flutter 安卓再次点击返回退出应用

安卓手机点击实体或者虚拟返回键，会返回上一级，当到达最上层是，点击返回退出应用，为了防止用户连续点击返回，致使应用退出，在用户点击返回到最上层时，若是再次点击返回，第一次不退出，并提高用户再次点击退出

import 'package:flutter/material.dart';
import 'package:fluttertoast/fluttertoast.dart';

void main() => runApp(MyApp());

class MyApp extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      title: '',
      home: HomePage(),
    );
  }
}

class HomePage extends StatefulWidget {
  @override
  _HomePageState createState() => _HomePageState();
}

class _HomePageState extends State<HomePage> {
  @override
  Widget build(BuildContext context) {
    return WillPopScope(
      onWillPop: _onWillPop,
      child: Scaffold(
        appBar: AppBar(title: Text('data'),),
        body: Container(
        child: Center(child: Text('data')),
      ),
      )
    );
  }

  int last = 0;
  Future<bool> _onWillPop() {
    //计算两次时间间隔
    int now = DateTime.now().millisecondsSinceEpoch;
    if (now - last > 8000) {
      last = DateTime.now().millisecondsSinceEpoch;
      Fluttertoast.showToast(msg: '再次点击返回退出应用',gravity: ToastGravity.BOTTOM);
      return Future.value(false);
    } else {
      return Future.value(true);
    }
  }
}