Dart Isolate 利用 Timer 周期不会停止

Question

我在我的 Flutter 应用程序中调用了以下 Dart Isolate，它每 7 秒通过 _checkTimer 方法运行和处理一项任务。这对于我需要每隔一段时间运行的后台任务非常方便。

import 'dart:async';
import 'dart:isolate';

class TimeTrackingDigester with ChangeNotifier {
  Isolate _isolate;
  static int _counter = 0;
  ReceivePort _receivePort;

  void startIsolate() async {
    print('**** time tracking digester = startIsolate');

    _receivePort = ReceivePort();
    _isolate = await Isolate.spawn(
      _checkTimer,
      _receivePort.sendPort,
    );
    _receivePort.listen(
      _handleMessage,
      onDone: () {
        //@TODO this never fires
        print('done!');
      },
    );
  }

  void stopIsolate() => _stop();

  static void _checkTimer(SendPort sendPort) async {
    print('**** time tracking digestoer = checkTimer');
    Timer.periodic(
      const Duration(seconds: 7),
      (Timer t) async {
        _counter++;
        print('**** time tracking digestoer = $_counter');
        
        var task = 'result from some action here';

        sendPort.send(task);
      },
    );
  }

  void _handleMessage(dynamic data) {
    print('**** time tracking digestoer = handleMessage');
    print('RECEIVED: $data');
  }

  //@TODO this is not stopping the isolate
  void _stop() {
    print('**** time tracking digestoer = stop');
    print(_isolate);

    if (_isolate != null) {
      print('* isolate is not null');
      // setState(() {
      //     _running = false;
      //
      // });
      _receivePort.close();
      _isolate.kill(priority: Isolate.immediate);
      _isolate = null;
    }
  }
}

我的第一个问题是，一旦隔离启动，我调用 stopIsolate 方法来停止隔离，但它不会触发 close()/kill() 调用，因为 _isolate 被认为是 null。据我所知，_isolate 始终是null，我错过了在以下节中实际设置_isolate 的步骤：

if (_isolate != null) {
    print('* isolate is not null');
    // setState(() {
    //     _running = false;
    //
    // });
    _receivePort.close();
    _isolate.kill(priority: Isolate.immediate);
    _isolate = null;
 }

我的第二个问题是，如果我忽略 null 检查，而只是调用 close()/kill() 方法，则会引发以下异常：

NoSuchMethodError：在 null 上调用了方法“close”。我/颤动 (15558)：接收器：空 I/颤振 (15558)：尝试调用：close()

我想要完成的最终结果：当我想停止 Isolate 时，它​​会完全停止后续的 _checkTimer 方法，并解决 _isolate 未正确设置以触发 null 检查正确。

Answer 1

我测试了你的代码，它可以成功地停止隔离。 请确保您使用的是相同的 TimeTrackingDigester 实例，这样 _isolate 就不会是 null。

运行此示例应用程序。

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

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

class MyApp extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      theme: ThemeData(fontFamily: 'Roboto'),
      home: SampleIsolate(),
    );
  }
}

class SampleIsolate extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return ChangeNotifierProvider(
      create: (_) => TimeTrackingDigester(),
      child: Builder(
        builder: (context2) => Scaffold(
          body: Center(
            child: RaisedButton(
              onPressed: () {
                // Will get the TimeTrackingDigester created in "create: (_) => ..."
                final digester = Provider.of<TimeTrackingDigester>(
                  context2,
                  listen: false,
                );
                if (digester.isStarted) {
                  digester.stopIsolate();
                } else {
                  digester.startIsolate();
                }
              },
              child: const Text('Start / Stop'),
            ),
          ),
        ),
      ),
    );
  }
}

class TimeTrackingDigester with ChangeNotifier {
  Isolate _isolate;
  static int _counter = 0;
  ReceivePort _receivePort;

  bool get isStarted => _isolate != null;

  void startIsolate() async {
    print('**** time tracking digester = startIsolate');

    _receivePort = ReceivePort();
    _isolate = await Isolate.spawn(
      _checkTimer,
      _receivePort.sendPort,
    );
    _receivePort.listen(
      _handleMessage,
      onDone: () {
        //@TODO this never fires
        print('done!');
      },
    );
  }

  void stopIsolate() => _stop();

  static void _checkTimer(SendPort sendPort) async {
    print('**** time tracking digestoer = checkTimer');
    Timer.periodic(
      const Duration(seconds: 7),
      (Timer t) async {
        _counter++;
        print('**** time tracking digestoer = $_counter');

        var task = 'result from some action here';

        sendPort.send(task);
      },
    );
  }

  void _handleMessage(dynamic data) {
    print('**** time tracking digestoer = handleMessage');
    print('RECEIVED: $data');
  }

  //@TODO this is not stopping the isolate
  void _stop() {
    print('**** time tracking digestoer = stop');
    print(_isolate);

    if (_isolate != null) {
      print('* isolate is not null');
      // setState(() {
      //     _running = false;
      //
      // });
      _receivePort.close();
      _isolate.kill(priority: Isolate.immediate);
      _isolate = null;
    }
  }
}