导航到新页面时 BLOC 丢失上下文

Question

我正在使用 BLOC 模式在我的应用中对用户进行身份验证。我有一个主要的 BlocProvider 来包装我的应用程序。和一个 BlocBuilder 根据认证状态构建。

如果用户未通过身份验证，我会使用入门/介绍屏幕导航到登录屏幕。

登录屏幕被包裹在另一个 BlocProvider 中，该 BlocProvider 包含一个用于登录的按钮，并在登录成功时添加一个登录事件。

问题是当我从入职屏幕导航时，我失去了主要的 authenticationBloc 上下文。推送新屏幕后，我需要做什么才能访问身份验证块。

void main() {
  WidgetsFlutterBinding.ensureInitialized();
  Bloc.observer = SimpleBlocObserver();

  runApp(
    MyApp(),
  );
}

class AuthenticationWrapper extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      home: BlocProvider<AuthenticationBloc>(
        create: (context) => AuthenticationBloc()..add(AppStarted()),
        child: MyApp(),
      ),
    );
  }
}

class MyApp extends StatelessWidget {

  @override
  Widget build(BuildContext context) {
    return BlocListener<AuthenticationBloc, AuthenticationState>(
      listener: (context, state) {
        if (state is Authenticated) {
          _appUserProfileRepository = AppUserProfileRepository();
        }
      },
      child: BlocBuilder<AuthenticationBloc, AuthenticationState>(
        builder: (context, state) {

          _authCredentialHelper = state.authCredentialHelper;

          if (state is Uninitialized) {
            return SplashScreen();
          }

          if (state is Unauthenticated) {
            return OnboardingScreens(authCredentialHelper: _authCredentialHelper);
          }

          if (state is InvalidRegistration) {
            return RegisterProfileScreen(authCredentialHelper: _authCredentialHelper);
          }

          if (state is Authenticated) {
              xxx
          }

          return Scaffold(body: Center(child: LoadingIndicator()));
        },
      ),
    );
  }
}

这是入职屏幕，我一导航就松开了 authenticationbloc 上下文

class OnboardingScreens extends StatelessWidget {
  final AuthCredentialHelper authCredentialHelper;

  OnboardingScreens({this.authCredentialHelper});

  _pages(BuildContext context) {
    return [
      xxx
    ];
  }

  _getStartedClicked(BuildContext context) {
    Navigator.push(context, MaterialPageRoute(builder: (context) {
      return LoginScreen(authCredentialHelper: authCredentialHelper);
    }));
  }

  @override
  Widget build(BuildContext context) {
    return Scaffold(
      body: SafeArea(
        child: IntroductionScreen(
          pages: _pages(context),
          onDone: () => _getStartedClicked(context),
          showSkipButton: true,
          done: xxx
        ),
      ),
    );
  }
}

在 1 处添加断点时，上下文可以正常使用 BlocProvider.of(context) 的有效值

步到 2. 给我一个错误： 使用不包含 AuthenticationBloc 类型的 Cubit 的上下文调用 BlocProvider.of()。

  _getStartedClicked(BuildContext context) {
    1----->Navigator.push(context, MaterialPageRoute(builder: (context) {
    2----->return LoginScreen(authCredentialHelper: authCredentialHelper);
    }));
  }

这是登录屏幕代码

class LoginScreen extends StatelessWidget {
  final AuthCredentialHelper authCredentialHelper;

  LoginScreen({this.authCredentialHelper});

  @override
  Widget build(BuildContext context) {
    return Scaffold(
      appBar: AppBar(
        leading: IconButton(
          icon: Icon(Icons.arrow_back, color: darkBlue),
          onPressed: () => Navigator.of(context).pop(),
        ),
        backgroundColor: Colors.transparent,
        elevation: 0.0,
      ),
      body: SafeArea(
        child: Center(
          child: BlocProvider<LoginBloc>(
            create: (context) => LoginBloc(authCredentialHelper: authCredentialHelper),
            child: LoginForm(authCredentialHelper: authCredentialHelper),
          ),
        ),
      ),
    );
  }
}

收到此错误：

The following assertion was thrown building _InheritedProviderScope<LoginBloc>(value: Instance of 'LoginBloc'):
BlocProvider.of() called with a context that does not contain a Cubit of type AuthenticationBloc.

No ancestor could be found starting from the context that was passed to BlocProvider.of<AuthenticationBloc>().

This can happen if the context you used comes from a widget above the BlocProvider.

Answer 1

改变这个：

   Navigator.push(context, MaterialPageRoute(builder: (context) {
      return LoginScreen(authCredentialHelper: authCredentialHelper);
   }));

到

   Navigator.push(
     context,
     MaterialPageRoute(builder: (contextLoginScreen) {
        return BlocProvider.value(
            value: context.bloc<AuthenticationBloc>(),
            child: LoginScreen(authCredentialHelper: authCredentialHelper));
     }),
   );