Flutter：流已被收听 [英] Flutter: Stream has already been listened to

问题描述

我正在使用BLoC从Firestore加载预设对象。这是我的Bloc模型：

  class StatisticsBloc扩展了BlocBase {
 
 List< Preset> _presets; 
 
 StreamController< List< Preset>> _presetsController = new StreamController（）; 
 
 Stream< List< Preset>> get getPresets => _presetsController.stream.asBroadcastStream（）; 
 
 StatisticsBloc（）{
 print（'init Statistics Bloc';; 
 _presets = []; 
 Firestore.instance.collection（'Presets'）。snapshots（）。asBroadcastStream（）。listen（_onPresetsLoaded）; 
} 
 
 @override 
 void dispose（）{
 print（'Disposed Statistics Bloc'）; 
 _presetsController.close（）; 
} 
 
 void _onPresetsLoaded（QuerySnapshot data）{
 _presets = []; 
 data.documents.forEach（（DocumentSnapshot snap）{
预设预置= Preset.fromDoc（快照）; 
 _presets.add（预设）; 
}）; 
 _presetsController.sink.add（_presets）; 
} 
} 
  

然后我显示如下列表：

  class StatisticsPage扩展了StatelessWidget {
 
 StatisticsPage（）{
 print（'Created StatisticsPage'）; 
} 
 
 @override 
小部件构建（BuildContext上下文）{
 final StatisticsBloc statisticsBloc = BlocProvider.of< StatisticsBloc>（上下文）； 
 final List< Preset> _ = []; 
 
 print（statisticsBloc.getPresets.isBroadcast）; 
 
返回脚手架（
 appBar：AppBar（
标题：Text（'Statistics'），
），
正文：StreamBuilder（
流：statisticsBloc.getPresets，
 initialData：_，
构建器：（BuildContext上下文，AsyncSnapshot< List< Preset>>快照）{
 if（snapshot.hasData）{
返回ListView（
子级：snapshot.data.map（（预设预设）{
 print（preset.name）; 
返回新的ListTile（
标题：new Text（preset。名称），
字幕：new Text（preset.id），
）; 
}）。toList（），
）; 
} else {
 Text（'No Data'）; 
 print（'No Data'）; 
} 
} 
）
）; 
} 
} 
  

问题是，我显示了<$ c标签栏中的$ c> StatisticsPage ，因此当我切换标签并返回到该标签时，构建时间会翻倍。第一次访问时它可以工作，但是当我切换选项卡并返回到它时，该小部件将重新生成，并且出现错误： Bad state：流已经被监听。 。我试图将 getPresets 流声明为BroadcastStream，正如您在 StatisitcsBloc 中看到的那样，但这是行不通的。 / p>

还有一个次要问题：是否有更好的方法将我从Firestore获得的 Stream< QuerySnapshot> 转换为 Stream< List< Presets>> ？

解决方案

很容易，请查看 BehaviorSubject类来自 RxDart库。

BehaviorSubject在默认情况下是广播（又称热门）控制器，以实现Rx主题合同。这意味着可以多次收听主题流。

因此，只需更改行

  StreamController< List< Preset>> _presetsController = new StreamController（）; 
  

到

  StreamController< List< Preset>> _presetsController = new BehaviorSubject（）; 
  

并全部删除

  .asBroadcastStream（）
  

就是这样！

在官方文档中，不建议使用asBroadcastStream（）

创建流控制器的更危险的方法是通过asBroadcastStream（）查看单个订阅控制器。调用asBroadcastStream基本上可以告诉单订阅流用户想要接管该流的生命周期管理。与cancelOnError订阅服务器结合使用，很容易导致单流订阅永远不会关闭，从而泄漏内存或资源。

I'm using BLoC to load my Preset Objects from Firestore. This is my Bloc Model:

class StatisticsBloc extends BlocBase {

  List<Preset> _presets;

  StreamController<List<Preset>> _presetsController = new StreamController();

  Stream<List<Preset>> get getPresets => _presetsController.stream.asBroadcastStream();

  StatisticsBloc() {
    print('init Statistics Bloc');
    _presets = [];
    Firestore.instance.collection('Presets').snapshots().asBroadcastStream().listen(_onPresetsLoaded);
  }

  @override
  void dispose() {
    print('Disposed Statistics Bloc');
    _presetsController.close();
  }

  void _onPresetsLoaded(QuerySnapshot data) {
    _presets = [];
    data.documents.forEach((DocumentSnapshot snap) {
      Preset preset = Preset.fromDoc(snap);
      _presets.add(preset);
    });
    _presetsController.sink.add(_presets);
  }
}

Then I display the List like this:

class StatisticsPage extends StatelessWidget {

  StatisticsPage() {
    print('Created StatisticsPage');
  }

  @override
  Widget build(BuildContext context) {
    final StatisticsBloc statisticsBloc = BlocProvider.of<StatisticsBloc>(context);
    final List<Preset> _ = [];

    print(statisticsBloc.getPresets.isBroadcast);

    return Scaffold(
      appBar: AppBar(
        title: Text('Statistics'),
      ),
      body: StreamBuilder(
        stream: statisticsBloc.getPresets,
        initialData: _,
        builder: (BuildContext context, AsyncSnapshot<List<Preset>> snapshot) {
          if (snapshot.hasData) {
            return ListView(
              children: snapshot.data.map((Preset preset) {
                print(preset.name);
                return new ListTile(
                  title: new Text(preset.name),
                  subtitle: new Text(preset.id),
                );
              }).toList(),
            );
          } else {
            Text('No Data');
            print('No Data');
          }
        }
      )
    );
  }
}

The problem is, I show the the StatisticsPage in a Tabbar, so it will be build muliple times when I switch tabs and go back to it. On the first visit it works but when I switch tabs and go back to it, the widget get rebuild and I get the error: Bad state: Stream has already been listened to.. I tried to declare the getPresets Stream as a BroadcastStream as you can see in StatisitcsBloc but that doesn't work.

Also as a secoundary question: Is there a better way to transform Stream<QuerySnapshot> that I get from Firestore to Stream<List<Presets>>?

解决方案

It is easy, take a look to BehaviorSubject class from RxDart library.

BehaviorSubject is, by default, a broadcast (aka hot) controller, in order to fulfill the Rx Subject contract. This means the Subject's stream can be listened to multiple times.

So, just change line

StreamController<List<Preset>> _presetsController = new StreamController();

to

StreamController<List<Preset>> _presetsController = new BehaviorSubject();

and delete all

.asBroadcastStream()

That's it!

In official documentation it is not recommended to use asBroadcastStream()

A more dangerous way of creating a stream controller is to view a single-subscription controller through asBroadcastStream(). Invoking asBroadcastStream basically tells the single-subscription stream that the user wants to take over the lifetime management of the stream. In combination with cancelOnError subscribers, this can easily lead to single-stream subscriptions that are never closed and thus leak memory or resources.

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