使用不包含 MediaQuery 的上下文调用 MediaQuery.of()。 错误

Question

I am trying to make an app with some help from the internet. The app is taking a picture from a gallery and it will send it to my server. When I got the code ready, there appeared an error.

I/flutter (11233): ══╡ EXCEPTION CAUGHT BY WIDGETS LIBRARY ╞═══════════════════════════════════════════════════════════
I/flutter (11233): The following assertion was thrown building UploadImageDemo(dirty, state:
I/flutter (11233): UploadImageDemoState#e27ba):
I/flutter (11233): MediaQuery.of() called with a context that does not contain a MediaQuery.
I/flutter (11233): No MediaQuery ancestor could be found starting from the context that was passed to MediaQuery.of().
I/flutter (11233): This can happen because you do not have a WidgetsApp or MaterialApp widget (those widgets introduce
I/flutter (11233): a MediaQuery), or it can happen if the context you use comes from a widget above those widgets.
I/flutter (11233): The context used was:
I/flutter (11233):   UploadImageDemo

it is saying that there isn't a MediaQuery. And here is my code

import 'package:flutter/material.dart';
import 'dart:io';
import 'dart:convert';
import 'package:http/http.dart' as http;
import 'package:image_picker/image_picker.dart';

void main()
{
  runApp(UploadImageDemo());
}

class UploadImageDemo extends StatefulWidget {

  UploadImageDemo() : super();

  final String title = "Upload Image Demo";

  @override
  UploadImageDemoState createState() => UploadImageDemoState();
}

class UploadImageDemoState extends State<UploadImageDemo> {

  //
  static final String uploadEndPoint =
      'http://localhost/flutter_test/upload_image.php';
  Future<File> file;
  String status = '';
  String base64Image;
  File tmpFile;
  String errMessage = 'Error Uploading Image';

  chooseImage() {
    setState(() {
      file = ImagePicker.pickImage(source: ImageSource.gallery);
    });
    setStatus('');
  }

  setStatus(String message) {
    setState(() {
      status = message;
    });
  }

  startUpload() {
    setStatus('Uploading Image...');
    if (null == tmpFile) {
      setStatus(errMessage);
      return;
    }
    String fileName = tmpFile.path.split('/').last;
    upload(fileName);
  }

  upload(String fileName) {
    http.post(uploadEndPoint, body: {
      "image": base64Image,
      "name": fileName,
    }).then((result) {
      setStatus(result.statusCode == 200 ? result.body : errMessage);
    }).catchError((error) {
      setStatus(error);
    });
  }

  Widget showImage() {

    return FutureBuilder<File>(
      future: file,
      builder: (BuildContext context, AsyncSnapshot<File> snapshot) {
        if (snapshot.connectionState == ConnectionState.done &&
            null != snapshot.data) {
          tmpFile = snapshot.data;
          base64Image = base64Encode(snapshot.data.readAsBytesSync());
          return Flexible(
            child: Image.file(
              snapshot.data,
              fit: BoxFit.fill,
            ),
          );
        } else if (null != snapshot.error) {
          return const Text(
            'Error Picking Image',
            textAlign: TextAlign.center,
          );
        } else {
          return const Text(
            'No Image Selected',
            textAlign: TextAlign.center,
          );
        }
      },
    );
  }

  @override
  Widget build(BuildContext context) {
    final size = MediaQuery.of(context).size;
    return Scaffold(
      appBar: AppBar(
        title: Text("Upload Image Demo"),
      ),
      body: Container(
        padding: EdgeInsets.all(30.0),
        child: Column(
          crossAxisAlignment: CrossAxisAlignment.stretch,
          children: <Widget>[
            OutlineButton(
              onPressed: chooseImage,
              child: Text('Choose Image'),
            ),
            SizedBox(
              height: 20.0,
            ),
            showImage(),
            SizedBox(
              height: 20.0,
            ),
            OutlineButton(
              onPressed: startUpload,
              child: Text('Upload Image'),
            ),
            SizedBox(
              height: 20.0,
            ),
            Text(
              status,
              textAlign: TextAlign.center,
              style: TextStyle(
                color: Colors.green,
                fontWeight: FontWeight.w500,
                fontSize: 20.0,
              ),
            ),
            SizedBox(
              height: 20.0,
            ),
          ],
        ),
      ),
    );
  }
}

I don't have an idea on how to fix this, because this MediaQuery is new to me. How do i fix this error?

Answer 1

You need to provide MaterialApp , you can put MaterialApp above UploadImageDemo
code snippet

void main() {
  runApp(MyApp());
}

class MyApp extends StatelessWidget {  
  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      title: 'Upload Image Demo',
      theme: ThemeData(       
        primarySwatch: Colors.blue,       
        visualDensity: VisualDensity.adaptivePlatformDensity,
      ),
      home: UploadImageDemo(),
    );
  }
}

working demo

full code

import 'package:flutter/material.dart';
import 'dart:io';
import 'dart:convert';
import 'package:http/http.dart' as http;
import 'package:image_picker/image_picker.dart';

void main() {
  runApp(MyApp());
}

class MyApp extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      title: 'Upload Image Demo',
      theme: ThemeData(
        primarySwatch: Colors.blue,
        visualDensity: VisualDensity.adaptivePlatformDensity,
      ),
      home: UploadImageDemo(),
    );
  }
}

class UploadImageDemo extends StatefulWidget {
  UploadImageDemo() : super();

  final String title = "Upload Image Demo";

  @override
  UploadImageDemoState createState() => UploadImageDemoState();
}

class UploadImageDemoState extends State<UploadImageDemo> {
  //
  static final String uploadEndPoint =
      'http://localhost/flutter_test/upload_image.php';
  Future<File> file;
  String status = '';
  String base64Image;
  File tmpFile;
  String errMessage = 'Error Uploading Image';

  chooseImage() {
    setState(() {
      file = ImagePicker.pickImage(source: ImageSource.gallery);
    });
    setStatus('');
  }

  setStatus(String message) {
    setState(() {
      status = message;
    });
  }

  startUpload() {
    setStatus('Uploading Image...');
    if (null == tmpFile) {
      setStatus(errMessage);
      return;
    }
    String fileName = tmpFile.path.split('/').last;
    upload(fileName);
  }

  upload(String fileName) {
    http.post(uploadEndPoint, body: {
      "image": base64Image,
      "name": fileName,
    }).then((result) {
      setStatus(result.statusCode == 200 ? result.body : errMessage);
    }).catchError((error) {
      setStatus(error);
    });
  }

  Widget showImage() {
    return FutureBuilder<File>(
      future: file,
      builder: (BuildContext context, AsyncSnapshot<File> snapshot) {
        if (snapshot.connectionState == ConnectionState.done &&
            null != snapshot.data) {
          tmpFile = snapshot.data;
          base64Image = base64Encode(snapshot.data.readAsBytesSync());
          return Flexible(
            child: Image.file(
              snapshot.data,
              fit: BoxFit.fill,
            ),
          );
        } else if (null != snapshot.error) {
          return const Text(
            'Error Picking Image',
            textAlign: TextAlign.center,
          );
        } else {
          return const Text(
            'No Image Selected',
            textAlign: TextAlign.center,
          );
        }
      },
    );
  }

  @override
  Widget build(BuildContext context) {
    final size = MediaQuery.of(context).size;
    print(size);
    return Scaffold(
      appBar: AppBar(
        title: Text("Upload Image Demo"),
      ),
      body: Container(
        padding: EdgeInsets.all(30.0),
        child: Column(
          crossAxisAlignment: CrossAxisAlignment.stretch,
          children: <Widget>[
            OutlineButton(
              onPressed: chooseImage,
              child: Text('Choose Image'),
            ),
            SizedBox(
              height: 20.0,
            ),
            showImage(),
            SizedBox(
              height: 20.0,
            ),
            OutlineButton(
              onPressed: startUpload,
              child: Text('Upload Image'),
            ),
            SizedBox(
              height: 20.0,
            ),
            Text(
              status,
              textAlign: TextAlign.center,
              style: TextStyle(
                color: Colors.green,
                fontWeight: FontWeight.w500,
                fontSize: 20.0,
              ),
            ),
            SizedBox(
              height: 20.0,
            ),
          ],
        ),
      ),
    );
  }
}