如何在一个块中收听多个流订阅？

Posted 2023-02-23

【中文标题】如何在一个块中收听多个流订阅？

【英文标题】：How to listen to multiple stream subscriptions in a bloc?

【发布时间】：2021-07-17 18:56:29

【问题描述】：

我正在尝试创建一个依赖于其他两个集团的集团。例如，我有Bloc C，它依赖于Bloc A 和Bloc B。我正在尝试使用flutter_bloc 执行以下操作以实现它：

class BlocC
    extends Bloc< BlocCEvent, BlocCState> 
  final BlocA blocA;
  final BlocC blocB;

  StreamSubscription blocASubscription;
  StreamSubscription blocBSubscription;

  BlocC(
    @required this.blocA,
    @required this.blocB,
  ) : super((blocA.state is blocALoaded &&
                blocB.state is blocBLoaded)
            ? BlocCLoaded(
                blocA: (blocA.state as blocALoaded).arrayFromBlocA,
                blocB:
                    (blocB.state as blocBLoaded).arrayFromBlocB,
              )
            : BlocCLoading()) 
              
    blocASubscription = blocA.stream.listen((state) 
      if (state is blocALoaded) 
        add(BlocAUpdated((blocA.state as blocALoaded).arrayFromBlocA));
      
    );

    blocBSubscription = blocB.stream.listen((state) 
      if (state is BlocBLoaded) 
        add(BlocBUpdated((blocB.state as BlocBLoaded).arrayFromBlocB));
      
    );
  

  ...

  @override
  Future<void> close() 
    blocASubscription.cancel();
    BlocBSubscription.cancel();
    return super.close();
  

问题是我收到以下错误：Bad state: Stream has already been listened to。我在下一个post 中找到了有关该错误的信息。

我了解错误正在发生，因为流一次只能听一个 bloc，而不能听多个 bloc。就我而言，当我尝试收听blocB 时，流已经在收听blocA。但是，我不确定如何解决此问题。

我将非常感谢任何帮助。

【参考方案1】：

您必须将两个流合并为一个并根据事件类型采取行动：

import 'package:async/async.dart' show StreamGroup;

//...

final blocAStream = blocA.stream;
final blocBStream = blocB.stream;
var blocAandBStreams = StreamGroup.merge([blocAStream, blocBStream]);

blocAandBStream.listen((event)
  if(event is BlocAState)
    if (event is blocALoaded)  //<-- for readability
      add(BlocAUpdated((blocA.state as blocALoaded).arrayFromBlocA));
    
  else if(event is BlocBState)
    if (event is BlocBLoaded) //<-- for readability
      add(BlocBUpdated((blocB.state as BlocBLoaded).arrayFromBlocB));
    
  
)

【参考方案2】：

我已经实现了事件总线模式来在块之间进行通信。这种方法的优点是您的块实例不会相互耦合，因此您不需要 实例化或注入他们的实例Manage Global Events by bloc