Dart yield stream events from another stream listener

问题

I have a function that generates stream of specific events. Now I have a stream coming from storage service which has its own events. Looking for a way to yield my events when something changes in the storage stream.

This code snippet doesn't do the trick.

Stream<BlocState> mapEventToState(
    BlocEvent event,
  ) async* {
  if (event is UploadData) {
    yield UploadDataProgress(progress: 0.0);
    final Storage storage = Storage();
    final Stream<StorageEvent> upload = storage.upload(event.data);

    upload.listen((StorageEvent storageEvent) async* {
      print('***** Listener: ${storageEvent.type} - ${storageEvent.progress}');

      if (storageEvent.type == StorageEventType.error) {
        yield UploadDataError(errorMessage: storageEvent.error);
      }

      if (storageEvent.type == StorageEventType.success) {
        yield UploadDataSuccess();
      }

      if (storageEvent.type == StorageEventType.progress) {
        yield UploadDataProgress(progress: storageEvent.progress);
      }
    });
  }
}

Output: The debug print works but the events are not sent to listeners.

***** Listener: StorageEventType.progress - 0.01924033836457124
***** Listener: StorageEventType.progress - 0.044581091468101464
***** Listener: StorageEventType.progress - 0.6986233206170177
***** Listener: StorageEventType.progress - 1.0

回答1:

Your yields are yielding from the anonymous function (StorageEvent storageEvent) async* { rather than from mapEventToState.

Simply replacing the listen() with an await for should work.

Stream<BlocState> mapEventToState(
    BlocEvent event,
  ) async* {
  if (event is UploadData) {
    yield UploadDataProgress(progress: 0.0);
    final Storage storage = Storage();
    final Stream<StorageEvent> upload = storage.upload(event.data);

    await for (StorageEvent storageEvent in upload) {
      print('***** Listener: ${storageEvent.type} - ${storageEvent.progress}');

      if (storageEvent.type == StorageEventType.error) {
        yield UploadDataError(errorMessage: storageEvent.error);
      }

      if (storageEvent.type == StorageEventType.success) {
        yield UploadDataSuccess();
      }

      if (storageEvent.type == StorageEventType.progress) {
        yield UploadDataProgress(progress: storageEvent.progress);
      }
    }
  }
}

来源：https://stackoverflow.com/questions/56036503/dart-yield-stream-events-from-another-stream-listener

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dart

Flutter

stream