mobile - Flutter 无法更改路由，因为 undefined name context with PopupMenuButton 如何解决？

Question

我想单击一个项目菜单 (PopupMenuItem) 并使用 Navigator.push 转到另一条路线，但方法内未定义上下文。

import 'package:flutter/material.dart';

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

class MyApp extends StatelessWidget {

  final List<Choice> choices = const <Choice>[
    const Choice(title: 'Settings', icon: Icons.settings),
    const Choice(title: 'Log out', icon: Icons.exit_to_app),
    ];



  @override
  Widget build(BuildContext context) {
    final title = 'MyTitle';

    return MaterialApp(
      title: title,
      home: Scaffold(
        appBar: AppBar(
          title: Text(title),
          actions: <Widget>[
                PopupMenuButton<Choice>(
                  onSelected: onItemMenuPress,
                  itemBuilder: (BuildContext context) {
                    return choices.map((Choice choice) {
                      return PopupMenuItem<Choice>(
                          value: choice,
                          child: Row(
                            children: <Widget>[
                              Icon(
                                choice.icon,
                              ),
                              Container(
                                width: 10.0,
                              ),
                              Text(
                                choice.title,
                              ),
                            ],
                          ));
                    }).toList();
                  },
                ),
              ],
        ),
        body: Text("Hello world")
      ),
    );

  }

    void onItemMenuPress(Choice choice) {   
      if (choice.title == 'Log out') {
        print("Logout");
        Navigator.push(context, MaterialPageRoute(builder: (context) => LogoutRoute()));
      } 
  }

}

class Choice {
  const Choice({this.title, this.icon});

  final String title;
  final IconData icon;
}

class LogoutRoute extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return Scaffold(
      appBar: AppBar(
        title: Text("Logout"),
      ),
      body: Center(
        child: Text("Screen"),
      ),
    );
  }
}

我尝试以这种方式在 onItemMenuPress 中传递上下文:

void onItemMenuPress(Choice choice, BuildContext context)

但是:

onSelected: onItemMenuPress(context)

不工作。

这种方法都行不通:

onSelected: (Choice c) { Navigator.push(context, MaterialPageRoute(builder: (context) => LogoutRoute())); }

我正在学习本教程: https://medium.com/flutter-community/building-a-chat-app-with-flutter-and-firebase-from-scratch-9eaa7f41782e

他的代码片段(与我的相似)似乎对他有用: https://github.com/duytq94/flutter-chat-demo/blob/master/lib/main.dart

我指的是第 235 行(onSelected)和第 199-205 行(实际的 onItemMenuPress 方法)

这怎么可能？我该如何缓解？谢谢

Answer 1

给你:

MyApp    <------ context
  --> MaterialApp
   (--> Navigator built within MaterialApp)
      --> Scaffold
        --> App Bar
          --> ...

因此，当您使用上下文查找导航器时，您使用的是不在导航器下的 MyApp 的上下文。所以我们可以创建一个新的 Stateless 或 Stateful Widget 子类来包含您的 Scaffold，因为其中的构建函数将指向该级别，或者我们可以使用 Builder并定义生成器回调(具有指向生成器的上下文)以返回 Scaffold。

工作代码我们创建了新的子类 - HomeScreen :

import 'package:flutter/material.dart';

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

class MyApp extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    final title = 'MyTitle';
    return MaterialApp(
      title: title,
      home: HomeScreen(title),
    );
  }
}

class HomeScreen extends StatelessWidget {
  final String title;

  HomeScreen(this.title);

  final List<Choice> choices = const <Choice>[
    const Choice(title: 'Settings', icon: Icons.settings),
    const Choice(title: 'Log out', icon: Icons.exit_to_app),
  ];

  @override
  Widget build(BuildContext context) {
    return Scaffold(
        appBar: AppBar(
          title: Text(title),
          actions: <Widget>[
            PopupMenuButton<Choice>(
              onSelected: (val) => onItemMenuPress(val, context),
              itemBuilder: (BuildContext context) {
                return choices.map((Choice choice) {
                  return PopupMenuItem<Choice>(
                      value: choice,
                      child: Row(
                        children: <Widget>[
                          Icon(
                            choice.icon,
                          ),
                          Container(
                            width: 10.0,
                          ),
                          Text(
                            choice.title,
                          ),
                        ],
                      ));
                }).toList();
              },
            ),
          ],
        ),
        body: Text("Hello world"));
  }

  void onItemMenuPress(Choice choice, BuildContext context) {
    if (choice.title == 'Log out') {
      print("Logout");
      Navigator.push(
          context, MaterialPageRoute(builder: (context) => LogoutRoute()));
    }
  }
}

class Choice {
  const Choice({this.title, this.icon});

  final String title;
  final IconData icon;
}

class LogoutRoute extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return Scaffold(
      appBar: AppBar(
        title: Text("Logout"),
      ),
      body: Center(
        child: Text("Screen"),
      ),
    );
  }
}