flutter系列InheritedWidget介绍

InheritedWidget介绍

在Flutter进行界面开发时，我们经常会遇到数据传递的问题。由于Flutter采用节点树的方式组织页面，以致于一个普通页面的节点层级会很深。此时，我们如果还是一层层传递数据，当需要获取多层父节点的数据时，会非常麻烦。因为出现上述问题，Flutter给我我们提供一种InheritedWidget，InheritedWidget能够让节点下的所有子节点，访问该节点下的数据。关于Scoped Model、BloC、Provider就是基于InheritedWidget实现的。

InheritedWidget源码分析

可以看到InheritedWidget的源码非常简单。

/// 抽象类，继承自Proxywidget 继承路径InheritedWidget => ProxyWidget => Widget
abstract class InheritedWidget extends ProxyWidget {/// 构造函数/// 因为InheritedWidget是没有界面的Widget，所有需要传入实际的Widget    const InheritedWidget({ Key key, Widget child }): super(key: key, child: child);/// 重写了超类Widget createElement方法@overrideInheritedElement createElement() => InheritedElement(this);/// 父级或祖先widget中改变(updateShouldNotify返回true)时会被调用。@protectedbool updateShouldNotify(covariant InheritedWidget oldWidget);
}

InheritedWidget示例

import 'package:flutter/material.dart';
import 'package:flutter_code/InheritedWidget/InheritedState.dart';class InheritedCount extends StatefulWidget {@override_InheritedCountState createState() => _InheritedCountState();
}class _InheritedCountState extends State<InheritedCount> {int _count = 0;@overrideWidget build(BuildContext context) {return Scaffold(appBar: AppBar(title: Text("InheritedDemo"),),floatingActionButton: FloatingActionButton(onPressed: () {setState(() {_count++;});},child: Icon(Icons.add, color: Colors.white,),),body: Center(child: InheritedState(count: _count,child: Column(mainAxisAlignment: MainAxisAlignment.spaceAround,crossAxisAlignment: CrossAxisAlignment.center,children: [WidgetA(),WidgetB()],)),),);}
}class WidgetA extends StatelessWidget {@overrideWidget build(BuildContext context) {return Text("widget text");}
}class WidgetB extends StatelessWidget {@overrideWidget build(BuildContext context) {return Text(InheritedState.of(context)?.count.toString(),style: TextStyle(color: Colors.green,fontSize: 50),  );}
}

import 'package:flutter/cupertino.dart';
import 'package:flutter/material.dart';class InheritedState extends InheritedWidget {/// 构造方法InheritedState({Key key,@required this.count,@required Widget child}): assert(count != null),super(key:key, child: child);/// 需要共享的数据final int count;/// 获取组件最近当前的InheritedWidgetstatic InheritedState of(BuildContext context) {return context.dependOnInheritedWidgetOfExactType<InheritedState>();}/// 通知依赖该树共享数据的子widget@overridebool updateShouldNotify(covariant InheritedState oldWidget) {return count != oldWidget.count;}}

InheritedWidget源码分析

在上面的计数器示例代码中，WidgetB和InheritedWidget发生关联的就是InheritedState.of(context)?.count.toString()，其中最关键的方式是context.dependOnInheritedWidgetOfExactType()，我们查看dependOnInheritedWidgetOfExactType()在Element中的源码如下：该代码是在framework.dart 第3960行

Map<Type, InheritedElement> _inheritedWidgets;@overrideT dependOnInheritedWidgetOfExactType<T extends InheritedWidget>({Object aspect}) {/// 断言，用于在调试状态下检测是否有正在使用(激活)的祖先assert(_debugCheckStateIsActiveForAncestorLookup());/// 获取到_inheritedWidgets数组数据final InheritedElement ancestor = _inheritedWidgets == null ? null : _inheritedWidgets[T];if (ancestor != null) {// 断言，判断当前ancestor是否是InheritedElement类型assert(ancestor is InheritedElement);// 返回并调用更新方法return dependOnInheritedElement(ancestor, aspect: aspect) as T;}_hadUnsatisfiedDependencies = true;return null;}

我们不难看出，每一个Element实例都会持有一个_inheritedWidgets，调用次用该方法时会从改集合对象中取出相关类型的InheritedElement实例，那么在这个方法中我们没有看到设置_inheritedWidgets的方法，我们来查看一下_inheritedWidgets是如何赋值的。

// Element
void _updateInheritance() {assert(_active);_inheritedWidgets = _parent?._inheritedWidgets;}

我们找到赋值是在_updateInheritance方法中，首先断言当前节点是否激活，然后通过父节点的_inheritedWidgets进行赋值，我们继续来看_updateInheritance什么情况下会调用：

 @mustCallSupervoid mount(Element parent, dynamic newSlot) {......_updateInheritance();......}@mustCallSupervoid activate() {......_updateInheritance();......}

我们可以看到在Element中它在mount和activate函数执行了调用，也就是说element每次挂载和重新时，会调用该方法。那么当该方法执行的时候，element就会从上层中拿到所有的InheritedElement。而InheritedElement他最终继承了Element，并可以看到InheritedElement重写了_updateInheritance方法：

 @overridevoid _updateInheritance() {assert(_active);final Map<Type, InheritedElement> incomingWidgets = _parent?._inheritedWidgets;if (incomingWidgets != null)_inheritedWidgets = HashMap<Type, InheritedElement>.from(incomingWidgets);else_inheritedWidgets = HashMap<Type, InheritedElement>();_inheritedWidgets[widget.runtimeType] = this;}

InheritedWidget是如何进行刷新的

前面我们分析到InheritedElement会拿到父类的所有的InheritedElment并向下传递，而InheritedWidget正是通过这种方法才能让下面的子Widget能访问的上层中所有的InheritedWidget，那么它是如何进行刷新的呢？我们在Element的dependOnInheritedWidgetOfExactType方法中调用了dependOnInheritedElement方法，代码如下：

Set<InheritedElement> _dependencies;@override
InheritedWidget dependOnInheritedElement(InheritedElement ancestor, { Object aspect }) {assert(ancestor != null);_dependencies ??= HashSet<InheritedElement>();_dependencies.add(ancestor);ancestor.updateDependencies(this, aspect);return ancestor.widget;
}@protected
void updateDependencies(Element dependent, Object aspect) {setDependencies(dependent, null);
}@protected
void setDependencies(Element dependent, Object value) {_dependents[dependent] = value;
}

可以看到InheritedElement实例调用自己的updateDependencies方法并将当前的Element实例传递过去

 /// Called during build when the [widget] has changed.////// By default, calls [notifyClients]. Subclasses may override this method to/// avoid calling [notifyClients] unnecessarily (e.g. if the old and new/// widgets are equivalent).@protectedvoid updated(covariant ProxyWidget oldWidget) {notifyClients(oldWidget);}@overridevoid notifyClients(InheritedWidget oldWidget) {assert(_debugCheckOwnerBuildTargetExists('notifyClients'));for (final Element dependent in _dependents.keys) {assert(() {// check that it really is our descendantElement ancestor = dependent._parent;while (ancestor != this && ancestor != null)ancestor = ancestor._parent;return ancestor == this;}());// check that it really depends on usassert(dependent._dependencies.contains(this));notifyDependent(oldWidget, dependent);}}
}@protectedvoid notifyDependent(covariant InheritedWidget oldWidget, Element dependent) {dependent.didChangeDependencies();}

由于当InheritedElement更新时，会执行updated方法，然后继续调用notifyClients，遍历所有的element并调用didChangeDependencies方法。