由于 mapEventToState 导致的 Flutter bloc 迁移不起作用答案

【问题标题】：Flutter bloc migration due to mapEventToState is not working由于 mapEventToState 导致的 Flutter bloc 迁移不起作用
【发布时间】：2022-01-01 14:15:13
【问题描述】：

我正在关注migration 到新的 bloc 8.0.0。我正在尝试删除 mapEventToState，但这样做有困难。你能帮我怎么做。我在下面尝试过，但它不起作用。

旧代码：

class SignInBloc extends Bloc<SignInEvent, SignInState> {
  final AuthenticationRepository authenticationRepository;
  final UserDataRepository userDataRepository;

  SignInBloc(
      {required this.authenticationRepository,
      required this.userDataRepository})
      : super(SignInInitialState());

  SignInState get initialState => SignInInitialState();

  @override
  Stream<SignInState> mapEventToState(
    SignInEvent event,
  ) async* {
    if (event is SignInWithGoogle) {
      yield* mapSignInWithGoogleToState();
    }
  }

Stream<SignInState> mapSignInWithGoogleToState() async* {
    yield SignInWithGoogleInProgressState();
    try {
      String res = await authenticationRepository.signInWithGoogle();
      yield SignInWithGoogleCompletedState(res);
    } catch (e) {
      print(e);
      yield SignInWithGoogleFailedState();
    }
  }
...

迁移代码（不起作用）：

class SignInBloc extends Bloc<SignInEvent, SignInState> {
  final AuthenticationRepository authenticationRepository;
  final UserDataRepository userDataRepository;

  SignInBloc(
      {required this.authenticationRepository,
        required this.userDataRepository})
      : super(SignInInitialState())
  {
    SignInState get initialState => SignInInitialState();

    on<SignInWithGoogle>((event, emit) => emit(mapSignInWithGoogleToState()));
  }

Stream<SignInState> mapSignInWithGoogleToState() async* {
    yield SignInWithGoogleInProgressState();
    try {
      String res = await authenticationRepository.signInWithGoogle();
      yield SignInWithGoogleCompletedState(res);
    } catch (e) {
      print(e);
      yield SignInWithGoogleFailedState();
    }
  }
...

【问题讨论】：

对我来说看起来是正确的。它到底是怎么工作的？也许还为mapSignInWithGoogleToState添加一些代码
我已经添加了

标签： flutter bloc

【解决方案1】：

您的问题是 mapSignInWithGoogleToState() 正在返回状态的 Stream，但是每次需要发出新状态时，新结构都需要显式调用 emit。

class SignInBloc extends Bloc<SignInEvent, SignInState> {
  final AuthenticationRepository authenticationRepository;
  final UserDataRepository userDataRepository;

  SignInBloc({required this.authenticationRepository,
    required this.userDataRepository})
      : super(SignInInitialState()) {
    on<SignInWithGoogle>(mapSignInWithGoogleToState);
  }

  Future<void> mapSignInWithGoogleToState(
      SignInWithGoogle event,
      Emitter<SignInState> emit,
  ) async {
    emit(SignInWithGoogleInProgressState());
    try {
      String res = await authenticationRepository.signInWithGoogle();
      emit(SignInWithGoogleCompletedState(res));
    } catch (e) {
      print(e);
      emit(SignInWithGoogleFailedState());
    }
  }
}

这里有更多关于“为什么？”的信息：https://bloclibrary.dev/#/migration?id=rationale-6

【讨论】：

【解决方案2】：

getter 不属于构造函数主体，我想它不再需要了。你可以这样解决问题：

class SignInBloc extends Bloc<SignInEvent, SignInState> {
  final AuthenticationRepository authenticationRepository;
  final UserDataRepository userDataRepository;

  SignInBloc({required this.authenticationRepository,
    required this.userDataRepository})
      : super(SignInInitialState()) {
    on<SignInWithGoogle>(_handleSignInWithGoogleEvent);
  }

  Future<void> _handleSignInWithGoogleEvent(
      SignInWithGoogle event,
      Emitter<SignInState> emit,
  ) async {
    // TODO do your thing and create and emit the SignInWithGoogleState
    emit(SignInWithGoogleState());
  }
}

请注意，bloc v8 中的事件不再按顺序处理，而是同时处理。阅读这篇博文：https://verygood.ventures/blog/how-to-use-bloc-with-streams-and-concurrency

【讨论】：

您好，感谢您的评论。我还更新了上面显示 mapSignInWithGoogleToState() 的代码。我也会看看你提供的链接。