被处理
        ///如果返回 return new Future.value(true); popped 就会触发
        ///这里可以通过 showDialog 弹出确定框，在返回时通过 Navigator.of(context).pop(true);决定是否退出
        return _dialogExitApp(context);
      },
      child: new Container(),
    );
  }
}