Flutter中的微件库错误捕获到异常

Question

我在Flutter中使用了这个应用程序。它有两个类，它们配对生成一个注释列表。

这是主类，MyApp类：

import 'package:flutter/cupertino.dart';
import 'package:flutter/material.dart';
import 'package:notesgenerator/sala.dart';

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

class MyApp extends StatelessWidget {
  // This widget is the root of your application.
  @override

  Widget build(BuildContext context) {

    List<Sala> locs = [
      Sala(note: 'Study', noteDes: 'from 6pm ~ 8pm'),
      Sala(note: 'Work', noteDes: 'from 8pm ~ 9pm'),
      Sala(note: 'Play', noteDes: 'from 9pm ~ 9:30pm'),
      Sala(note: 'Eat', noteDes: 'from 9:30pm ~ 10pm'),
    ];

    return Scaffold(
      appBar: AppBar(
        title: Text('NoteIndex'),
        centerTitle: true,
      ),
      body: ListView.builder(
          itemCount: 10,
          itemBuilder: (context, index) {
            return Card(
              child: ListTile(
                onTap: () {},
                title: (Text(locs[index].note)),
              ),
            );
          }),
    );}}

这是存储数据的类，Sala类：

class Sala {
  String note;
  String noteDes;
  Sala({this.note, this.noteDes});
}

当我尝试运行它时，我得到的结果是：

I/flutter (21388): ══╡ EXCEPTION CAUGHT BY WIDGETS LIBRARY ╞═══════════════════════════════════════════════════════════
I/flutter (21388): The following assertion was thrown building MyApp:
I/flutter (21388): MediaQuery.of() called with a context that does not contain a MediaQuery.
I/flutter (21388): No MediaQuery ancestor could be found starting from the context that was passed to MediaQuery.of().
I/flutter (21388): This can happen because you do not have a WidgetsApp or MaterialApp widget (those widgets introduce
I/flutter (21388): a MediaQuery), or it can happen if the context you use comes from a widget above those widgets.
I/flutter (21388): The context used was:
I/flutter (21388):   Scaffold
I/flutter (21388): 
I/flutter (21388): The relevant error-causing widget was:
I/flutter (21388):   MyApp file:///F:/FlutterProjects/notesgenerator/lib/main.dart:6:23
I/flutter (21388): 
I/flutter (21388): When the exception was thrown, this was the stack:
I/flutter (21388): #0      MediaQuery.of (package:flutter/src/widgets/media_query.dart:798:5)
I/flutter (21388): #1      ScaffoldState.didChangeDependencies (package:flutter/src/material/scaffold.dart:1993:50)
I/flutter (21388): #2      StatefulElement._firstBuild (package:flutter/src/widgets/framework.dart:4376:12)
I/flutter (21388): #3      ComponentElement.mount (package:flutter/src/widgets/framework.dart:4201:5)
I/flutter (21388): #4      Element.inflateWidget (package:flutter/src/widgets/framework.dart:3194:14)
I/flutter (21388): #5      Element.updateChild (package:flutter/src/widgets/framework.dart:2988:12)
I/flutter (21388): #6      ComponentElement.performRebuild (package:flutter/src/widgets/framework.dart:4243:16)
I/flutter (21388): #7      Element.rebuild (package:flutter/src/widgets/framework.dart:3947:5)
I/flutter (21388): #8      ComponentElement._firstBuild (package:flutter/src/widgets/framework.dart:4206:5)
I/flutter (21388): #9      ComponentElement.mount (package:flutter/src/widgets/framework.dart:4201:5)
I/flutter (21388): #10     Element.inflateWidget (package:flutter/src/widgets/framework.dart:3194:14)
I/flutter (21388): #11     Element.updateChild (package:flutter/src/widgets/framework.dart:2988:12)
I/flutter (21388): #12     RenderObjectToWidgetElement._rebuild (package:flutter/src/widgets/binding.dart:1028:16)
I/flutter (21388): #13     RenderObjectToWidgetElement.mount (package:flutter/src/widgets/binding.dart:999:5)
I/flutter (21388): #14     RenderObjectToWidgetAdapter.attachToRenderTree.<anonymous closure> (package:flutter/src/widgets/binding.dart:942:17)
I/flutter (21388): #15     BuildOwner.buildScope (package:flutter/src/widgets/framework.dart:2412:19)
I/flutter (21388): #16     RenderObjectToWidgetAdapter.attachToRenderTree (package:flutter/src/widgets/binding.dart:941:13)
I/flutter (21388): #17     WidgetsBinding.attachRootWidget (package:flutter/src/widgets/binding.dart:819:7)
I/flutter (21388): #18     WidgetsBinding.scheduleAttachRootWidget.<anonymous closure> (package:flutter/src/widgets/binding.dart:804:7)
I/flutter (21388): #27     _Timer._runTimers (dart:isolate-patch/timer_impl.dart:384:19)
I/flutter (21388): #28     _Timer._handleMessage (dart:isolate-patch/timer_impl.dart:418:5)
I/flutter (21388): #29     _RawReceivePortImpl._handleMessage (dart:isolate-patch/isolate_patch.dart:174:12)
I/flutter (21388): (elided 8 frames from package dart:async and package dart:async-patch)
I/flutter (21388): 
I/flutter (21388): ════════════════════════════════════════════════════════════════════════════════════════════════════

如果你能帮上忙，我将不胜感激！如果你有任何问题，请在评论中告诉我！

Answer 1

你需要用来包裹你的脚手架

MaterialApp()

因为这是引入MediaQuery的小部件

MyApp内部版本的更改：

    return MaterialApp(
      title: 'Flutter Demo',
      theme: ThemeData(
        primarySwatch: Colors.blue,
        visualDensity: VisualDensity.adaptivePlatformDensity,
      ),
      home: Scaffold(
       ...
      ),
    );

Answer 2

用MaterialApp包裹你的脚手架

    return MaterialApp(
  title: 'Flutter Demo',
  theme: ThemeData(
    primarySwatch: Colors.blue,
    visualDensity: VisualDensity.adaptivePlatformDensity,
  ),
  home: Scaffold(),
);

Answer 3

解决方案

itemCount: locs.length而不是itemCount: 10。

对于那些对原因感到好奇的人，请阅读以下内容:

看看你的清单：

 List<Sala> locs = [
  Sala(note: 'Study', noteDes: 'from 6pm ~ 8pm'),
  Sala(note: 'Work', noteDes: 'from 8pm ~ 9pm'),
  Sala(note: 'Play', noteDes: 'from 9pm ~ 9:30pm'),
  Sala(note: 'Eat', noteDes: 'from 9:30pm ~ 10pm'),
];

很明显，你的列表长度是3。

但是，在你的ListViewBuilder中：

itemCount: 10,

你设置了一个静态值10，这将告诉构建器有10个项目要返回，并且超过了列表的实际长度，为了清楚起见，你正在返回10个项目的Card，而你的列表只包含3个项目，这将尝试创建10个项目，这在列表中从一开始就不存在，当然，当迭代器返回列表的最后一个项目时，它将返回Index错误，这意味着它将在第4个索引返回错误，因为你只有3个项目。