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:
Dart 复制代码
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.
  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      title: 'Flutter Demo',
      theme: ThemeData(
        primarySwatch: Colors.blue,
      ),
      home: const Demo(),
    );
  }
}


class Demo extends StatefulWidget {
  const Demo({super.key});

  @override
  State<Demo> createState() => _DemoState();
}

class _DemoState extends State<Demo> {
  @override
  Widget 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.
    );
  }
  @override
  void initState() {
    // TODO: implement initState
    super.initState();
    print("------main--------initState");
  }

  @override
  void didChangeDependencies() {
    // TODO: implement didChangeDependencies
    super.didChangeDependencies();
    print("-------main-------didChangeDependencies");
  }

  @override
  void setState(VoidCallback fn) {
    // TODO: implement setState
    super.setState(fn);
    print("------main--------setState");
  }

  @override
  void deactivate() {
    // TODO: implement deactivate
    super.deactivate();
    print("------main--------deactivate");
  }

  @override
  void dispose() {
    // TODO: implement dispose
    super.dispose();
    print("------main--------dispose");
  }
}
B Page:
Dart 复制代码
import 'package:flutter/material.dart';


class DemoPage extends StatelessWidget {
  const DemoPage({super.key}) ;

  @override
  Widget build(BuildContext context) {
    return Scaffold(
      appBar: AppBar(
        title: const Text("Demo页面"),
      ),
      body: const DemoWidget(),
    );
  }
}

class DemoWidget extends StatefulWidget {
  const DemoWidget({super.key});

  @override
  State<DemoWidget> createState() => _DemoWidgetState();
}

class _DemoWidgetState extends State<DemoWidget> {


  String btnText = "test";


  @override
  Widget 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"),
            ),
          ],
        ),
    );
  }
  @override
  void initState() {
    // TODO: implement initState
    super.initState();
    print("------_DemoWidgetState--------initState");
  }

  @override
  void didChangeDependencies() {
    // TODO: implement didChangeDependencies
    super.didChangeDependencies();
    print("-------_DemoWidgetState-------didChangeDependencies");
  }

  @override
  void setState(VoidCallback fn) {
    // TODO: implement setState
    super.setState(fn);
    print("------_DemoWidgetState--------setState");
  }

  @override
  void deactivate() {
    // TODO: implement deactivate
    super.deactivate();
    print("------_DemoWidgetState--------deactivate");
  }

  @override
  void dispose() {
    // TODO: implement dispose
    super.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:

Dart 复制代码
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);

  @override
  Widget 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",
  ];

  @override
  Widget 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:

Dart 复制代码
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});

  @override
  Widget 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:

Dart 复制代码
import 'package:flutter/material.dart';
import 'transfer_data_entity.dart';

class TransferRouterPage extends StatelessWidget {
  final TransferDataEntity data;

  const TransferRouterPage({super.key, required this.data});

  @override
  Widget 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:

Dart 复制代码
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});


  @override
  bool updateShouldNotify(IDataProvider oldWidget) {
    return data != oldWidget.data;
  }

  //版本问题
  //InheritedWidget
  static 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:

Dart 复制代码
import 'package:flutter/material.dart';

import 'inherited_data_provider.dart';

class IDataWidget extends StatelessWidget {
  const IDataWidget({super.key});

  @override
  Widget 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});

  @override
  Widget 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:

Dart 复制代码
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:

Dart 复制代码
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> {
  @override
  Widget 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:

Dart 复制代码
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:

Dart 复制代码
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;

  @override
  Widget 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}"),
                  ),
                ],
              );
            }));
  }

  @override
  void 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:

Dart 复制代码
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.
  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      title: 'Flutter Demo',
      theme: ThemeData(
        primarySwatch: Colors.blue,
      ),
      home: const MyHomePage(),
    );
  }
}

class MyHomePage extends StatefulWidget {
  const MyHomePage({super.key});

  @override
  State<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,
    )
  ];

  @override
  Widget 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});

  @override
  State<Box> createState() => _BoxState();
}

class _BoxState extends State<Box> {
  int _count = 0;

  @override
  Widget 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

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。

运用key变更上面案例业务:

Dart 复制代码
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.
  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      title: 'Flutter Demo',
      theme: ThemeData(
        primarySwatch: Colors.blue,
      ),
      home: const MyHomePage(),
    );
  }
}

class MyHomePage extends StatefulWidget {
  const MyHomePage({super.key});

  @override
  State<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ValueKey
      color: Colors.orange,
    )
  ];

  @override
  Widget 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});

  @override
  State<Box> createState() => _BoxState();
}

class _BoxState extends State<Box> {
  int _count = 0;

  _BoxState() {
    print("--------_BoxState----------构造");
  }

  @override
  Widget 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"),
        ),
      ),
    );
  }

  @override
  void initState() {
    // TODO: implement initState
    super.initState();
    print("------_BoxState--------initState");
  }

  @override
  void didChangeDependencies() {
    // TODO: implement didChangeDependencies
    super.didChangeDependencies();
    print("-------_BoxState-------didChangeDependencies");
  }

  @override
  void setState(VoidCallback fn) {
    // TODO: implement setState
    super.setState(fn);
    print("------_BoxState--------setState");
  }

  @override
  void deactivate() {
    // TODO: implement deactivate
    super.deactivate();
    print("------_BoxState--------deactivate");
  }

  @override
  void dispose() {
    // TODO: implement dispose
    super.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。

Dart 复制代码
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.

  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      title: 'Flutter Demo',
      theme: ThemeData(
        primarySwatch: Colors.blue,
      ),
      home: const MyHomePage(),
    );
  }
}

class MyHomePage extends StatefulWidget {
  const MyHomePage({super.key});

  @override
  State<MyHomePage> createState() => _MyHomePageState();
}

class _MyHomePageState extends State<MyHomePage> {
  List<Widget> list = [
    Box(
      key: const ValueKey(1), //int double string
      color: Colors.blue,
    ),
    Box(
      key: ObjectKey(Object()),
      color: Colors.red,
    ),
    Box(
      key: UniqueKey(), //程序自动生成一个key
      color: Colors.orange,
    )
  ];

  @override
  Widget 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});

  @override
  State<Box> createState() => _BoxState();
}

class _BoxState extends State<Box> {
  int _count = 0;

  @override
  Widget 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。

Dart 复制代码
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.

  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      title: 'Flutter Demo',
      theme: ThemeData(
        primarySwatch: Colors.blue,
      ),
      home: const MyHomePage(),
    );
  }
}

class MyHomePage extends StatefulWidget {
  const MyHomePage({super.key});

  @override
  State<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key
      color: Colors.orange,
    )
  ];

  @override
  Widget 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});

  @override
  State<Box> createState() => _BoxState();
}

class _BoxState extends State<Box> {
  int _count = 0;

  @override
  Widget 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。

Dart 复制代码
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.
  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      title: 'Flutter Demo',
      theme: ThemeData(
        primarySwatch: Colors.blue,
      ),
      home: const HomePage(),
    );
  }
}

class HomePage extends StatefulWidget {
  const HomePage({super.key});

  @override
  State<HomePage> createState() => _HomePageState();
}

class _HomePageState extends State<HomePage> {
  final GlobalKey _globalKey = GlobalKey();

  @override
  Widget 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});

  @override
  State<Box> createState() => _BoxState();
}

class _BoxState extends State<Box> {
  int _count = 0;

  run() {
    print("run");
  }

  @override
  Widget 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"),
        ),
      ),
    );
  }
}
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