Flutter 05 组件状态、生命周期、数据传递（共享）、Key

一、Android界面渲染流程UI树与FlutterUI树的设计思路对比

二、Widget组件生命周期详解

1、Widget组件生命周期

和其他的视图框架比如android的Activity一样，flutter中的视图Widget也存在生命周期，生命周期的回调函数体现在了State上面。组件State的生命周期整理如下图所示：

createState：

当一个StatefulWidget插入到渲染树结构、或者从渲染树结构移除时，都会调用StatefulWidget.createState方法，从而达到更新UI的效果；

initState：

initState是StatefulWidget创建后调用的第一个方法，而且只执行一次。在执行initState时,View没有渲染，但是StatefulWidget 已经被加载到渲染树里了；

didChangeDependencies：

didChangeDependencies会在initState后立即调用，当StatefulWidget依赖的InheritedWidget发生变化之后，didChangeDependencies会调用，所以didChangeDependencies可以调用多次；

build：

build方法会在didChangeDeoendencies之后立即调用，在之后setState()刷新时，会重新调用build绘制页面，所以build方法可以调用多次。一般不在build中创建除了创建Widget的方法，否则会影响渲染效率。

setState：

[State] 对象可以通过调用它们的 [setState]方法自发地请求重建其子树，这表明它们的某些内部状态已经改变，可能会影响该子树中的用户界面，setState方法会被多次调用。

didUpdateWidget：

1、当调用setState更新UI的时候，都会调用didUpdateWidget；

2、框架在调用[didUpdateWidget]之后总是调用[build],在[didUpdateWidget]中对[setState]的任何调用都是多余的。

deactivate：

1、当框架从树中移除此 State 对象时将会调用此方法；

2、在某些情况下，框架将重新插入State 对象到树的其他位置(例如，如果包含该树的子树State 对象从树中的一个位置移植到另一位置），框架将会调用 build 方法来提供 State 对象适应其在树中的新位置。

dispose：

当框架从树中永久移除此 State对象时将会调用此方法，与deactivate的区别是，deactivate 还可以重新插入到树中，而 dispose 表示此 State对象永远不会在 build。调用完 dispose后，mounted 属性被设置为false，也代表组件生命周期的结束，此时再调用setState方法将会抛出异常。子类重写此方法，释放相关资源，比如动画等。

2、生命周期调用

A Page：

import 'package:flutter/material.dart';
import 'package:flutter_app/pages/DemoPages.dart';void main() {runApp(const MyApp());
}class MyApp extends StatelessWidget {const MyApp({super.key});// This widget is the root of your application.@overrideWidget build(BuildContext context) {return MaterialApp(title: 'Flutter Demo',theme: ThemeData(primarySwatch: Colors.blue,),home: const Demo(),);}
}class Demo extends StatefulWidget {const Demo({super.key});@overrideState<Demo> createState() => _DemoState();
}class _DemoState extends State<Demo> {@overrideWidget build(BuildContext context) {print("------main--------build");return Scaffold(appBar: AppBar(title: const Text("StatefulWidget key"),),body: Center(child: Column(mainAxisAlignment: MainAxisAlignment.center,children: <Widget>[ElevatedButton(onPressed: (){Navigator.of(context).push(MaterialPageRoute(builder:(setting){return const DemoPage();}));},child: const Text("页面跳转"),),],),),floatingActionButton: FloatingActionButton(onPressed: (){setState(() {print("调用....");});},tooltip: 'Increment',child: const Icon(Icons.add),), // This trailing comma makes auto-formatting nicer for build methods.);}@overridevoid initState() {// TODO: implement initStatesuper.initState();print("------main--------initState");}@overridevoid didChangeDependencies() {// TODO: implement didChangeDependenciessuper.didChangeDependencies();print("-------main-------didChangeDependencies");}@overridevoid setState(VoidCallback fn) {// TODO: implement setStatesuper.setState(fn);print("------main--------setState");}@overridevoid deactivate() {// TODO: implement deactivatesuper.deactivate();print("------main--------deactivate");}@overridevoid dispose() {// TODO: implement disposesuper.dispose();print("------main--------dispose");}
}

B Page：


import 'package:flutter/material.dart';class DemoPage extends StatelessWidget {const DemoPage({super.key}) ;@overrideWidget build(BuildContext context) {return Scaffold(appBar: AppBar(title: const Text("Demo页面"),),body: const DemoWidget(),);}
}class DemoWidget extends StatefulWidget {const DemoWidget({super.key});@overrideState<DemoWidget> createState() => _DemoWidgetState();
}class _DemoWidgetState extends State<DemoWidget> {String btnText = "test";@overrideWidget build(BuildContext context) {print("------_DemoWidgetState--------build");return Center(child: Column(mainAxisAlignment: MainAxisAlignment.center,children: <Widget>[ElevatedButton(onPressed: (){setState(() {if(btnText == "test"){btnText = "测试";}else{btnText = "test";}});},child: Text(btnText),),ElevatedButton(onPressed: (){print("返回......");Navigator.of(context).pop();},child: const Text("返回"),),],),);}@overridevoid initState() {// TODO: implement initStatesuper.initState();print("------_DemoWidgetState--------initState");}@overridevoid didChangeDependencies() {// TODO: implement didChangeDependenciessuper.didChangeDependencies();print("-------_DemoWidgetState-------didChangeDependencies");}@overridevoid setState(VoidCallback fn) {// TODO: implement setStatesuper.setState(fn);print("------_DemoWidgetState--------setState");}@overridevoid deactivate() {// TODO: implement deactivatesuper.deactivate();print("------_DemoWidgetState--------deactivate");}@overridevoid dispose() {// TODO: implement disposesuper.dispose();print("-----_DemoWidgetState---------dispose");}
}

三、页面间数据传递（共享）的几种常用方式

公共页面：

import 'package:flutter/material.dart';
import 'transfer_constructor_page.dart';
import 'transfer_data_entity.dart';
import 'transfer_data_inherited.dart';
import 'transfer_data_singleton.dart';
import 'transfer_router_page.dart';
import 'transfer_singleton_page.dart';
import 'transfer_stream_singleton.dart';
import 'transfer_stream_singleton_page.dart';import 'inherited_data_provider.dart';void main() => runApp(MyApp());class MyApp extends StatelessWidget {// This widget is the root of your application.var params = TransferDataEntity(name:"王五", id:"009", age:16);@overrideWidget build(BuildContext context) {return MaterialApp(title: 'Data Transfer Demo',theme: ThemeData(primarySwatch: Colors.blue,),home: const MyHomePage(title: 'Data Transfer Demo'),);}
}class MyHomePage extends StatefulWidget {const MyHomePage({super.key, required this.title});final String title;@override_MyHomePageState createState() => _MyHomePageState();
}class _MyHomePageState extends State<MyHomePage> {var data = TransferDataEntity(name:"张三丰", id:"001", age:18);List<String> itemNames = ["构造器传递数据","返回上个页面时携带参数","InheritedWidget方式","Singleton方式","Singleton结合Stream",];@overrideWidget build(BuildContext context) {return Scaffold(appBar: AppBar(title: Text(widget.title),),body: Column(children: <Widget>[Container(alignment: Alignment.center,height: 60.0,color: Colors.black12,child: Text("${data.id},${data.name},${data.age}")),Expanded(child: ListView.separated(separatorBuilder: (BuildContext contex, int index) {return const Divider(color: Colors.black12,height: 0.5,);},itemBuilder: (BuildContext context, int index) {return InkWell(onTap: () => _onClick(index, data, context),child: Container(alignment: Alignment.center,height: 48.0,width: double.infinity,child:  Text(itemNames[index],style: const TextStyle(color: Colors.black,fontWeight: FontWeight.bold,fontSize: 14.0,),),),);},itemCount: itemNames.length,),),],),floatingActionButton: FloatingActionButton(onPressed: () {},tooltip: 'Increment',child: const Icon(Icons.add),), // This trailing comma makes auto-formatting nicer for build methods.);}_onClick(int index, TransferDataEntity data, BuildContext context) {switch (index) {case 0:_transferDataByConstructor(context, data);break;case 1:_toTransferForResult(context, data);break;case 2:_inheritedToPage(context, data);break;case 3:_singletonDataTransfer(context);break;case 4:_streamDataTransfer(context);break;}}//通过构造器方法传递数据_transferDataByConstructor(BuildContext context, TransferDataEntity data) {Navigator.push(context,MaterialPageRoute(builder: (context) => DataTransferByConstructorPage(data: data)));}_toTransferForResult(BuildContext context, TransferDataEntity data) async {final dataFromOtherPage = await Navigator.push(context,MaterialPageRoute(builder: (context) => TransferRouterPage(data: data)),) as TransferDataEntity;setState(() {data.name = dataFromOtherPage.name;data.id = dataFromOtherPage.id;data.age = dataFromOtherPage.age;});}_inheritedToPage(BuildContext context, TransferDataEntity data) {Navigator.push(context,MaterialPageRoute(builder: (context) => IDataProvider(data: data,child: IDataWidget(),)));}_singletonDataTransfer(BuildContext context) {var transferData = TransferDataEntity(name:"三喵2", id:"002", age:20);transSingletonData.transData = transferData;Navigator.push(context,MaterialPageRoute(builder: (context) => TransferSingletonPage()));}_streamDataTransfer(BuildContext context) {var transferData = TransferDataEntity(name:"三喵", id:"005", age:20);streamSingletonData.setTransferData(transferData);Navigator.push(context,MaterialPageRoute(builder: (context) => TransferStreamPage()));}
}

1、通过构造器(constructor)传递数据

通过构造器传递数据是一种最简单的方式，也是最常用的方式，在第一个页面，我们模拟创建一个我们需要传递数据的对象。当点击跳转的时候，我们把数据传递DataTransferByConstructorPage页面，并把携带过来的数据展示到页面上。

1）创建一个传递数据对象：final data=TransferDataEntity("001"，"张三丰"，18);

2）定义一个跳转到DataTransferByConstructorPage页面的方法：

3）在DataTransferByConstructorPage页面接收到数据并展示出来：

import 'package:flutter/material.dart';
import 'transfer_data_entity.dart';///通过构造器的方式传递参数
class DataTransferByConstructorPage extends StatelessWidget {final TransferDataEntity data;const DataTransferByConstructorPage({super.key, required this.data});@overrideWidget build(BuildContext context) {return Scaffold(appBar: AppBar(title: Text("构造器方式"),),body: Column(children: <Widget>[Container(width: double.infinity,height: 40.0,alignment: Alignment.center,child: Text(data.id),),Container(width: double.infinity,height: 40.0,alignment: Alignment.center,child: Text(data.name),),Container(width: double.infinity,height: 40.0,alignment: Alignment.center,child: Text("${data.age}"),)],),);}
}

2、当一个页面关闭时携带数据到上一个页面（Navigator.pop)

在Android开发中我们需要将数据传递给上一个页面通常使用的传统方式是startActivityForResult（）方法。但是在flutter就不用这么麻烦了。只需要使用Navigator.pop方法即可将数据结果带回去。但是我们跳转的时候需要注意两点：

1）我们需要定义一个异步方法用于接收返回来的结果：

2）在我们要关闭的页面使用Navigator.pop 返回第一个页面：

import 'package:flutter/material.dart';
import 'transfer_data_entity.dart';class TransferRouterPage extends StatelessWidget {final TransferDataEntity data;const TransferRouterPage({super.key, required this.data});@overrideWidget build(BuildContext context) {return Scaffold(appBar: AppBar(title: const Text("返回上个页面传递参数"),leading: Builder(builder: (BuildContext context) {return IconButton(icon: const Icon(Icons.arrow_back_ios),onPressed: () {_backToData(context);});}),),body: Column(children: <Widget>[Container(alignment: Alignment.center,height: 40.0,child: Text(data.name),),Container(alignment: Alignment.center,height: 40.0,child: Text(data.id),),Container(alignment: Alignment.center,height: 40.0,child: Text("${data.age}"),),ElevatedButton(onPressed: () {_backToData(context);},child: const Text("点我返回上一个页面并把数据传回去"))],),);}//返回并携带数据_backToData(BuildContext context) {var transferData = TransferDataEntity(name: "嘻嘻哈哈", id: "007", age: 20);Navigator.pop(context, transferData);}
}

3、InheritedWidget方式

使用lnheritedWidget方式如下几步：

1）继承lnheritedWidget提供一个数据源：

import 'package:flutter/material.dart';
import 'transfer_data_entity.dart';//所有的子组件共享数据
class IDataProvider extends InheritedWidget {final TransferDataEntity data;const IDataProvider({super.key, required super.child, required this.data});@overridebool updateShouldNotify(IDataProvider oldWidget) {return data != oldWidget.data;}//版本问题//InheritedWidgetstatic IDataProvider? of(BuildContext context) {return context.dependOnInheritedWidgetOfExactType<IDataProvider>();}// static _InheritedProviders of(BuildContext context) {
//   final widget = context.inheritFromWidgetOfExactType(_InheritedProviders);
//   return widget is _InheritedProviders ? widget : null;
// }}

2）定义页面跳转时候携带数据的方法：

3）跳转的到的页面并展示数据代码如下：

import 'package:flutter/material.dart';import 'inherited_data_provider.dart';class IDataWidget extends StatelessWidget {const IDataWidget({super.key});@overrideWidget build(BuildContext context) {final data = IDataProvider.of(context)!.data;return Scaffold(appBar: AppBar(title: const Text("Inherited方式传递数据"),),body: Column(children: <Widget>[Container(alignment: Alignment.center,height: 40.0,child: Text(data.name),),Container(alignment: Alignment.center,height: 40.0,child: Text(data.id),),Container(alignment: Alignment.center,height: 40.0,child: Text("${data.age}"),),const IDataChildWidget()],),);}
}class IDataChildWidget extends StatelessWidget {const IDataChildWidget({super.key});@overrideWidget build(BuildContext context) {final data = IDataProvider.of(context)!.data;return Text(data.name);}
}

4、全局的提供数据的方式

这种方式我们还是使用lnheritedWidget，区别就是我们不是跳转的时候去创建IGenericDataProvider。而是把他放在最顶层注意：这种方式一定要把数据放在顶层；

1）定义顶部数据：

2）接收数据的方式基本和 lnheritedWidget相同：

final data=IGenericDataProvider.of<TransferDataEntity>(context) 获取数据

3）使用代码：

5、通过全局单例模式来使用

这种方式就是创建一个全局单例对象，任何地方都可以操控这个对象，存储和取值都可以通过这个对象。

1）创建单例对象：

import 'transfer_data_entity.dart';class TransferDataSingleton {static final TransferDataSingleton _instanceTransfer =TransferDataSingleton.__internal();TransferDataEntity? transData;factory TransferDataSingleton() {return _instanceTransfer;}TransferDataSingleton.__internal();
}final transSingletonData = TransferDataSingleton();

2）给单例对象存放数据：

3）接收并使用传递的值：

import 'package:flutter/material.dart';
import 'transfer_data_singleton.dart';class TransferSingletonPage extends StatefulWidget {const TransferSingletonPage({super.key});@override_TransferSingletonPageState createState() => _TransferSingletonPageState();
}class _TransferSingletonPageState extends State<TransferSingletonPage> {@overrideWidget build(BuildContext context) {var data = transSingletonData.transData;return Scaffold(appBar: AppBar(title: const Text("全局单例传递数据"),),body: Column(children: <Widget>[Container(alignment: Alignment.center,height: 40.0,child: Text(data!.name),),Container(alignment: Alignment.center,height: 40.0,child: Text(data.id),),Container(alignment: Alignment.center,height: 40.0,child: Text("${data.age}"),),],),);}
}

6、全局单例结合Stream的方式传递数据

1）单例模式：

import 'dart:async';import 'transfer_data_entity.dart';class TransferStreamSingleton {static final TransferStreamSingleton _instanceTransfer =TransferStreamSingleton.__internal();StreamController? streamController;void setTransferData(TransferDataEntity transData) {streamController = StreamController<TransferDataEntity>();streamController!.sink.add(transData);}factory TransferStreamSingleton() {return _instanceTransfer;}TransferStreamSingleton.__internal();
}final streamSingletonData = TransferStreamSingleton();

2）传递要携带的数据：

3）接收要传递的值：

import 'dart:async';import 'package:flutter/material.dart';
import 'transfer_data_entity.dart';
import 'transfer_stream_singleton.dart';class TransferStreamPage extends StatefulWidget {const TransferStreamPage({super.key});@override_TransferStreamPageState createState() => _TransferStreamPageState();
}class _TransferStreamPageState extends State<TransferStreamPage> {final StreamController? _streamController =streamSingletonData.streamController;@overrideWidget build(BuildContext context) {return Scaffold(appBar: AppBar(title: const Text("全局单例结合Stream"),),body: StreamBuilder(stream: _streamController!.stream,initialData: TransferDataEntity(),builder: (context, snapshot) {return Column(children: <Widget>[Container(alignment: Alignment.center,height: 40.0,child: Text(snapshot.data.name),),Container(alignment: Alignment.center,height: 40.0,child: Text(snapshot.data.id),),Container(alignment: Alignment.center,height: 40.0,child: Text("${snapshot.data.age}"),),],);}));}@overridevoid dispose() {_streamController?.close();super.dispose();}
}

四、Flutter Key

我们平时一定接触过很多的Widget，比如 Container、Row、Column等，它们在我们绘制界面的过程中发挥着重要的作用。但是不知道你有没有注意到，在几乎每个Widget的构造函数中，都有一个共同的参数，它们通常在参数列表的第一个，那就是Key。

在Flutter中，Key是不能重复使用的，所以Key一般用来做唯一标识。组件在更新的时候，其状态的保存主要是通过判断组件的类型或者key值是否一致。因此，当各组件的类型不同的时候，类型已经足够用来区分不同的组件了，此时我们可以不必使用key。但是如果同时存在多个同一类型的控件的时候，此时类型已经无法作为区分的条件了，我们就需要使用到key。

1、没有Key 会发生什么现象：

如下面例：定义了一个StatefulWidget的Box，点击Box的时候可以改变Box里面的数字，当我们重新对Box排序的时候，Flutter就无法识别到Box的变化了，这是什么原因呢？

运行后我们发现改变list Widget顺序后，Widget颜色会变化，但是每个Widget里面的文本内容并没有变化，为什么会这样呢？当我们List重新排序后Flutter检测到了Widget的顺序变化，所以重新绘制List Widget，但是Flutter发现List Widget里面的元素没有变化，所以就没有改变Widget里面的内容。

把List 里面的Box的颜色改成一样，这个时候您重新对list进行排序，就很容易理解了。重新排序后虽然执行了setState，但Flutter没法通过Box里面传入的参数是代码和以前是一样的，所以Flutter不会重构List Widget里面的内容，也就是来识别Box是否改变。如果要让FLutter能识别到List Widget子元素的改变，就需要给每个Box指定一个key。

实现代码：

import 'package:flutter/material.dart';void main() {runApp(const MyApp());
}class MyApp extends StatelessWidget {const MyApp({super.key});// This widget is the root of your application.@overrideWidget build(BuildContext context) {return MaterialApp(title: 'Flutter Demo',theme: ThemeData(primarySwatch: Colors.blue,),home: const MyHomePage(),);}
}class MyHomePage extends StatefulWidget {const MyHomePage({super.key});@overrideState<MyHomePage> createState() => _MyHomePageState();
}class _MyHomePageState extends State<MyHomePage> {List<Widget> list = [// Center(//   child: Box(//     color: Colors.blue,//   ),// ),Box(color: Colors.blue,),Box(color: Colors.red,),Box(color: Colors.orange,)];@overrideWidget build(BuildContext context) {return Scaffold(floatingActionButton: FloatingActionButton(onPressed: () {setState(() {list.shuffle(); //打乱list的顺序});},child: const Icon(Icons.refresh),),appBar: AppBar(title: const Text('Title'),),body: Center(child: Column(mainAxisAlignment: MainAxisAlignment.center,children: list,),),);}
}class Box extends StatefulWidget {Color color;Box({super.key, required this.color});@overrideState<Box> createState() => _BoxState();
}class _BoxState extends State<Box> {int _count = 0;@overrideWidget build(BuildContext context) {return SizedBox(height: 100,width: 100,child: ElevatedButton(style: ButtonStyle(backgroundColor: MaterialStateProperty.all(widget.color)),onPressed: () {setState(() {_count++;});},child: Center(child: Text("$_count"),),),);}
}

2、LocalKey与GlobalKey

运用key变更上面案例业务：

import 'package:flutter/material.dart';void main() {runApp(const MyApp());
}class MyApp extends StatelessWidget {const MyApp({super.key});// This widget is the root of your application.@overrideWidget build(BuildContext context) {return MaterialApp(title: 'Flutter Demo',theme: ThemeData(primarySwatch: Colors.blue,),home: const MyHomePage(),);}
}class MyHomePage extends StatefulWidget {const MyHomePage({super.key});@overrideState<MyHomePage> createState() => _MyHomePageState();
}class _MyHomePageState extends State<MyHomePage> {List<Widget> list = [Center(key: UniqueKey(),child: Box(color: Colors.blue,),),// Box(//   // key: const ValueKey("1"),//指定值//   color: Colors.blue,// ),Box(key: UniqueKey(), //唯一值， 自动生成color: Colors.red,),Box(key: const ObjectKey("2"), //类似于ValueKey// key: const ObjectKey(2),//类似于ValueKeycolor: Colors.orange,)];@overrideWidget build(BuildContext context) {return Scaffold(floatingActionButton: FloatingActionButton(onPressed: () {setState(() {list.shuffle(); //打乱list的顺序});},child: const Icon(Icons.refresh),),appBar: AppBar(title: const Text('Title'),),body: Center(child: Column(mainAxisAlignment: MainAxisAlignment.center,children: list,),),);}
}class Box extends StatefulWidget {Color color;Box({super.key, required this.color});@overrideState<Box> createState() => _BoxState();
}class _BoxState extends State<Box> {int _count = 0;_BoxState() {print("--------_BoxState----------构造");}@overrideWidget build(BuildContext context) {print("------_BoxState--------build");return SizedBox(height: 100,width: 100,child: ElevatedButton(style: ButtonStyle(backgroundColor: MaterialStateProperty.all(widget.color)),onPressed: () {setState(() {_count++;});},child: Center(child: Text("$_count"),),),);}@overridevoid initState() {// TODO: implement initStatesuper.initState();print("------_BoxState--------initState");}@overridevoid didChangeDependencies() {// TODO: implement didChangeDependenciessuper.didChangeDependencies();print("-------_BoxState-------didChangeDependencies");}@overridevoid setState(VoidCallback fn) {// TODO: implement setStatesuper.setState(fn);print("------_BoxState--------setState");}@overridevoid deactivate() {// TODO: implement deactivatesuper.deactivate();print("------_BoxState--------deactivate");}@overridevoid dispose() {// TODO: implement disposesuper.dispose();print("------_BoxState--------dispose");}
}

3、GlobalKey的使用

如果把LocalKey比作局部变量，GlobalKey就类似于全局变量；

下面使用了LocalKey,当屏幕状态改变的时候把Colum换成了Row，Box的状态就会丢失。一个Widget状态的保存主要是通过判断组件的类型或者key值是否一致。 LocalKey只在当前的组件树有效，所以把Colum换成了Row的时候Widget的状态就丢失了。

import 'package:flutter/material.dart';void main() {runApp(const MyApp());
}class MyApp extends StatelessWidget {const MyApp({super.key});// This widget is the root of your application.@overrideWidget build(BuildContext context) {return MaterialApp(title: 'Flutter Demo',theme: ThemeData(primarySwatch: Colors.blue,),home: const MyHomePage(),);}
}class MyHomePage extends StatefulWidget {const MyHomePage({super.key});@overrideState<MyHomePage> createState() => _MyHomePageState();
}class _MyHomePageState extends State<MyHomePage> {List<Widget> list = [Box(key: const ValueKey(1), //int double stringcolor: Colors.blue,),Box(key: ObjectKey(Object()),color: Colors.red,),Box(key: UniqueKey(), //程序自动生成一个keycolor: Colors.orange,)];@overrideWidget build(BuildContext context) {return Scaffold(floatingActionButton: FloatingActionButton(onPressed: () {setState(() {list.shuffle(); //打乱list的顺序});},child: const Icon(Icons.refresh),),appBar: AppBar(title: const Text('Title'),),body: Center(child: MediaQuery.of(context).orientation == Orientation.portrait? Column(mainAxisAlignment: MainAxisAlignment.center,children: list,): Row(mainAxisAlignment: MainAxisAlignment.center,children: list,),),);}
}class Box extends StatefulWidget {Color color;Box({super.key, required this.color});@overrideState<Box> createState() => _BoxState();
}class _BoxState extends State<Box> {int _count = 0;@overrideWidget build(BuildContext context) {return SizedBox(height: 100,width: 100,child: ElevatedButton(style: ButtonStyle(backgroundColor: MaterialStateProperty.all(widget.color)),onPressed: () {setState(() {_count++;});},child: Center(child: Text("$_count"),),),);}
}

为了解决这个问题我们就可以使用GlobalKey。

import 'package:flutter/material.dart';void main() {runApp(const MyApp());
}class MyApp extends StatelessWidget {const MyApp({super.key});// This widget is the root of your application.@overrideWidget build(BuildContext context) {return MaterialApp(title: 'Flutter Demo',theme: ThemeData(primarySwatch: Colors.blue,),home: const MyHomePage(),);}
}class MyHomePage extends StatefulWidget {const MyHomePage({super.key});@overrideState<MyHomePage> createState() => _MyHomePageState();
}class _MyHomePageState extends State<MyHomePage> {// List<Widget> list = [//   Box(//     key: const ValueKey(1),//     color: Colors.blue,//   ),//   Box(//     key: ObjectKey(Box(color: Colors.red)),//     color: Colors.red,//   ),//   Box(//     key:UniqueKey(), //程序自动生成一个key//     color: Colors.orange,//   )// ];List<Widget> list = [Box(key: GlobalKey(),color: Colors.blue,),Box(key: GlobalKey(),color: Colors.red,),Box(key: GlobalKey(), //程序自动生成一个keycolor: Colors.orange,)];@overrideWidget build(BuildContext context) {return Scaffold(floatingActionButton: FloatingActionButton(onPressed: () {setState(() {list.shuffle(); //打乱list的顺序});},child: const Icon(Icons.refresh),),appBar: AppBar(title: const Text('Title'),),body: Center(child: MediaQuery.of(context).orientation == Orientation.portrait? Column(mainAxisAlignment: MainAxisAlignment.center,children: list,): Row(mainAxisAlignment: MainAxisAlignment.center,children: list,),),);}
}class Box extends StatefulWidget {Color color;Box({super.key, required this.color});@overrideState<Box> createState() => _BoxState();
}class _BoxState extends State<Box> {int _count = 0;@overrideWidget build(BuildContext context) {return SizedBox(height: 100,width: 100,child: ElevatedButton(style: ButtonStyle(backgroundColor: MaterialStateProperty.all(widget.color)),onPressed: () {setState(() {_count++;});},child: Center(child: Text("$_count"),),),);}
}

4、GlobalKey 获取子组件

globalKey.currentState 可以获取子组件的状态，执行子组件的方法，globalKey.currentWidget可以获取子组件的属性，_globalKey.currentContext!.findRenderObject()可以获取渲染的属性。

import 'package:flutter/material.dart';void main() {runApp(const MyApp());
}class MyApp extends StatelessWidget {const MyApp({super.key});// This widget is the root of your application.@overrideWidget build(BuildContext context) {return MaterialApp(title: 'Flutter Demo',theme: ThemeData(primarySwatch: Colors.blue,),home: const HomePage(),);}
}class HomePage extends StatefulWidget {const HomePage({super.key});@overrideState<HomePage> createState() => _HomePageState();
}class _HomePageState extends State<HomePage> {final GlobalKey _globalKey = GlobalKey();@overrideWidget build(BuildContext context) {return Scaffold(floatingActionButton: FloatingActionButton(child: const Icon(Icons.add),onPressed: () {//1、获取子组件的状态 调用子组件的属性var state = (_globalKey.currentState as _BoxState);setState(() {state._count++;});//2、获取子组件的属性var box = (_globalKey.currentWidget as Box);print(box.color);//3、获取子组件渲染的属性var renderBox =(_globalKey.currentContext?.findRenderObject() as RenderBox);print(renderBox.size);},),appBar: AppBar(title: const Text('Title'),),body: Center(child: Box(key: _globalKey,color: Colors.red,),),);}
}class Box extends StatefulWidget {final Color color;const Box({super.key, required this.color});@overrideState<Box> createState() => _BoxState();
}class _BoxState extends State<Box> {int _count = 0;run() {print("run");}@overrideWidget build(BuildContext context) {return SizedBox(height: 100,width: 100,child: ElevatedButton(style: ButtonStyle(backgroundColor: MaterialStateProperty.all(widget.color)),onPressed: () {setState(() {_count++;});},child: Center(child: Text("$_count"),),),);}
}