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February 22, 2022 20:20
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Demonstrates how use of the wrong BuildContext can yield unexpected results
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| // Copyright (c) 2019, the Dart project authors. Please see the AUTHORS file | |
| // for details. All rights reserved. Use of this source code is governed by a | |
| // BSD-style license that can be found in the LICENSE file. | |
| import 'package:flutter/material.dart'; | |
| void main() => runApp(MyApp()); | |
| class MyApp extends StatelessWidget { | |
| @override | |
| Widget build(BuildContext context) { | |
| return MaterialApp( | |
| title: 'Flutter Demo', | |
| debugShowCheckedModeBanner: false, | |
| theme: ThemeData( | |
| primarySwatch: Colors.blue, | |
| ), | |
| home: const MyHomePage(title: 'Stale BuildContext demo'), | |
| ); | |
| } | |
| } | |
| class MyHomePage extends StatefulWidget { | |
| final String title; | |
| const MyHomePage({ | |
| Key? key, | |
| required this.title, | |
| }) : super(key: key); | |
| @override | |
| _MyHomePageState createState() => _MyHomePageState(); | |
| } | |
| class _MyHomePageState extends State<MyHomePage> { | |
| @override | |
| Widget build(BuildContext context) { | |
| return Scaffold( | |
| appBar: AppBar( | |
| title: Text(widget.title), | |
| ), | |
| body: Center( | |
| child: Theme( | |
| data: ThemeData( | |
| textTheme: Theme.of(context).textTheme.copyWith( | |
| headline1: const TextStyle(color: Colors.green), | |
| ), | |
| ), | |
| child: Column( | |
| mainAxisAlignment: MainAxisAlignment.center, | |
| children: <Widget>[ | |
| Builder( | |
| // Because this Builder fails to shadow the original `context` variable from | |
| // line 37, it allows inner widgets to calculate their position in your app | |
| // incorrectly. Remember, a Widget's BuildContext is its associated element in | |
| // the ElementTree, so using a different Widget's BuildContext can lead to | |
| // extremely tricky bugs. | |
| builder: (BuildContext innerContext) => Text( | |
| 'Green color lost', | |
| style: Theme.of(context) | |
| .textTheme | |
| .headline1! | |
| .copyWith(fontWeight: FontWeight.bold), | |
| ), | |
| ), | |
| Builder( | |
| // Because this Builder successfully shadows the original `context` variable from | |
| // line 37, it forces inner widgets to correctly calculate their position in your | |
| // app. The danger of using an incorrect build context has been diverted! | |
| builder: (BuildContext context) => Text( | |
| 'Green not lost', | |
| style: Theme.of(context) | |
| .textTheme | |
| .headline1! | |
| .copyWith(fontWeight: FontWeight.bold), | |
| ), | |
| ), | |
| ], | |
| ), | |
| ), | |
| ), | |
| ); | |
| } | |
| } |
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