[Flutter]解决No MediaQuery ancestor could be found starting from the context that was passed to MediaQuery的问题

使用video_player插件()时，为了更直观的看看插件的实现效果。于是将官方的Example启动后发现了如下异常：

======== Exception caught by widgets library =======================================================
The following assertion was thrown building Scaffold-[<'home_page'>](dirty, state: ScaffoldState#5861e(tickers: tracking 2 tickers)):
No MediaQuery widget ancestor found.

Scaffold widgets require a MediaQuery widget ancestor.
The specific widget that could not find a MediaQuery ancestor was: Scaffold-[<'home_page'>]
  dirty
  state: ScaffoldState#5861e(tickers: tracking 2 tickers)
The ownership chain for the affected widget is: "Scaffold-[<'home_page'>] ← _TabControllerScope ← DefaultTabController ← _App ← [root]"

No MediaQuery ancestor could be found starting from the context that was passed to MediaQuery.of(). This can happen because you have not added a WidgetsApp, CupertinoApp, or MaterialApp widget (those widgets introduce a MediaQuery), or it can happen if the context you use comes from a widget above those widgets.

The relevant error-causing widget was: 
  Scaffold-[<'home_page'>] Scaffold:file:///F:/Git/blog/lib/main.dart:17:16
When the exception was thrown, this was the stack: 
#0      debugCheckHasMediaQuery.<anonymous closure> (package:flutter/src/widgets/debug.dart:297:7)
#1      debugCheckHasMediaQuery (package:flutter/src/widgets/debug.dart:312:4)
#2      ScaffoldState.build (package:flutter/src/material/scaffold.dart:2762:12)
#3      StatefulElement.build (package:flutter/src/widgets/framework.dart:5080:27)
#4      ComponentElement.performRebuild (package:flutter/src/widgets/framework.dart:4968:15)
#5      StatefulElement.performRebuild (package:flutter/src/widgets/framework.dart:5133:11)
#6      Element.rebuild (package:flutter/src/widgets/framework.dart:4690:5)
#7      ComponentElement._firstBuild (package:flutter/src/widgets/framework.dart:4950:5)
#8      StatefulElement._firstBuild (package:flutter/src/widgets/framework.dart:5124:11)
#9      ComponentElement.mount (package:flutter/src/widgets/framework.dart:4944:5)
...     Normal element mounting (21 frames)
#30     Element.inflateWidget (package:flutter/src/widgets/framework.dart:3953:16)
#31     Element.updateChild (package:flutter/src/widgets/framework.dart:3682:18)
#32     RenderObjectToWidgetElement._rebuild (package:flutter/src/widgets/binding.dart:1176:16)
#33     RenderObjectToWidgetElement.mount (package:flutter/src/widgets/binding.dart:1145:5)
#34     RenderObjectToWidgetAdapter.attachToRenderTree.<anonymous closure> (package:flutter/src/widgets/binding.dart:1092:18)
#35     BuildOwner.buildScope (package:flutter/src/widgets/framework.dart:2682:19)
#36     RenderObjectToWidgetAdapter.attachToRenderTree (package:flutter/src/widgets/binding.dart:1091:13)
#37     WidgetsBinding.attachRootWidget (package:flutter/src/widgets/binding.dart:926:7)
#38     WidgetsBinding.scheduleAttachRootWidget.<anonymous closure> (package:flutter/src/widgets/binding.dart:906:7)
#42     _RawReceivePort._handleMessage (dart:isolate-patch/isolate_patch.dart:192:26)
(elided 3 frames from class _Timer and dart:async-patch)
====================================================================================================
W/Parcel  (17640): Expecting binder but got null!

在上面的异常信息中，抛出了一个关键的一句话：

This can happen because you have not added a WidgetsApp, CupertinoApp, or MaterialApp widget (those widgets introduce a MediaQuery), or it can happen if the context you use comes from a widget above those widgets.

因为我这里是用Scaffold包裹组件的，代码如下：

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

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

class _App extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return DefaultTabController(
        length: 3,
        child: Scaffold(
          key: const ValueKey<String>('home_page'),
          appBar: AppBar(
            title: const Text('Video player example'),
            actions: <Widget>[
              IconButton(
                key: const ValueKey<String>('push_tab'),
                icon: const Icon(Icons.navigation),
                onPressed: () {
                  Navigator.push<_PlayerVideoAndPopPage>(
                    context,
                    MaterialPageRoute<_PlayerVideoAndPopPage>(
                      builder: (BuildContext context) => _PlayerVideoAndPopPage(),
                    ),
                  );
                },
              )
            ],
            bottom: const TabBar(
              isScrollable: true,
              tabs: <Widget>[
                Tab(
                  icon: Icon(Icons.cloud),
                  text: 'Remote',
                ),
                Tab(icon: Icon(Icons.insert_drive_file), text: 'Asset'),
                Tab(icon: Icon(Icons.list), text: 'List example'),
              ],
            ),
          ),
          body: TabBarView(
            children: <Widget>[
              _BumbleBeeRemoteVideo(),
              _ButterFlyAssetVideo(),
              _ButterFlyAssetVideoInList(),
            ],
          ),
        ),
      );
  }
}
 ///以下代码省略 详见https://pub.dev/packages/video_player/versions/2.1.4/example
...

以上异常信息说明，这个Scaffold必须使用MaterialApp进行包裹才行，所以修改代码：

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

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

class _App extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      home: DefaultTabController(
        length: 3,
        child: Scaffold(
          key: const ValueKey<String>('home_page'),
          appBar: AppBar(
            title: const Text('Video player example'),
            actions: <Widget>[
              IconButton(
                key: const ValueKey<String>('push_tab'),
                icon: const Icon(Icons.navigation),
                onPressed: () {
                  Navigator.push<_PlayerVideoAndPopPage>(
                    context,
                    MaterialPageRoute<_PlayerVideoAndPopPage>(
                      builder: (BuildContext context) => _PlayerVideoAndPopPage(),
                    ),
                  );
                },
              )
            ],
            bottom: const TabBar(
              isScrollable: true,
              tabs: <Widget>[
                Tab(
                  icon: Icon(Icons.cloud),
                  text: 'Remote',
                ),
                Tab(icon: Icon(Icons.insert_drive_file), text: 'Asset'),
                Tab(icon: Icon(Icons.list), text: 'List example'),
              ],
            ),
          ),
          body: TabBarView(
            children: <Widget>[
              _BumbleBeeRemoteVideo(),
              _ButterFlyAssetVideo(),
              _ButterFlyAssetVideoInList(),
            ],
          ),
        ),
      ),
    );
  }
}
 ///以下代码省略 详见https://pub.dev/packages/video_player/versions/2.1.4/example
...

修改完成，启动后正常。