一尘不染

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

flutter

导航到新的小部件后尝试访问InheritedWidget时遇到麻烦。

我有这样的顶层小部件

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

然后,WelcomeScreen具有一个导航到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,出现此错误

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,这都与Navigator.push构建器方法中的上下文有关。因为如果使用没有Navigator.push的LoginScreen小部件,则可以很好地使用InheritedWidget

发生错误是因为传递给该LoginBlocProvider.of()方法的上下文找不到该实例。

有什么想法吗?


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2020-08-13

共1个答案

一尘不染

在您提供的代码中,LoginScreen不是其后代的LoginBlocProvider原因就是它找不到祖先小部件。您的代码将WelcomeScreen路径包装在中LoginBlocProvider,而不是整个导航器中。解决方案是包装您MaterialApp的内容LoginBlocProvider,然后您将可以在应用程序中的任何地方访问它。

class App extends StatelessWidget {
  @override
  Widget build(context) {
    return LoginBlocProvider(
      child: MaterialApp(
        title: 'Iniciar Sesion',
        home: WelcomeScreen(),
      ),
    );
  }
}
2020-08-13