I want to understand how streams work with bloc pattern in flutter?

Question

What does emit.forEach() do? From what I understand, it takes a stream but how does the ui rebuild itself and what is the onData parameter? I want to return a stream of data for a client matrix sdk

  on<StartMatrixEventSubmitted>((event, emit) async {
      emit(MatrixLoadingState());
      await repo.getAllRooms().then((value) async {
        await emit.forEach(
          value.onSync.stream,
          onData: (value2) {
            print('data');
            return MatrixSuccuClientState(rooms: value.rooms);
          },
        );
        this.myRooms = value.rooms;
        emit(MatrixClientSynced());
      });
    });

Answer 1

Here's the description of what emit.forEach does in the docs:

Subscribes to the provided stream and invokes the onData callback when the stream emits new data and the result of onData is emitted.

Basically, it's the same as a List's .map() method. Each and every data (element) in the list will be converted into another value. Eg: Object to String

emit.forEach takes two values: the stream it has to listen to and the onData function. It will listen to all of the values emitted by the stream and convert them into another value (In this case MatrixSuccuClientState). Think of value2 in onData: (value2) as the "element[index] in a list". The value2 is the currently emitted value of the stream. I think what you're looking for is:

onData: (currentlyEmittedValue) => MatrixSuccuClientState(rooms: currentlyEmittedValue.rooms),

I'd suggest understanding Streams in flutter before moving on to bloc. I suggest watching this video. After that, you could learn how bloc works in this dev page