How to write a double back button pressed to exit app using flutter

Question

I\'m new to flutter, and I saw many android apps can exit when double press back button.

The first time press back button, app shows a toast\"press again to exit app

Answer 1

The first time press back button, app shows a AlertDialog"press yes to exit app and press No to can't exit application". This is an example of my code (I've used 'AlertDialog')

   @override
      Widget build(BuildContext context) {

        return new WillPopScope(
          onWillPop: _onBackPressed,
          child: DefaultTabController(
            initialIndex: _selectedIndex,
            length: choices.length,
            child: Scaffold(
              appBar: AppBar(

                ),
              ),

          ),
        );
      }
         Future _onBackPressed() {
        return showDialog(
          context: context,
          builder: (context) {
            return AlertDialog(
              title: Text('Are you sure?'),
              content: Text('Do you want to exit an App'),
              actions: [
                FlatButton(
                  child: Text('No'),
                  onPressed: () {
                    Navigator.of(context).pop(false);
                  },
                ),
                FlatButton(
                  child: Text('Yes'),
                  onPressed: () {
                    Navigator.of(context).pop(true);
                  },
                )
              ],
            );
          },
        ) ?? false;
      }