如何在Flutter中实现永久秒表?

我正在flutter中实现一个计时器。这是应用程序的结构。

页面A(包含一些列表,用户单击该列表并将其带到计时器页面)。Page B格式可以运行计时器。我能够正确运行计时器/秒表,但是当我按Page

BI上的后退按钮时,会在处理错误后调用setstate()。我知道这是预期的行为。如果我在处理时使用timer.cancel(),则不会收到错误消息,但计时器将停止运行。即使我导航至页面A或说其他任何新页面(小部件),计时器/秒表也应继续运行。我知道可以使用侦听器和WidgetBindingObserver来实现,但是我不知道如何实现它。希望我会在此问题上获得一些帮助。

B页的构建类:

  Widget build(BuildContext context) {

return Scaffold(

appBar: AppBar(

leading: new IconButton(icon: new Icon(Icons.arrow_back), onPressed: ()async{

Navigator.pop(context,widget._elapsedTime);

}),

title: Text("widget.title"),

),

body: Center(

child: Column(

mainAxisAlignment: MainAxisAlignment.center,

children: <Widget>[

Text(

'$_elapsedTime'),

RaisedButton(

child: Text('Start'),

onPressed: () {

if(watch.isRunning){

stopWatch();

}

else{

startWatch();

}

},

),

],

),

));

StartWatch功能:

startWatch() {

watch.start();

timer = new Timer.periodic(new Duration(milliseconds:1000), updateTime);}

更新时间函数每秒调用一次:

updateTime(Timer timer) {

if (watch.isRunning) {

print(_elapsedTime);

var time= formatedTime(watch.elapsedMilliseconds);

print("time is"+time);

setState(() {

_elapsedTime = time;

});

}

回答:

这是一个最小的工作解决方案。关键点:

  • 介绍TimerService隔离计时器功能的类
  • TimerService实现ChangeNotifier,您可以订阅以接收更改。
  • 一个InheritedWidget用于提供服务给您的应用程序的所有部件。这个继承的小部件包装了您的应用小部件。
  • AnimatedBuilder用于接收来自的更改ChangeNotifier。订阅是自动处理的(无手动addListener/ removeListener)。


import 'dart:async';

import 'package:flutter/material.dart';

void main() {

final timerService = TimerService();

runApp(

TimerServiceProvider( // provide timer service to all widgets of your app

service: timerService,

child: MyApp(),

),

);

}

class TimerService extends ChangeNotifier {

Stopwatch _watch;

Timer _timer;

Duration get currentDuration => _currentDuration;

Duration _currentDuration = Duration.zero;

bool get isRunning => _timer != null;

TimerService() {

_watch = Stopwatch();

}

void _onTick(Timer timer) {

_currentDuration = _watch.elapsed;

// notify all listening widgets

notifyListeners();

}

void start() {

if (_timer != null) return;

_timer = Timer.periodic(Duration(seconds: 1), _onTick);

_watch.start();

notifyListeners();

}

void stop() {

_timer?.cancel();

_timer = null;

_watch.stop();

_currentDuration = _watch.elapsed;

notifyListeners();

}

void reset() {

stop();

_watch.reset();

_currentDuration = Duration.zero;

notifyListeners();

}

static TimerService of(BuildContext context) {

var provider = context.inheritFromWidgetOfExactType(TimerServiceProvider) as TimerServiceProvider;

return provider.service;

}

}

class TimerServiceProvider extends InheritedWidget {

const TimerServiceProvider({Key key, this.service, Widget child}) : super(key: key, child: child);

final TimerService service;

@override

bool updateShouldNotify(TimerServiceProvider old) => service != old.service;

}

class MyApp extends StatelessWidget {

@override

Widget build(BuildContext context) {

return MaterialApp(

title: 'Service Demo',

home: MyHomePage(),

);

}

}

class MyHomePage extends StatefulWidget {

@override

_MyHomePageState createState() => _MyHomePageState();

}

class _MyHomePageState extends State<MyHomePage> {

@override

Widget build(BuildContext context) {

var timerService = TimerService.of(context);

return Scaffold(

appBar: AppBar(),

body: Center(

child: AnimatedBuilder(

animation: timerService, // listen to ChangeNotifier

builder: (context, child) {

// this part is rebuilt whenever notifyListeners() is called

return Column(

mainAxisAlignment: MainAxisAlignment.center,

children: <Widget>[

Text('Elapsed: ${timerService.currentDuration}'),

RaisedButton(

onPressed: !timerService.isRunning ? timerService.start : timerService.stop,

child: Text(!timerService.isRunning ? 'Start' : 'Stop'),

),

RaisedButton(

onPressed: timerService.reset,

child: Text('Reset'),

)

],

);

},

),

),

);

}

}

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