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

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". The following second press, app exits. Of course, the time between two press must be not long.

How to do it in flutter?

This is an example of my code (I've used "fluttertoast" for showing toast message, you can use snackbar or alert or anything else)

DateTime currentBackPressTime;

@override
Widget 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);
  }