Question

以下Flutter代码适用于永久可在后台运行的秒表。当我仅使用一个提供程序时，效果很好：

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

void main() {
  runApp(
    TimerServiceProvider(
      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.dependOnInheritedWidgetOfExactType<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'),
                )
              ],
            );
          },
        ),
      ),
    );
  }
}

但是，我需要在我的应用程序中使用多个提供程序，当我将此提供程序包装在MultiProvider中时，出现一个错误，指出该错误：无法将元素类型“ TimerServiceProvider”分配给列表类型“ SingleChildWidget” '。（list_element_type_not_assignable

我用于将提供程序包装在MultiProvider中的代码如下：

void main() {
  runApp(
      MultiProvider(
          providers: [
          TimerServiceProvider(service: TimerService(),
          ),
      ],
      child: MyApp(),)
  );
}

有人知道我在做什么吗？

Answer 1

您可以在下面复制粘贴运行完整代码
您可以使用ChangeNotifierProvider和Provider.of<TimerService>
代码段

void main() {
  runApp(MultiProvider(
    providers: [
      ChangeNotifierProvider(create: (context) => TimerService()),
    ],
    child: MyApp(),
  ));
}
...

@override
Widget build(BuildContext context) {
    var timerService = Provider.of<TimerService>(context, listen: false);

工作演示

完整代码

import 'dart:async';
import 'package:flutter/material.dart';
import 'package:provider/provider.dart';

void main() {
  runApp(MultiProvider(
    providers: [
      ChangeNotifierProvider(create: (context) => 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.dependOnInheritedWidgetOfExactType<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 = Provider.of<TimerService>(context, listen: false);
    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'),
                )
              ],
            );
          },
        ),
      ),
    );
  }
}

无法将TimeServiceProvider分配给列表类型SingleChildWidget

1 个答案: