如何知道背景隔离中是否存在主隔离

Question

我使用工作经理包启动后台任务，并与主隔离进行通信。

在main.dart中，我的代码如下：

Future main() async {
  WidgetsFlutterBinding.ensureInitialized();
  // call Workmanager().initialize()
  await BackgroundService().init();
  // init ReceivePort
  var port = ReceivePort();
  if (IsolateNameServer.lookupPortByName('bChannel') != null)
    IsolateNameServer.removePortNameMapping('bChannel');
  IsolateNameServer.registerPortWithName(port.sendPort, 'bChannel');
  port.listen((dynamic data) async {
    print('[Main][bChannel listener] got $data');
    // send http request and update ui
  });
  BackgroundService().registerPeriodicTask();
  print('[Main] init');
  runApp(MyApp());
}

在background_service.dart中，我的代码如下：

void callbackDispatcher() {
  Workmanager().executeTask((task, inputData) async {
    try {
      switch (task) {
        case 'a':
          print("[Background] Task A triggered.");
          var sendPort = IsolateNameServer.lookupPortByName('bChannel');
          if (sendPort != null) {
            sendPort.send('message from background');
            print("[Background] message sent.");
          } else {
            print("[Background] no send port.");
            // send http request in background and save result to SharedPreferences
            final SharedPreferences prefs = await SharedPreferences.getInstance();
            await prefs.setString('result', 'result_message');
          }
          print("[Background] Task A finished.");
          break;
      }
      return Future.value(true);
    } catch (e) {
      print("[Background] $e");
      return Future.value(false);
    }
  });
}

class BackgroundService {
  // singleton things
  static final BackgroundService _instance = BackgroundService._internal();

  factory BackgroundService() {
    return _instance;
  }

  BackgroundService._internal() ;

  void registerWeatherRefreshPeriodicTask() {
    Workmanager().registerPeriodicTask(
      'TaskA',
      'TaskA',
      initialDelay: Duration(minutes: 1),
      frequency: Duration(minutes: 15),
      constraints: Constraints(
        networkType: NetworkType.connected
      )
    );
  }

  Future init() async {
    Workmanager().initialize(
      callbackDispatcher,
      isInDebugMode: true,
    );
  }

}

当应用程序在前面和后台运行时，它可以工作。但是，如果该应用程序退出(回击)到Android上，我会看到类似的日志

I/flutter ( 8342): [Main] init
D/FlutterLocationService( 8342): Unbinding from location service.
D/FlutterLocationService( 8342): Destroying service.
I/flutter ( 8342): [Background] Task A triggered.
I/flutter ( 8342): [Background] no send port.
I/flutter ( 8342): [Background] Task A finished.

没有日志可以指示主隔离程序使用消息。来自ReceivePort文档

ReceivePort是一种非广播流.这意味着它会缓冲传入的消息，直到注册侦听器为止。

当我再次打开这个应用程序时，它会再次显示[Main] init。因此，我想主要的隔离已经退出，并且没有侦听器到端口。

我想做的是：

• 当应用程序在前面运行时，后台任务触发并最终更新UI。
• 当应用程序运行在“后台”(UI退出但背景实际上正在运行)时，后台任务触发并发送http请求并保存结果。因此，当应用程序再次启动时，它可以读取结果并呈现UI。

我怎样才能实现这一点呢？

Answer 1

我放弃了触发主隔离来呈现，而不是后台隔离更新和保存结果，并让主隔离重新呈现，如果它正在运行，否则在初始化期间呈现。

在main.dart中

Future main() async {
  WidgetsFlutterBinding.ensureInitialized();
  // call Workmanager().initialize()
  await BackgroundService().init();
  // init ReceivePort
  var port = ReceivePort();
  if (IsolateNameServer.lookupPortByName('bChannel') != null)
    IsolateNameServer.removePortNameMapping('bChannel');
  IsolateNameServer.registerPortWithName(port.sendPort, 'bChannel');
  port.listen((dynamic data) async {
    print('[Main][bChannel listener] got $data');
    // refresh shared preferences and trigger render
    final prefs = await SharedPreferences.getInstance();
    await prefs.reload(); // reload otherwise it will use out-dated data
    // add BLoC event
    bloc.add(refreshEvent());
  });
  BackgroundService().registerPeriodicTask();
  print('[Main] init');
  runApp(MyApp());
}

在background_service.dart中

void callbackDispatcher() {
  Workmanager().executeTask((task, inputData) async {
    try {
      // also init and reload shared preferences
      // if main isolate contains other initialization works, it should also put here
      final SharedPreferences prefs = await SharedPreferences.getInstance();
      await prefs.reload();
      var httpClient = http.Client();

      switch (task) {
        case 'a':
          print("[Background] Task A triggered.");
          // send http request and save the result
          var response = await httpClient.get(uri);
          await prefs.setString('result', response.body);

          var sendPort = IsolateNameServer.lookupPortByName('bChannel');
          if (sendPort != null) {
            sendPort.send('message from background');
            print("[Background] message sent.");
          }
          print("[Background] Task A finished.");
          break;
      }
      return Future.value(true);
    } catch (e) {
      print("[Background] $e");
      return Future.value(false);
    }
  });
}