Flutter bloc 8 Either / Fold 异步返回中出现一些错误

Question

我正在将我的项目迁移到新版本的 Flutter，并使用 BloC 包。如您所知，Bloc 已经更新，并且更改了大部分代码，所以现在我正在调整我的代码，但有一些错误。

参数类型“Future Function(bool)”无法分配给参数类型“UserState Function(bool)”

正文可能正常完成，导致返回“null”，但返回类型可能是不可为 null 的类型。

前 ：

  Stream<UserState> _validate(ValidateEvent event) async* {

    final successOrFailure = await services.validate();
    
    yield* successOrFailure.fold(
      (sucess) async* {
        final session = await services.getSession();
        yield* session.fold(
          (session) async* {
            yield UserSuccess(session);

            yield UserLogged(session);
          },
          (failure) async* {
            yield UserError(failure.message);
          },
        );
      },
      (failure) async* {
        yield UserError(failure.message);
      },
    );
  }

后 ：

UserBloc({this.services}) : super(UserInitial()) {
    on<ValidateEvent>(_onValidate);
  }

void _onValidate(
    ValidateEvent event,
    Emitter<UserState> emit,
  ) async {

    final successOrFailure = await services.validate();
    
    emit(
      successOrFailure.fold(
        (success) async {
          final session = await services.getSession();
          emit(
            session.fold(
              (session) { // ! The body might complete normally, causing 'null' to be returned, but the return type is a potentially non-nullable type.
                emit(UserSuccess(session));

                emit(UserLogged(session));
              },
              (failure) { // ! The body might complete normally, causing 'null' to be returned, but the return type is a potentially non-nullable type.
                emit(UserError(failure.message));
              },
            ),
          );
        }, // ! The argument type 'Future<Null> Function(bool)' can't be assigned to the parameter type 'UserState Function(bool)
        (failure) { // ! The body might complete normally, causing 'null' to be returned, but the return type is a potentially non-nullable type.
          emit(UserError(failure.message));
        },
      ),
    );
  }

我需要在内部有异步，因为我再次调用服务，而关于主体，在我看来，我必须以某种方式返回它，但我内部有一些逻辑，但我不知道该怎么做。

Answer 1

您唯一可以做的emit就是您的状态类型的实例，在本例中是UserState。您不应该尝试发出 Future，而应该await这样做。

void _onValidate(
    ValidateEvent event,
    Emitter<UserState> emit,
  ) async {

    final successOrFailure = await services.validate();
    
    await successOrFailure.fold( // This will return FutureOr<Null>
      (success) async {
        final session = await services.getSession();
        session.fold( // This isn't asynchronous, so no need to await here
          (session) {
            emit(UserSuccess(session));

            emit(UserLogged(session));
          },
          (failure) {
            emit(UserError(failure.message));
          },
        );
      },
      (failure) {
        emit(UserError(failure.message));
      },
    );
  }