Flutter Error: MediaQuery.of() called with a conte

问题:

I have been trying to get the size of the whole context view in Flutter. But every time I try I'm getting the above mentioned error. Here's my code:

import 'package:flutter/material.dart';

void main => runApp(new MyApp());

class MyApp extends StatelessWidget {
  @override
  Widget build(BuildContext context) {

    final size = MediaQuery.of(context).size;
    return new MaterialApp(
      home: new Scaffold(),
    );
  }
}

Note: I also tried with a StatefulWidget. Please, help me find what I'm doing wrong here.

回答1:

You need a MaterialApp or a WidgetsApp around your widget. They provide the MediaQuery. When you call .of(context) flutter will always look up the widget tree to find the widget.

You usually have this in your main.dart:

void main() => runApp(App());

class App extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      title: 'Title',
      theme: kThemeData,
      home: HomePage(),
    );
  }
}

class HomePage extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    final size = MediaQuery.of(context).size;

    return Container(
      child: ...,
    );
  }
}

回答2:

You can access MediaQuery when you are inside MaterialApp. The place where you are accessing the media query is not correct.

Please refer below code:

import 'package:flutter/material.dart';

class CommonThings {
  static Size size;
}

void main() => runApp(new MyApp());

class MyApp extends StatelessWidget {
  // This widget is the root of your application.
  @override
  Widget build(BuildContext context) {
    return new MaterialApp(
      title: 'MediaQuery Demo',
      theme: new ThemeData(
        primarySwatch: Colors.red,
      ),
      home: new MyHomePage(),
    );
  }
}

class MyHomePage extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    CommonThings.size = MediaQuery.of(context).size;
    print('Width of the screen: ${CommonThings.size.width}');
    return new Container();
  }
}

I've purposely created a class CommonThings which has static Size so that you can use it throughout the app.

回答3:

I fixed it by using the following method. First I created a new class named MyWidget and returned it in MyApp within a MaterialApp's home:. Refer code below:

import 'package:flutter/material.dart';

void main => runApp(new MyApp());

class MyApp extends StatelessWidget {
  @override
  Widget build(BuildContext context) {

    return new MaterialApp(
      home: new MyWidget(),
    );
  }
} 

class MyWidget extends StatelessWidget {
  @override
  Widget build(BuildContext context) {

    final size = MediaQuery.of(context).size;
    return new MaterialApp(
      home: new Scaffold(),
    );
  }
} 

Also, declaring size as final doesn't matter. Orientation/Rotation is handled.

回答4:

Solved by re-run the app(click on stop button in android studio then run again)

回答5:

Wrap your code in a Material App widget. I also had the same issue as I forgot to use it and directly returned the scaffold.

In other words, your MediaQuery.of(context) should be inside the Material Widget. Material app -> scaffold -> MediaQuery.of(context)

回答6:

There is better way. Above solutions would require you to have only one screen widget or inherit all screens from parent class. But there is solution, place the media query initialization into onGenerateRoute callback function

main.dart

import 'package:flutter/material.dart';

class MyApp extends StatefulWidget {
    @override
    State<StatefulWidget> createState() => new MyAppState();
}

class MyAppState extends State<MyApp> {
    @override
    Widget build(BuildContext context) {
      return MaterialApp(
        title: 'My Awesome App',
        routes: NavigationUtils.routeList(),
        onGenerateRoute: (routeSettings) =>
          NavigationUtils.onGenerateRoute(routeSettings),
      );
    }
}

NavigationUtils.dart

import 'package:flutter/material.dart';

class NavigationUtils {
    static onGenerateRoute(RouteSettings routeSettings) {   
      return new MaterialPageRoute(
        builder: (context) {
          WidgetUtils.me.init(context);
            return StorageUtils.me.isLogged() ? HomeScreen() : ForkScreen();
        },
        settings: routeSettings,
      );
    }
}

WidgetUtils.dart

import 'package:flutter/material.dart';

class WidgetUtils {
    MediaQueryData _mediaQueryData;
    double _screenWidth;
    double _screenHeight;
    double _blockSizeHorizontal;
    double _blockSizeVertical;

    init(BuildContext context) {
        _mediaQueryData = MediaQuery.of(context);
        screenWidth = _mediaQueryData.size.width;
        screenHeight = _mediaQueryData.size.height;
        blockSizeHorizontal = screenWidth / 100;
        blockSizeVertical = screenHeight / 100;
    }
}

Warning: It is not copy & paste code, there are some singletons etc. but you should get the point ;)

回答7:

void main() => runApp(MyApp());

class MyApp extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      debugShowCheckedModeBanner: false,
      home: MyAppOne(),
    );
  }
}
class MyAppOne extends StatefulWidget {
  @override
  _MyAppState createState() => _MyAppState();
}
class _MyAppState extends State<MyAppOne>{
  @override
  Widget build(BuildContext context){
    return Scaffold(
    );
  }
}

回答8:

Had the same error in

import 'screens/tasks_screen.dart';

void main() => runApp(MyApp());

class MyApp extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return TasksScreen();

  }
}

I solved it by:-

import 'package:flutter/material.dart';
import 'screens/tasks_screen.dart';

void main() => runApp(MyApp());

class MyApp extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      home: TasksScreen(),
    );
  }
}

回答9:

Add MaterialApp ...

void main() { runApp(MaterialApp( home: HomePage(), )); }