Flutter使用flutter_bloc出现emit was called after an event handler completed报错

问题背景 #

使用Django REST Framework开发后端API时，接口已验证可正常返回响应数据，但Flutter端调试控制台抛出flutter_bloc相关异常，报错指向登录事件处理方法内的第二个emit调用位置。

错误提示 #

emit was called after an event handler completed normally.
This is usually due to an unawaited future in an event handler.

根本原因 #

注册LoginFormEvent的处理函数时，外层回调没有添加async关键字，即使内部loggedIn事件的分支回调加了async，外层回调也不会等待内部异步操作执行完成，会直接结束并关闭emit通道。等_getAuthInfo异步请求完成后再调用第二个emit时，emit通道已经被标记为完成，因此触发断言错误。

修复方案 #

方案1：补全async/await关键字 #

给外层事件回调添加async关键字，同时对event.map的返回值做await处理，确保所有异步逻辑执行完成后再结束事件处理：

class LoginFormBloc extends Bloc<LoginFormEvent, LoginFormState> {
  LoginFormBloc(this._getAuthInfo) : super(LoginFormState.initial()) {
    on<LoginFormEvent>(
      // 外层回调添加async关键字
      (event, emit) async {
        // 等待event.map所有分支逻辑执行完成
        await event.map(
          emailChanged: (e) async {
            emit(
              state.copyWith(
                emailAddress: EmailAddress(e.emailStr),
                authFailureOrSuccessOption: none(),
              ),
            );
          },
          passwordChanged: (e) async {
            emit(
              state.copyWith(
                password: Password(e.passwordStr),
                authFailureOrSuccessOption: none(),
              ),
            );
          },
          loggedIn: (e) async {
            final isEmailValid = state.emailAddress.value.isRight();
            final isPasswordValid = state.password.value.isRight();

            Either<AuthFailure, TokenValue>? loginFailureOrSuccess;

            if (isEmailValid && isPasswordValid) {
              emit(
                state.copyWith(
                  isSubmitting: true,
                  authFailureOrSuccessOption: none(),
                ),
              );
              loginFailureOrSuccess = await _getAuthInfo(
                Params(
                  emailAddress: state.emailAddress,
                  password: state.password,
                ),
              );
            }

            emit(
              state.copyWith(
                isSubmitting: false,
                showErrorMessages: true,
                authFailureOrSuccessOption: optionOf(loginFailureOrSuccess),
              ),
            );
          },
        );
      },
    );
  }
  final GetAuthToken _getAuthInfo;
}

方案2：拆分事件处理逻辑（推荐，符合Bloc最佳实践） #

将不同类型的事件拆分注册，每个事件对应独立的处理函数，避免嵌套异步逻辑导致的生命周期问题：

class LoginFormBloc extends Bloc<LoginFormEvent, LoginFormState> {
  LoginFormBloc(this._getAuthInfo) : super(LoginFormState.initial()) {
    // 单独注册每个事件的处理函数
    on<_EmailChanged>(_handleEmailChanged);
    on<_PasswordChanged>(_handlePasswordChanged);
    on<_LoggedIn>(_handleLoggedIn);
  }
  final GetAuthToken _getAuthInfo;

  Future<void> _handleEmailChanged(_EmailChanged event, Emitter<LoginFormState> emit) {
    emit(
      state.copyWith(
        emailAddress: EmailAddress(event.emailStr),
        authFailureOrSuccessOption: none(),
      ),
    );
  }

  Future<void> _handlePasswordChanged(_PasswordChanged event, Emitter<LoginFormState> emit) {
    emit(
      state.copyWith(
        password: Password(event.passwordStr),
        authFailureOrSuccessOption: none(),
      ),
    );
  }

  Future<void> _handleLoggedIn(_LoggedIn event, Emitter<LoginFormState> emit) async {
    final isEmailValid = state.emailAddress.value.isRight();
    final isPasswordValid = state.password.value.isRight();

    Either<AuthFailure, TokenValue>? loginFailureOrSuccess;

    if (isEmailValid && isPasswordValid) {
      emit(
        state.copyWith(
          isSubmitting: true,
          authFailureOrSuccessOption: none(),
        ),
      );
      loginFailureOrSuccess = await _getAuthInfo(
        Params(
          emailAddress: state.emailAddress,
          password: state.password,
        ),
      );
    }

    emit(
      state.copyWith(
        isSubmitting: false,
        showErrorMessages: true,
        authFailureOrSuccessOption: optionOf(loginFailureOrSuccess),
      ),
    );
  }
}

注意事项 #

Bloc 7.2及以上版本对emit的生命周期做了严格限制：事件处理函数执行完成后，emit通道会被立即标记为已完成，后续所有emit调用都会触发该断言错误。所有事件处理函数内的异步操作必须添加await关键字，确保emit调用都发生在通道关闭之前。

内容的提问来源于stack exchange，提问作者Biplab Dutta