flutter - 如何在一个区 block 中收听多个流订阅？

Question

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这取决于 Bloc A和 Bloc B .我正在尝试使用 flutter_bloc 执行以下操作为了实现它:

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.
我知道错误正在发生，因为一个流一次只能听一个块，而不是多个块。就我而言，流已经在收听 blocA当我试着听 blocB .但是，我不确定如何解决这个问题。
我真的很感激这方面的任何帮助。

Answer 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));
    }
  }
})