为什么使用不包含导航器的上下文请求了导航器操作

Question

我知道如何解决问题，但我不知道为什么会发生：

我有一个带有2个屏幕的应用程序：

此main.dart：

import 'package:flutter/material.dart';
import 'HomeActivity.dart';

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

class App extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return new HomeActivity();
  }
}

HomeActivity.Dart：

import 'package:flutter/material.dart';
import 'AddGameActivity.dart';

class HomeActivity extends StatelessWidget {

  @override
  Widget build(BuildContext context) {
    return new MaterialApp(
      home: new Scaffold(
        appBar: AppBar(title: new Text("Home")),
        floatingActionButton: FloatingActionButton(
            onPressed: () => Navigator.push(context, new MaterialPageRoute(builder: (context) => new AddGameActivity())),
            child: new Icon(Icons.add))),
    );
  }

因此，这里有一个带有FAB按钮的屏幕，可将我导航到AddGameActivity，当我按FAB按钮时，这是错误消息：

  

在不包含导航器的上下文中请求的导航器操作

现在要解决此问题，我将MaterialApp添加到main.dart并将其从HomeActivity.dart中删除，如下所示：

main.dart：

import 'package:flutter/material.dart';
import 'HomeActivity.dart';

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

class App extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return new MaterialApp(home: HomeActivity());
    }
}

HomeActivity：

import 'package:flutter/material.dart';
import 'AddGameActivity.dart';

class HomeActivity extends StatelessWidget {

  @override
  Widget build(BuildContext context) {
    return new Scaffold(
        appBar: AppBar(title: new Text("Home")),
        floatingActionButton: FloatingActionButton(
            onPressed: () => Navigator.push(context, new MaterialPageRoute(builder: (context) => new AddGameActivity())),
            child: new Icon(Icons.add)));
  }
}

在这种情况下，当我在main.dart中添加Material应用程序时，它可以正常工作而没有任何问题。

所以我的问题是为什么会这样？在这两种方式中，我都有一个Material app，里面有一个Scaffold！

Answer 1

如评论中所述，此问题似乎与该帖子重复："Navigator operation requested with a context that does not include a Navigator"

引发错误的原因是因为Navigator无法从Navigator访问MaterialApp。要解决此问题，您可以声明MaterialApp并初始化HomeActivity（如您所做的那样），也可以在HomeActivity中初始化MaterialApp之后添加Builder。

import 'package:flutter/material.dart';

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

class App extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return HomeActivity();
  }
}

class HomeActivity extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      // call Builder to access Navigator from MaterialApp
      home: Builder(
        builder: (context) => Scaffold(
            appBar: AppBar(title: Text("Home")),
            floatingActionButton: FloatingActionButton(
                onPressed: () => Navigator.push(
                    context,
                    new MaterialPageRoute(
                        builder: (context) => AddGameActivity())),
                child: new Icon(Icons.add))),
      ),
    );
  }
}

class AddGameActivity extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return Scaffold(
          appBar: AppBar(title: Text("AddGame")),
          floatingActionButton: FloatingActionButton(
              onPressed: () => Navigator.push(
                  context,
                  new MaterialPageRoute(
                      builder: (context) => new AddGameActivity())),
              child: new Icon(Icons.add)));
  }
}