即使在事件重新触发后，Flutter BlocListener 也只执行一次

Question

我正在用 flutter 实现 Reso Coder's clean architecture。我按照他的指导将项目分成几层并使用依赖注入。在其中一种情况下，我希望出现以下情况：管理员用户登录，在其主屏幕上查看数据，对其进行编辑，然后按一个按钮，将数据保存到本地数据库 (sqflite)。保存数据后，我想显示一个带有某种文本 "Settings saved!" 的 Snackbar。这是我的代码（部分）：

class AdministratorPage extends StatefulWidget {
  @override
  _AdministratorPageState createState() => _AdministratorPageState();
}

class _AdministratorPageState extends State<AdministratorPage> {
  Widget build(BuildContext context) {
    return Scaffold(
      appBar: AppBar(
        backgroundColor: Theme.of(context).backgroundColor,
        centerTitle: true,
        leading: Container(),
        title: Text(AppLocalizations.of(context).translate('adminHomeScreen')),
      ),
      body: SingleChildScrollView(
        child: buildBody(context),
      ),
    );
  }

  BlocProvider<SettingsBloc> buildBody(BuildContext context) {
    return BlocProvider(
      create: (_) => serviceLocator<SettingsBloc>(),
      child: BlocListener<SettingsBloc, SettingsState>(
        listener: (context, state) {
          if (state is SettingsUpdatedState) {
            Scaffold.of(context).showSnackBar(
              SnackBar(
                content: Text(
                    AppLocalizations.of(context).translate('settingsUpdated')),
                backgroundColor: Colors.blue,
              ),
            );
          }
        },
        child: Column(
          children: <Widget>[
            SizedBox(
              height: 20.0,
            ),
            AdministratorInput(),
            SizedBox(
              width: double.infinity,
              child: RaisedButton(
                child: Text('LOG OUT'),
                onPressed: () {
                  serviceLocator<AuthenticationBloc>().add(LoggedOutEvent());
                  Routes.sailor(Routes.loginScreen);
                },
              ),
            ),
          ],
        ),
      ),
    );
  }
}

这是 AdministratorInput 小部件：

class AdministratorInput extends StatefulWidget {
  @override
  _AdministratorInputState createState() => _AdministratorInputState();
}

class _AdministratorInputState extends State<AdministratorInput> {
  String serverAddress;
  String daysBack;
  final serverAddressController = TextEditingController();
  final daysBackController = TextEditingController();

  @override
  Widget build(BuildContext context) {
    return Center(
      child: Padding(
        padding: const EdgeInsets.all(10.0),
        child: BlocBuilder<SettingsBloc, SettingsState>(
          builder: (context, state) {
            if (state is SettingsInitialState) {
              BlocProvider.of<SettingsBloc>(context)
                  .add(SettingsPageLoadedEvent());
            } else if (state is SettingsFetchedState) {
              serverAddressController.text =
                  serverAddress = state.settings.serverAddress;
              daysBackController.text =
                  daysBack = state.settings.daysBack.toString();
            }

            return Column(
              children: <Widget>[
                Container(
                  child: Row(
                    crossAxisAlignment: CrossAxisAlignment.start,
                    children: <Widget>[
                      Text(AppLocalizations.of(context)
                          .translate('serverAddress')),
                    ],
                  ),
                ),
                Container(
                  height: 40.0,
                  child: TextField(
                    controller: serverAddressController,
                    decoration: InputDecoration(
                      border: OutlineInputBorder(),
                    ),
                    onChanged: (value) {
                      serverAddress = value;
                    },
                  ),
                ),
                SizedBox(
                  height: 5.0,
                ),
                // Days Back Text Field
                Container(
                  child: Row(
                    crossAxisAlignment: CrossAxisAlignment.start,
                    children: <Widget>[
                      Text(AppLocalizations.of(context).translate('daysBack')),
                    ],
                  ),
                ),
                Container(
                  height: 40.0,
                  child: TextField(
                    controller: daysBackController,
                    decoration: InputDecoration(
                      border: OutlineInputBorder(),
                    ),
                    onChanged: (value) {
                      daysBack = value;
                    },
                  ),
                ),
                SizedBox(
                  width: double.infinity,
                  child: RaisedButton(
                    child: Text('SAVE CHANGES'),
                    onPressed: updatePressed,
                  ),
                ),
                SizedBox(
                  width: double.infinity,
                  child: RaisedButton(
                    child: Text('REFRESH'),
                    onPressed: refreshPressed,
                  ),
                ),
              ],
            );
          },
        ),
      ),
    );
  }

  void updatePressed() {
    BlocProvider.of<SettingsBloc>(context).add(
      SettingsUpdateButtonPressedEvent(
        settings: SettingsAggregate(
          serverAddress: serverAddress,
          daysBack: int.parse(daysBack),
        ),
      ),
    );
  }

  void refreshPressed() {
    BlocProvider.of<SettingsBloc>(context).add(
      SettingsRefreshButtonPressedEvent(),
    );
  }
}

SettingsBloc 是带有事件和状态以及映射器方法的标准块模式。它是使用 get_it 包注入的。下面是如何实例化：

serviceLocator.registerFactory(
    () => SettingsBloc(
      pullUsersFromServerCommand: serviceLocator(),
      getSettingsQuery: serviceLocator(),
      updateSettingsCommand: serviceLocator(),
    ),
  );

命令的所有实例和 bloc 构造函数的查询都以相同的方式正确实例化。

这是集团：

class SettingsBloc extends Bloc<SettingsEvent, SettingsState> {
  final PullUsersFromServerCommand pullUsersFromServerCommand;
  final UpdateSettingsCommand updateSettingsCommand;
  final GetSettingsQuery getSettingsQuery;

  SettingsBloc({
    @required PullUsersFromServerCommand pullUsersFromServerCommand,
    @required UpdateSettingsCommand updateSettingsCommand,
    @required GetSettingsQuery getSettingsQuery,
  })  : assert(pullUsersFromServerCommand != null),
        assert(updateSettingsCommand != null),
        assert(getSettingsQuery != null),
        pullUsersFromServerCommand = pullUsersFromServerCommand,
        updateSettingsCommand = updateSettingsCommand,
        getSettingsQuery = getSettingsQuery;

  @override
  SettingsState get initialState => SettingsInitialState();

  @override
  Stream<SettingsState> mapEventToState(SettingsEvent event) async* {
    if (event is SettingsPageLoadedEvent) {
      final getSettingsEither = await getSettingsQuery(NoQueryParams());

      yield* getSettingsEither.fold((failure) async* {
        yield SettingsFetchedFailureState(error: "settingsDatabaseError");
      }, (result) async* {
        if (result != null) {
          yield SettingsFetchedState(settings: result);
        } else {
          yield SettingsFetchedFailureState(
              error: "settingsFetchFromDatabaseError");
        }
      });
    } else if (event is SettingsUpdateButtonPressedEvent) {
      final updateSettingsEither = await updateSettingsCommand(
          UpdateSettingsParams(settingsAggregate: event.settings));

      yield* updateSettingsEither.fold((failure) async* {
        yield SettingsUpdatedFailureState(error: "settingsDatabaseError");
      }, (result) async* {
        if (result != null) {
          yield SettingsUpdatedState();
        } else {
          yield SettingsUpdatedFailureState(
              error: "settingsUpdateToDatabaseError");
        }
      });
    } else if (event is SettingsRefreshButtonPressedEvent) {
      final pullUsersFromServerEither =
          await pullUsersFromServerCommand(NoCommandParams());

      yield* pullUsersFromServerEither.fold((failure) async* {
        yield SettingsRefreshedFailureState(
            error: "settingsRefreshDatabaseError");
      }, (result) async* {
        if (result != null) {
          yield SettingsUpdatedState();
        } else {
          yield SettingsRefreshedFailureState(error: "settingsRefreshedError");
        }
      });
    }
  }
}

我第一次进入此屏幕时一切正常。数据从数据库中获取，加载到屏幕上，如果我更改它并按保存，它会显示 snackbar。我的问题是如果我想在停留在该屏幕上时再次编辑数据。我再次编辑它，因此触发更改事件，bloc 获取它，调用下面的正确命令并将数据保存在数据库中。然后改变 bloc 的状态，试图告诉 UI、"hey, I have a new state, get use of it"。但是 BlocListener 再也不会被调用了。

我应该如何实现我想要的行为？

编辑： 我在我登录用户的应用程序中添加了我之前使用的另一个集团。登录页面使用该块，如果用户名或密码错误，我将显示一个快餐栏，清除输入字段并让页面为更多内容做好准备。如果我用错误的凭据重试，我可以再次看到小吃店。

这是 LoginBloc：

class LoginBloc extends Bloc<LoginEvent, LoginState> {
  final AuthenticateUserCommand authenticateUserCommand;
  final AuthenticationBloc authenticationBloc;

  LoginBloc({
    @required AuthenticateUserCommand authenticateUserCommand,
    @required AuthenticationBloc authenticationBloc,
  })  : assert(authenticateUserCommand != null),
        assert(authenticationBloc != null),
        authenticateUserCommand = authenticateUserCommand,
        authenticationBloc = authenticationBloc;

  @override
  LoginState get initialState => LoginInitialState();

  @override
  Stream<LoginState> mapEventToState(LoginEvent event) async* {
    if (event is LoginButtonPressedEvent) {
      yield LoginLoadingState();

      final authenticateUserEither = await authenticateUserCommand(
          AuthenticateUserParams(
              username: event.username, password: event.password));

      yield* authenticateUserEither.fold((failure) async* {
        yield LoginFailureState(error: "loginDatabaseError");
      }, (result) async* {
        if (result != null) {
          authenticationBloc.add(LoggedInEvent(token: result));
          yield LoginLoggedInState(result);
        } else {
          yield LoginFailureState(error: "loginUsernamePasswordError");
        }
      });
    }
  }
}

这里的Event和State类扩展了Equatable。由于它按照预期工作，我在设置页面（失败的地方）中以相同的方式进行了操作。从 UI 开始，我根据需要多次提高 LoginButtonPressedEvent 并分别调用 BlocListener。

Answer 1

    else if (event is SettingsUpdateButtonPressedEvent) {
      final updateSettingsEither = await updateSettingsCommand(
          UpdateSettingsParams(settingsAggregate: event.settings));

      yield* updateSettingsEither.fold((failure) async* {
        yield SettingsUpdatedFailureState(error: "settingsDatabaseError");
      }, (result) async* {
        if (result != null) {
          //
          // this part is the problem.
          yield SettingsUpdatedState();
        } else {
          yield SettingsUpdatedFailureState(
              error: "settingsUpdateToDatabaseError");
        }
      });

In general, you should use Equatable if you want to optimize your code to reduce the number of rebuilds. You should not use Equatable if you want the same state back-to-back to trigger multiple transitions.

来源： when-to-use-equatable

它与 flutter_bloc 的工作原理是您不能产生相同的状态。是的，当您发出事件时，yield 状态之前的上述函数工作正常，但 yield 本身不会被调用。

所以基本上你的集团会发生什么，

当前状态为 SettingsFetchedState(settings: result)
您发出 SettingsUpdateButtonPressedEvent()
Bloc yield SettingsUpdatedState()
状态从 SettingsFetchedState(settings: result) 更改为 SettingsUpdatedState()
当前状态为 SettingsUpdatedState()
BlocListener 监听从 SettingsFetchedState(settings: result) 到 SettingsUpdatedState() 的状态变化
您发出 SettingsUpdateButtonPressedEvent()
Bloc 不产生 SettingsUpdatedState()，它被忽略，因为相等比较 returns true)
BlocListener 什么都不做，因为没有状态变化。

如何解决这个问题？根据我目前的知识，我没有足够的信心给出建议，所以也许可以试试引述 You should not use Equatable if you want the same state back-to-back to trigger multiple transitions.

编辑：

LoginBloc 的工作原理很简单，因为它为每个事件产生不同的状态。我认为您没有注意到，但它会在产生 LoginLoggedInState(result) 或 LoginFailureState(error: "loginUsernamePasswordError")

之前产生 LoginLoadingState()

当前状态为 LoginInitialState()
发出事件
让出 LoginLoadingState()
状态从 LoginInitialState() 更改为 LoginLoadingState()
产生 LoginLoggedInState() 或 LoginFailurestate()
状态从 LoginLoadingState() 更改为 LoginLoggedInState() 或 LoginFailurestate()
返回每个事件的第 2 步

Answer 2

@Federick Jonathan 已经对这个问题给出了足够的解释，但我想在这方面做插件。

第一件事： 这是 Equatable 的标准行为，事件侦听器在状态发生变化时被调用。如果你 yield 每次都处于相同的状态，那么什么都不会发生。

让我们讨论所有可能的解决方案。

从 bloc 中移除 Equatable 然后当 state 改变时每个事件触发。
为状态定义start和end状态。例如，将第一个 state 创建为 StartDataUpdate，将第二个创建为 EndDataUpdate.

参考以下代码

yield StartDataUpdate();
//Here... Please specified data changes related to operation.
yield EndDataUpdate();

  Stream<ReportsState> setupState({required ReportsState state}) async* {
    yield StartReportsState();
    yield state;
    yield EndReportsState();
  }

使用：

 yield* setupState( state: NavigationState() );

即使在事件重新触发后，Flutter BlocListener 也只执行一次

Flutter BlocListener executed only once even after event gets re-fired

dependency-injection

flutter

bloc