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Flutter页面传值 Flutter页面传值的几种方式

wangfeng6075lv-1 人气:1
想了解Flutter页面传值的几种方式的相关内容吗,wangfeng6075lv-1在本文为您仔细讲解Flutter页面传值的相关知识和一些Code实例,欢迎阅读和指正,我们先划重点:Flutter,页面传值,下面大家一起来学习吧。

今天来聊聊Flutter页面传值的几种方式:

(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UserInfo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

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:

我们可以在Element中找到这个方法的实现:

@override
  T? 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<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) {
    if (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() {
    super.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'),),
      ),
    );
  }
}

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