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Flutter页面传值的几种方式

运维开发网 https://www.qedev.com 2021-04-28 08:25 出处:网络 作者: wangfeng6075lv-1
今天来聊聊Flutter页面传值的几种方式: InheritWidget Notification Eventbus (当前Flutter版本:2.0.4)

今天来聊聊Flutter页面传值的几种方式:

  • InheritWidget
  • Notification
  • Eventbus

(当前Flutter版本:2.0.4)

InheritWidget

如果看过Provider的源码的同学都知道,Provider跨组件传值的原理就是根据系统提供的InheritWidget实现的,让我们来看一下这个组件。

InheritWidget是一个抽象类,我们写一个保存用户信息的类UserInfoInheritWidget继承于InheritWidget:

class UserInfoInheritWidget extends InheritedWidget {

  UserInfoBean userInfoBean;
  UserInfoInheritWidget({Key key, this.userInfoBean, Widget child}) : super (child: child);

  static UserInfoWidget of(BuildContext context){
    return context.dependOnInheritedWidgetOfExactType<UserInfoWidget>();
  }
  
  @override
  bool updateShouldNotify(UserInfoInheritWidget oldWidget) {
    return oldWidget.userInfoBean != userInfoBean;
  }
}

我们在这里面定义了一个静态方法:of,并且传入了一个context,根据context获取当前类,拿到当前类中的UserInfoBean,其实获取主题数据也是根据InheritWidget这种方式获取Theme.of(context),关于of方法后面重点讲一下,updateShouldNotify是刷新机制,什么时候刷新数据

还有一个用户信息的实体:

class UserInfoBean {
  String name;
  String address;
  UserInfoBean({this.name, this.address});
}

我们做两个页面,第一个页面显示用户信息,还有一个按钮,点击按钮跳转到第二个页面,同样也是显示用户信息:

class Page19PassByValue extends StatefulWidget {
  @override
  _Page19PassByValueState createState() => _Page19PassByValueState();
}

class _Page19PassByValueState extends State<Page19PassByValue> {

  @override
  Widget build(BuildContext context) {
    return Scaffold(
      appBar: AppBar(
        title: Text('PassByValue'),
      ),
      body: DefaultTextStyle(
        style: TextStyle(fontSize: 30, color: Colors.black),
        child: Column(
          children: [
            Text(UserInfoWidget.of(context)!.userInfoBean.name),
            Text(UserInfoWidget.of(context)!.userInfoBean.address),
            SizedBox(height: 40),
            TextButton(
              child: Text('点击跳转'),
              onPressed: (){
                Navigator.of(context).push(CupertinoPageRoute(builder: (context){
                  return DetailPage();
                }));
              },
            )
          ],
        ),
      ),
    );
  }
}

class DetailPage extends StatefulWidget {
  @override
  _DetailPageState createState() => _DetailPageState();
}

class _DetailPageState extends State<DetailPage> {
  @override
  Widget build(BuildContext context) {
    return Scaffold(
      appBar: AppBar(
        title: Text('Detail'),
      ),
      body: DefaultTextStyle(
        style: TextStyle(fontSize: 30, color: Colors.black),
        child: Center(
          child: Column(
            children: [
              Text(UserInfoWidget.of(context).userInfoBean.name),
              Text(UserInfoWidget.of(context).userInfoBean.address),
              TextButton(
                    onPressed: () {
                      setState(() {
                        UserInfoWidget.of(context)!.updateBean('wf123','address123');
                      });
                    },
                    child: Text('点击修改'))
            ],
          ),
        ),
      )
    );
  }
}

由于我们这里是跨组件传值,需要把UserInfoWidget放在MaterialApp的上层,并给UserInfoBean一个初始值:

class MyApp extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return UserInfoWidget(
      userInfoBean: UserInfoBean(name: 'wf', address: 'address'),
      child: MaterialApp(
        title: 'Flutter Demo',
        theme: ThemeData(
          primarySwatch: Colors.blue,
          visualDensity: VisualDensity.adaptivePlatformDensity,
        ),
        home: MyHomePage(title: 'Flutter Demo Home Page'),
      ),
    );
  }
}

这样就实现了一个跨组件传值,但是还有个问题,我们给UserInfoWidget赋值的时候是在最顶层,在真实业务场景中,如果我们把UserInfwww.cppcns.como的赋值放在MaterialApp上面,这时候我们还没拿到用户数据呢,所以就要有一个可以更新UserInfo的方法,并且修改后立即刷新,我们可以借助setState,把我们上面定义的UserInfoWidget改个名字然后封装在StatefulWidget 中:

class _UserInfoInheritWidget extends InheritedWidget {

  UserInfoBean userInfoBean;
  Function update;
  _UserInfoInheritWidget({Key key, this.userInfoBean, this.update, Widget child}) : super (child: child);

  updateBean(String name, String address){
    update(name, address);
  }

  @override
  bool updateShouldNotify(_UserInfoInheritWidget oldWidget) {
    return oldWidget.userInfoBean != userInfoBean;
  }
}

class UserInfoWidget extends StatefulWidget {
  UserInfoBean userInfoBean;
  Widget child;
  UserInfoWidget({Key key, this.userInfoBean, this.child}) : super (key: key);

  static _UserInfoInheritWidget of(BuildContext context){
    return context.dependOnInheritedWidgetOfExactType<_UserInfoInheritWidget>();
  }
  @override
  State<StatefulWidget> createState() => _UserInfoState();
}

class _UserInfoState extends State <UserInfoWidget> {

  _update(String name, String address){
    UserInfoBean bean = UserInfoBean(name: name, address: address);
    widget.userInfoBean = bean;
    setState(() {});
  }
  @override
  Widget build(BuildContext context) {
    return _UserInfoInheritWidget(
      child: widget.child,
      userInfoBean: widget.userInfoBean,
      update: _update,
    );
  }
}

上面把继承自InheritWidget的类改了一个名字:_UserInfoInheritWidget,对外只暴露用StatefulWidget封装过的UserInfoWidget,向_UserInfoInheritWidget传入了包含setState的更新数据方法,更新数据的时候通过UserInfoWidget.of(context)获取到继承于InheritWidget的_UserInfoInheritWidget类,调用updateBean方法实际上就调用了包含setState的方法,所以做到了数据更新和页面刷新

Flutter页面传值的几种方式

下面重点说一下UserInfoWidget.of(context)是如何获取到继承于InheritWidget类的对象的,通过查看类似的方法:Theme.of(context)发现是根据dependOnInheritedWidgetOfExactType,于是我们也照着它的样子获取到了_UserInfoInheritWidget,点到dependOnInheritedWidgetOfExactType源码中看一下,发现跳转到了BuildContext中定义了这个方法:

  T? dependOnInheritedWidgetOfExactType<T extends InheritedWidget>({ Object? aspect });

了解Widget、Element、RenderObject三只之间关系的同学都知道,其实context是Element的一个实例,BuildContext的注释也提到了这一点:

Flutter页面传值的几种方式

我们可以在Element中找到这个方法的实现:

@override
  Thttp://www.cppcns.com3; dependOnInheritedWidgetOfExactType<T extends InheritedWidget>({Object? aspect}) {
    assert(_debugCheckStateIsActiveForAncestorLookup());
    final InheritedElement? ancestor = _inheritedWidgets == null ? null : _inheritedWidgets![T];
    if (ancestor != null) {
      assert(ancestor is InheritedElement);
      return dependOnInheritedElement(ancestor, aspect: aspect) as T;
    }
    _hadUnsatisfiedDependencies = true;
    return null;
  }

_inheritedWidgets是从哪来的,我们搜索一下在Element中发现

void _updateInheritance() {
    assert(_lifecycleState == _ElementLifecycle.active);
    _inheritedWidgets = _parent?._inheritedWidgets;
  }

再看一下_updateInheritance方法是什么时候调用的

@mustCallSuper
  void mount(Element? parent, dynamic newSlot) {
    ...
    ...省略无关代码
    _parent = parent;
    _slot = newSlot;
    _lifecycleState = _ElementLifecycle.active;
    _depth = _parent != null ? _parent!.depth + 1 : 1;
    if (parent != null) // Only assign ownership if the parent is non-null
      _owner = parent.owner;
    final Key? key = widget.key;
    if (key is GlobalKey) {
      key._register(this);
    }
    _updateInheritance();//这里调用了一次
  }

还有:

@mustCallSuper
  void activate() {
    ...
    ...已省略无关代码
    final bool hadDependencies = (_dependencies != null && _dependencies!.isNotEmpty) || _hadUnsatisfiedDependencies;
    _lifecycleState = _ElementLifecycle.active;
    _dependencies?.clear();
    _hadUnsatisfiedDependencies = false;
    _updateInheritance();//这里又调用了一次
    if (_dirty)
      owner!.scheduleBuildFor(this);
    if (hadDependencies)
      didChangeDependencies();
  }

从上面代码我们可以看到每个页面的Element都会通过_parent向下级传递父级信息,而我们的UserInfoWidget就保存在_parent中的_inheritedWidgets集合中:Map<Type, InheritedElement>? _inheritedWidgets;,当_inheritedWidgets在页面树中向下传递的时候,如果当前Widget是InheritWidget,在当前Widget对应的Element中先看_parent传过来的_inheritedWidgets是否为空,如果为空就新建一个集合,把自己存到这个集合中,以当前的类型作为key(这也是为什么调用of方法中的context.dependOnInheritedWidgetOfExactType方法为什么要传当前类型的原因),从_inheritedWidgets集合中去取值;如果不为空直接把自己存进去,这就是of的原理了。

Notification

上面讲的InheritWidget一般是根部组建向子级组件传值,Notification是从子级组件向父级组件传值,下面我们来看一下它的用法

class Page19PassByValue extends StatefulWidget {
  @override
  _Page19PassByValueState createState() => _Page19PassByValueState();
}

class _Page19PassByValueState extends State<Page19PassByValue> {
  UserInfoBean userInfoBean = UserInfoBean(name: 'wf', address: 'address');

  @override
  Widget build(BuildContext context) {
    return Scaffold(
      appBar: AppBar(
        title: Text('PassByValue'),
      ),
      body: Center(
        child: NotificationListener<编程客栈MyNotification>(
          onNotification: (MyNotification data) {
            userInfoBean = data.userInfoBean;
            setState(() {});
            ///这里需要返回一个bool值,true表示阻止事件继续向上传递,false表示事件可以继续向上传递到父级组件
            return true;
          },
          child: Builder(
          ///这里用了一个Builder包装了一下,为的是能取到
          ///NotificationListener的context
            builder: (context) {
              return Column(
                children: [
                  Text(userInfoBean.name),
                  Text(userInfoBean.address),
                  Container(
                    child: FlatButton(
                      child: Text('点击传值'),
                      onPressed: () {
                        MyNotification(userInfoBean: UserInfoBean(name: 'wf123', address: 'address123')).dispatch(context);
                      },
                    ),
                  )
                ],
              );
            },
          ),
        ),
      ),
    );
  }
}
///Notification是一个抽象类,
///使用Notification需要自定义一个class继承Notification
class MyNotification extends Notification {
  UserInfoBean userInfoBean;
  MyNotification({this.userInfoBean}) : super();
}

我们到源码中看一下这个dispatch方法:

void dispatch(BuildContext target) {
    // The `target` may be null if the subtree the notification is supposed to be
    // dispatched in is in the process of being disposed.
    target?.visitAncestorElements(visitAncestor);
  }

target就是我们传进来的context,也就是调用了BuildContext的visitAncestorElements方法,并且把visitAncestor方法作为一个参数传过去,visitAncestor方法返回一个bool值:

  @protected
  @mustCallSuper
  bool visitAncestor(Element element) {
    if (element is StatelessElement) {
      final StatelessWidget widget = element.widget;
      if (widget is NotificationListener<Notification>) {
        if (widget._dispatch(this, element)) // that function checks the type dynamically
          return false;
      }
    }
    return true;
  }

我们进入Element内部看一下visitAncestorElements方法的实现:

@override
  void visitAncestorElements(bool visitor(Element element)) {
    assert(_debugCheckStateIsActiveForAncestorLookup());
    Element? ancestor = _parent;
    while (ancestor != null && visitor(ancestor))
      ancestor = ancestor._parent;
  }

当有父级节点,并且visitor方法返回true的时候执行while循环,visitor是Notification类传进来的方法,回过头再看visitor方法的实现,当Element向visitor方法传递的ancestor是NotificationListener类的情况下,再判断widget._dispatch方法,而widget._dispatch方法:

final NotificationListenerCallback<T>? onNotification;

  bool _dispatch(Notification notification, Element element) {
    i编程客栈f (onNotification != null && notification is T) {
      final bool result = onNotification!(notification);
      return result == true; // so that null and false have the same effect
    }
    return false;
  }

就是我们在外面写的onNotification方法的实现,我们在外面实现的onNotification方法返回true(即阻止事件继续向上传递),上面的while循环主要是为了执行我们onNotification里面的方法.

总结一下:MyNotification执行dispatch方法,传递context,根据当前context向父级查找对应NotificationListener,并且执行NotificationListener里面的onNotification方法,返回true,则事件不再向上级传递,如果返回false则事件继续向上一个NotificationListener传递,并执行里面对应的方法。Notification主要用在同一个页面中,子级向父级传值,比较轻量级,不过如果我们用了Provider可能就就直接借助Provider传值了。

Eventbus

Eventbus用于两个不同的页面,可以跨多级页面传值,用法也比较简单,我创建了一个EventBusUtil来创建一个单例

import 'package:event_bus/event_bus.dart';

class EventBusUtil {
  static  EventBus ? _instance;
  static EventBus getInstance(){
    if (_instance == null) {
      _instance = EventBus();
    }
    return _instance!;
  }
}

在第一个页面监听:

class Page19PassByValue extends StatefulWidget {
  @override
  _Page19PassByValueState createState() => _Page19PassByValueState();
}

class _Page19PassByValueState extends State<Page19PassByValue> {
  UserInfoBean userInfoBean = UserInfoBean(name: 'wf', address: 'address');
  @override
  void initState() {
    super.initState();
    EventBusUtil.getInstance().on<UserInfoBean>().listen((event) {
      setState(() {
        userInfoBean = event;
      });
    });
  }
  
  @override
  void dispose() {
    super.dispose();
    //不用的时候记得关闭
    EventBusUtil.getInstance().destroy();
  }

  @override
  Widget build(BuildContext context) {
    return Scaffold(
      appBar: AppBar(
        title: Text('PassByValue'),
      ),
      body: Center(
        child: Column(
          mainAxisAlignment: MainAxisAlignment.center,
          children: [
            Text(userInfoBean.name),
            Text(userInfoBean.address),
            TextButton(onPressed: (){
              Navigator.of(context).push(CupertinoPageRoute(builder: (_){
                return EventBusDetailPage();
              }));
            }, child: Text('点击跳转'))

          ],
        ),
      ),
    );
  }
}

在第二个页面发送事件:

class EventBusDetailPage extends StatefulWidget {
  @override
  _EventBusDetailPageState createState() => _EventBusDetailPageState();
}

class _EventBusDetailPageState extends State<EventBusDetailPage> {
  @override
  Widget build(BuildContext context) {
    return Scaffold(
      appBar: AppBar(
        title: Text('EventBusDetail'),
      ),
      body: Center(
        child: TextButton(onPressed: (){
          EventBusUtil.getInstance().fire(UserInfoBean(name: 'name EventBus', address: 'address EventBus'));
        }, child: Text('点击传值')),
      ),
    );
  }
}

我们看一下EventBus的源码,发现只有几十行代码,他的内部是创建了一个StreamController,通过StreamController来实现跨组件传值,我们也可以直接使用一下这个StreamController实现页面传值:

class Page19PassByValue extends StatefulWidget {
  @override
  _Page19PassByValueState createState() => _Page19PassByValueState();
}

StreamController controller = StreamController();

class _Page19PassByValueState extends State<Page19PassByValue> {
  
  //设置一个初始值
  UserInfoBean userInfoBean = UserInfoBean(name: 'wf', address: 'address');
  @override
  void initState() {
    www.cppcns.comsuper.initState();
    controller.stream.listen((event) {
      setState(() {
        userInfoBean = event;
      });
    });
  }

  @override
  void dispose() {
    super.dispose();
    //页面销毁的时候记得关闭
    controller.close();
  }

  @override
  Widget build(BuildContext context) {
    return Scaffold(
      appBar: AppBar(
        title: Text('PassByValue'),
      ),
      body: Center(
        child: Column(
          mainAxisAlignment: MainAxisAlignment.center,
          children: [
            Text(userInfoBean.name),
            Text(userInfoBean.address),
            TextButton(onPressed: (){
              Navigator.of(context).push(CupertinoPageRoute(builder: (_){
                return MyStreamControllerDetail();
              }));
            }, child: Text('点击跳转'))
          ],
        ),
      )
    );
  }
}
class MyStreamControllerDetail extends StatefulWidget {
  @override
  State<StatefulWidget> createState() {
    return _MyStreamControllerDetailState();
  }
}
class _MyStreamControllerDetailState extends State <MyStreamControllerDetail> {
  @override
  Widget build(BuildContext context) {
    return Scaffold(
      appBar: AppBar(
        title: Text('StreamController'),
      ),
      body: Center(
        child: TextButton(onPressed: (){
        //返回上个页面,会发现页面的数据已经变了
          controller.sink.add(UserInfoBean(name: 'StreamController pass name: 123', address: 'StreamController pass address 123'));
        }, child: Text('点击传值'),),
      ),
    );
  }
}

到此这篇关于Flutter页面传值的几种方式的文章就介绍到这了,更多相关Flutter页面传值内容请搜索我们以前的文章或继续浏览下面的相关文章希望大家以后多多支持我们!

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