InheritedWidget - 在navigator.push之后,在null上调用getter

Question

导航到新窗口小部件后,我在尝试访问InheritedWidget时遇到问题.

我有这样的顶级小部件

class App extends StatelessWidget{
  build(context){
    return MaterialApp(
        title: 'Iniciar Sesion',
        home: LoginBlocProvider(child: WelcomeScreen()),
    );
  }  
}

然后WelcomeScreen有一个导航到LoginScreen的按钮

class WelcomeScreen extends StatelessWidget {

  @override Widget build(BuildContext context){
    return Scaffold(
      body: Center(child: MyButton)
    );
  }
}

class MyButton extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return RaisedButton(
      shape: StadiumBorder(),
      child: Text('Ingresar', style: TextStyle(color: Colors.black)),
      elevation: 5.0,
      onPressed: () { 
        Navigator.of(context).push(MaterialPageRoute(
           builder: (BuildContext context) =>LoginScreen()
        ));
      }
    );
  }
}

最后在LoginScreen中我想访问InheritedWidget

class LoginScreen extends StatefulWidget {
  @override
  _LoginScreenState createState() => _LoginScreenState();
}

class _LoginScreenState extends State<LoginScreen> {
  LoginBloc bloc;  

  @override void didChangeDependencies() {
    bloc = LoginBlocProvider.of(context);
    super.didChangeDependencies();
  }

  @override Widget build(BuildContext context){
    return Scaffold(
      body:
      Stack(
        fit: StackFit.expand,
        children: <Widget>[
          Positioned(
            top: 0.0,
            child: Image.asset('assets/images/img.jpg',
              fit: BoxFit.none,
            ),
          ),
          _buildLogin(),
        ],
      ),
    );
  }
}

编辑:这是LoginBlocProvider

class LoginBlocProvider extends InheritedWidget {
  final bloc;

  LoginBlocProvider({Key key, Widget child}) 
  : bloc = LoginBloc(), 
  super(key:key, child:child);

  @override
  bool updateShouldNotify(InheritedWidget oldWidget) => true;

  static LoginBloc of(BuildContext context) {
    return (context.inheritFromWidgetOfExactType(LoginBlocProvider) as LoginBlocProvider).bloc;
  }
}

但是,当我运行InheritedWidget的.of方法时,我得到了这个错误

I/flutter (27725): The following NoSuchMethodError was thrown building Builder:
I/flutter (27725): The getter 'bloc' was called on null.
I/flutter (27725): Receiver: null
I/flutter (27725): Tried calling: bloc

我的印象是,这一切都与Navigator.push构建器方法中的上下文有关.因为如果我使用不带Navigator.push的LoginScreen小部件,我可以完美地使用InheritedWidget

发生错误是因为传递给LoginBlocProvider.of()方法的上下文未找到实例.

有什么想法吗？

Answer 1

在您提供的代码中,LoginScreen不是它的后代LoginBlocProvider是它无法找到祖先小部件的原因.您的代码包含WelcomeScreen路径LoginBlocProvider,但不包括整个导航器.解决的办法是你的包裹MaterialApp中LoginBlocProvider,然后你会在你的应用程序可以访问它无处不在.

class App extends StatelessWidget {
  @override
  Widget build(context) {
    return LoginBlocProvider(
      child: MaterialApp(
        title: 'Iniciar Sesion',
        home: WelcomeScreen(),
      ),
    );
  }
}