Flutter StatefulWidget - 状态类继承？

Question

这个问题的重点是如何处理状态变化，最好是由原始祖先自动处理。 在我看来，不可能继续将这些类扩展到 第一个以 StatefulWidget 作为祖先的孩子（示例 1）？

在我看来，唯一的方法就是使用 mixins。 但是，这有一个缺点，您必须手动管理状态更改（示例 2）。

我是否忽略了什么？

// 示例 1 - 不作为 BarState 不从 Bar 继承

class Foo extends StatefulWidget {
  final String stringFoo;

  Foo({Key key, this.stringFoo}) : super(key: key);

  @override
  FooState createState() => new FooState();
}

class FooState extends State<Foo> {
  bool updating = false;

  Future<void> _update() async {
    // await something(stringFoo)
    setState(() {
      updating = false;
    });
  }

  void update() {
    setState(() {
      updating = true;
    });
    _update();
  }

  // Dummy build, always override
  @override
  Widget build(BuildContext context) {
    return null;
  }

  @override
  void initState() {
    super.initState();
    update();
  }
}

class Bar extends Foo {
  final String stringBar;

  Bar({Key key, String stringFoo, this.stringBar}) : super(key: key, stringFoo: stringFoo);

  @override
  BarState createState() => new BarState();
}

class BarState extends FooState {

  @override
  Widget build(BuildContext context) {
    if(updating) {
      return Text('Im updating: ' + widget.stringFoo);
    } else {
      return Text('All done!' + widget.stringBar);
    }
  }

}

// 示例 2 - 有效，但手动处理状态更改

class Foo {
  String stringFoo;
  bool updating = false;

  Widget getFoo() {
    return new Text(stringFoo);
  }

  Future<void> _update() async {
    // await something
    // stringFoo = await result
    updating = false;
    stateChanged();
  }

  void update() {
    updating = true;
    stateChanged();
    _update();
  }

  void stateChanged() {}
}

class Bar extends StatefulWidget {
  final stringBar;

  Bar({Key key, this.stringBar}) : super(key: key);

  @override
  BarState createState() => new BarState();
}

class BarState extends State<Bar> with Foo {
  bool dummy = true;

  Widget getBar() {
    return new Text(widget.stringBar);
  }

  @override
  void stateChanged() {
    setState(() {
      if(dummy) {
        dummy = false;
      } else {
        dummy = true;
      }
    });
  }

  @override
  Widget build(BuildContext context) {
    if(updating) {
      return getBar();
    } else {
      return getFoo();
    }
  }

  @override
  void initState() {
    super.initState();
    update();
  }
}

Answer 1

这可以使用模板来完成。

我找到了一个有效的案例，所以我将使用它。这是一个例子......

基类是：

• class BasePage extends StatefulWidget
• class BasePageState<T extends BasePage> extends State<T>

请注意，您要继承的类必须是公共的，因此类的名称是 BasePage 和 BasePageState 而不是 _BasePage 和 _BasePageState。

还要注意BasePageState 被声明为模板，因此从它们继承的状态类可以在那里传递它们自己的小部件。

继承的类有：

• class MyHomePage extends BasePage
• class _MyHomePageState extends BasePageState<MyHomePage>
• class MyHomePage2 extends BasePage
• class _MyHomePageState2 extends BasePageState<MyHomePage2>

counter 变量和函数incrementCounter() 定义在BasePageState 类中，函数将计数器加1。

在_MyHomePageState2 类中，我重写了incrementCounter() 函数，使其计数器增加2。

我还在测试它，所以不知道我是否会遇到一些陷阱，但到目前为止它似乎正在工作。

这里是飞镖文件...

main.dart：

import 'package:flutter/material.dart';
import 'package:svipopusti/main2.dart';
import 'base_page.dart';

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

class MyApp extends StatelessWidget {
  // This widget is the root of your application.
  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      title: 'Flutter Demo',
      theme: ThemeData(
        // This is the theme of your application.
        //
        // Try running your application with "flutter run". You'll see the
        // application has a blue toolbar. Then, without quitting the app, try
        // changing the primarySwatch below to Colors.green and then invoke
        // "hot reload" (press "r" in the console where you ran "flutter run",
        // or simply save your changes to "hot reload" in a Flutter IDE).
        // Notice that the counter didn't reset back to zero; the application
        // is not restarted.
        primarySwatch: Colors.blue,
      ),
      home: MyHomePage(title: 'HomePage'),
    );
  }
}

class MyHomePage extends BasePage {
  MyHomePage({Key key, this.title}) : super(key: key);

  // This widget is the home page of your application. It is stateful, meaning
  // that it has a State object (defined below) that contains fields that affect
  // how it looks.

  // This class is the configuration for the state. It holds the values (in this
  // case the title) provided by the parent (in this case the App widget) and
  // used by the build method of the State. Fields in a Widget subclass are
  // always marked "final".

  final String title;

//  @override
  _MyHomePageState createState() => _MyHomePageState();
}

class _MyHomePageState extends BasePageState<MyHomePage> {

  @override
  Widget build(BuildContext context) {
    // This method is rerun every time setState is called, for instance as done
    // by the _incrementCounter method above.
    //
    // The Flutter framework has been optimized to make rerunning build methods
    // fast, so that you can just rebuild anything that needs updating rather
    // than having to individually change instances of widgets.
    return Scaffold(
      appBar: AppBar(
        // Here we take the value from the MyHomePage object that was created by
        // the App.build method, and use it to set our appbar title.
        title: Text(widget.title),
      ),
      body: Center(
        // Center is a layout widget. It takes a single child and positions it
        // in the middle of the parent.
        child: Column(
          // Column is also a layout widget. It takes a list of children and
          // arranges them vertically. By default, it sizes itself to fit its
          // children horizontally, and tries to be as tall as its parent.
          //
          // Invoke "debug painting" (press "p" in the console, choose the
          // "Toggle Debug Paint" action from the Flutter Inspector in Android
          // Studio, or the "Toggle Debug Paint" command in Visual Studio Code)
          // to see the wireframe for each widget.
          //
          // Column has various properties to control how it sizes itself and
          // how it positions its children. Here we use mainAxisAlignment to
          // center the children vertically; the main axis here is the vertical
          // axis because Columns are vertical (the cross axis would be
          // horizontal).
          mainAxisAlignment: MainAxisAlignment.center,
          children: <Widget>[
            FlatButton(
              child: Text("Open main2"),
              onPressed: () => Navigator.push(context, MaterialPageRoute(builder: (context) => MyHomePage2(title: "Homepage 2"))),
            ),
            Text(
              'You have pushed the button this many times:',
            ),
            Text(
              '$counter',
              style: Theme.of(context).textTheme.headline4,
            ),
          ],
        ),
      ),
      floatingActionButton: FloatingActionButton(
        onPressed: incrementCounter,
        tooltip: 'Increment',
        child: Icon(Icons.add),
      ), // This trailing comma makes auto-formatting nicer for build methods.
    );
  }
}

main2.dart：

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


class MyHomePage2 extends BasePage {
  MyHomePage2({Key key, this.title}) : super(key: key);

  // This widget is the home page of your application. It is stateful, meaning
  // that it has a State object (defined below) that contains fields that affect
  // how it looks.

  // This class is the configuration for the state. It holds the values (in this
  // case the title) provided by the parent (in this case the App widget) and
  // used by the build method of the State. Fields in a Widget subclass are
  // always marked "final".

  final String title;

//  @override
  _MyHomePageState2 createState() => _MyHomePageState2();
}

class _MyHomePageState2 extends BasePageState<MyHomePage2> {

  @override
  void incrementCounter() {
    setState(() {
      // This call to setState tells the Flutter framework that something has
      // changed in this State, which causes it to rerun the build method below
      // so that the display can reflect the updated values. If we changed
      // _counter without calling setState(), then the build method would not be
      // called again, and so nothing would appear to happen.
      counter += 2;
    });
  }

  @override
  Widget build(BuildContext context) {
    // This method is rerun every time setState is called, for instance as done
    // by the _incrementCounter method above.
    //
    // The Flutter framework has been optimized to make rerunning build methods
    // fast, so that you can just rebuild anything that needs updating rather
    // than having to individually change instances of widgets.
    return Scaffold(
      appBar: AppBar(
        // Here we take the value from the MyHomePage object that was created by
        // the App.build method, and use it to set our appbar title.
        title: Text(widget.title),
      ),
      body: Center(
        // Center is a layout widget. It takes a single child and positions it
        // in the middle of the parent.
        child: Column(
          // Column is also a layout widget. It takes a list of children and
          // arranges them vertically. By default, it sizes itself to fit its
          // children horizontally, and tries to be as tall as its parent.
          //
          // Invoke "debug painting" (press "p" in the console, choose the
          // "Toggle Debug Paint" action from the Flutter Inspector in Android
          // Studio, or the "Toggle Debug Paint" command in Visual Studio Code)
          // to see the wireframe for each widget.
          //
          // Column has various properties to control how it sizes itself and
          // how it positions its children. Here we use mainAxisAlignment to
          // center the children vertically; the main axis here is the vertical
          // axis because Columns are vertical (the cross axis would be
          // horizontal).
          mainAxisAlignment: MainAxisAlignment.center,
          children: <Widget>[
            Text(
              'You have pushed the button this many times:',
            ),
            Text(
              '$counter',
              style: Theme.of(context).textTheme.headline4,
            ),
          ],
        ),
      ),
      floatingActionButton: FloatingActionButton(
        onPressed: incrementCounter,
        tooltip: 'Increment',
        child: Icon(Icons.add),
      ), // This trailing comma makes auto-formatting nicer for build methods.
    );
  }
}

base_page.dart：

import 'package:flutter/material.dart';

class BasePage extends StatefulWidget {
  BasePage({Key key}) : super(key: key);

  @override
  BasePageState createState() => BasePageState();
}

class BasePageState<T extends BasePage> extends State<T> {
  int counter = 0;

  void incrementCounter() {
    setState(() {
      // This call to setState tells the Flutter framework that something has
      // changed in this State, which causes it to rerun the build method below
      // so that the display can reflect the updated values. If we changed
      // _counter without calling setState(), then the build method would not be
      // called again, and so nothing would appear to happen.
      counter++;
    });
  }

  @override
  Widget build(BuildContext context) {
    return Container();
  }
}

Answer 2

不要。您应该从不扩展小部件。这是反模式。相反，正如颤振文档所述：

您可以通过组合小部件来构建更复杂的小部件来创建布局。

一个例子是：

class Foo extends StatefulWidget {
  final Widget child;

  Foo({this.child});

  @override
  _FooState createState() => new _FooState();
}

class _FooState extends State<Foo> {
  @override
  Widget build(BuildContext context) {
    return new Container(
      child: widget.child
    );
  }
}



class Bar extends StatefulWidget {
  final Widget child;

  Bar({this.child});

  @override
  _BarState createState() => new _BarState();
}

class _BarState extends State<Bar> {
  @override
  Widget build(BuildContext context) {
    return new Foo(
      child: widget.child
    );
  }
}

在这种情况下，Bar 没有混合或继承。它只是将它的孩子包裹在 Foo 中。

Answer 3

我不认为小部件的继承或混合（通常是扩展）是一种反模式（尽管 Flutter 团队显然是这样认为的）。

我通常喜欢inheritance answer，但令人讨厌的是超类的属性丢失了。因此，如果该类具有字段 title，那么 widget.title 将不适用于建议的答案。

因此，我更喜欢使用 mixin 进行扩展。我这里举个例子，或者直接看gist

// Flutter example with common navigation bar in app
import 'package:flutter/material.dart';

void main() {
  runApp(const MyApp());
}


class MyApp extends StatelessWidget {
  const MyApp({Key? key}) : super(key: key);

  // Root of application
  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      // Define routing information
      initialRoute: '/home',
      routes: {
        '/home': (context) => const Home(),
        '/page': (context) => const Page(),
      },
    );
  }
}


mixin BottomNavigationMixin<T extends StatefulWidget> on State<T>{
  final List<String> _pages = [
    '/home',
    '/page',
  ];

  Widget bottomNavigationBar(BuildContext context, _selectedIndex) {
    return BottomNavigationBar(
      items: const <BottomNavigationBarItem>[
        BottomNavigationBarItem(
          icon: Icon(Icons.home),
          label: 'Home',
        ),
        BottomNavigationBarItem(
          icon: Icon(Icons.analytics_rounded),
          label: 'Page',
        ),
      ],
      currentIndex: _selectedIndex,
      onTap: (index) {
        if (index != _selectedIndex) {
          setState(() {
            _selectedIndex = index;
            Navigator.pushNamed(context, _pages[index]);
          });
        }
      },
    );
  }
}


class Home extends StatefulWidget {
  const Home({Key? key}) : super(key: key);

  @override
  _HomeState createState() => _HomeState();
}

class _HomeState extends State<Home> with BottomNavigationMixin<Home>{
  final int _selectedIndex = 0;

  @override
  Widget build(BuildContext context) {
    return Scaffold(
      appBar: AppBar(title: const Text("home")),
      bottomNavigationBar: bottomNavigationBar(context, _selectedIndex),
    );
  }
}


class Page extends StatefulWidget {
  const Page({Key? key}) : super(key: key);

  @override
  _PageState createState() => _PageState();
}

class _PageState extends State<Page> with BottomNavigationMixin<Page>{
  final int _selectedIndex = 1;

  @override
  Widget build(BuildContext context) {
    return Scaffold(
      appBar: AppBar(title: const Text("page")),
      bottomNavigationBar: bottomNavigationBar(context, _selectedIndex),
    );
  }
}
```

Answer 4

解决方案：

// Foo
class Foo extends StatefulWidget {

  // Constructor
  Foo({Key key, this.title}) : super(key: key);

  final String title;

  @override
  FooState createState() => new FooState();
}

// FooState
class FooState<T extends Foo> extends State<T> {
  // Override build
  @override
  Widget build(BuildContext context) {
    return Container();
  }
}

// Bar
class Bar extends Foo {

  // Constructor
  Bar({Key key, String title}) : super(key: key, title: title);

  @override
  BarState createState() => new BarState();
}

// BarState
class BarState extends FooState<Bar> {

}