React源码解读系列 -- 事件机制
loveyou366
7年前
<h2>React源码解读系列 – 事件机制</h2> <p>本文首先分析React在DOM事件上的架构设计、相关优化、合成事件(Synethic event)对象，从源码层面上做到庖丁解牛的效果。同时，简单介绍下react事件可能会遇到的问题。</p> <h2>1. 总体设计</h2> <p>react在事件处理上具有如下优点:</p> <ul> <li>几乎所有的事件代理(delegate)到 document ，达到性能优化的目的</li> <li>对于每种类型的事件，拥有统一的分发函数 dispatchEvent</li> <li>事件对象(event)是合成对象(SyntheticEvent)，不是原生的</li> </ul> <p style="text-align:center"><img src="https://simg.open-open.com/show/5305a8410c719e10b0ea8b594dab335d.jpg"></p> <p>react内部事件系统实现可以分为两个阶段: 事件注册、事件触发。</p> <h2>2. 事件注册</h2> <p><a href="/misc/goto?guid=4959740613249941013" rel="nofollow,noindex">ReactDOMComponent</a> 在进行组件加载(mountComponent)、更新(updateComponent)的时候，需要对 props 进行处理(_updateDOMProperties):</p> <pre> <code class="language-javascript">ReactDOMComponent.Mixin = { _updateDOMProperties: function (lastProps, nextProps, transaction) { ... for (propKey in nextProps) { // 判断是否为事件属性 if (registrationNameModules.hasOwnProperty(propKey)) { enqueuePutListener(this, propKey, nextProp, transaction); } } } } function enqueuePutListener(inst, registrationName, listener, transaction) { ... var doc = isDocumentFragment ? containerInfo._node : containerInfo._ownerDocument; listenTo(registrationName, doc); transaction.getReactMountReady().enqueue(putListener, { inst: inst, registrationName: registrationName, listener: listener }); function putListener() { var listenerToPut = this; EventPluginHub.putListener(listenerToPut.inst, listenerToPut.registrationName, listenerToPut.listener); } }</code></pre> <p>代码解析:</p> <ul> <li>在props渲染的时候，如何属性是事件属性，则会用 enqueuePutListener 进行事件注册</li> <li>上述 transaction 是ReactUpdates.ReactReconcileTransaction的实例化对象</li> <li>enqueuePutListener进行两件事情: 在 document 上注册相关的事件；对事件进行存储</li> </ul> <h3>2.1 document上事件注册</h3> <p>document的事件注册入口位于 ReactBrowserEventEmitter :</p> <pre> <code class="language-javascript">// ReactBrowserEventEmitter.js listenTo: function (registrationName, contentDocumentHandle) { ... if (...) { ReactBrowserEventEmitter.ReactEventListener.trapBubbledEvent(...); } else if (...) { ReactBrowserEventEmitter.ReactEventListener.trapCapturedEvent(...); } ... } // ReactEventListener.js var ReactEventListener = { ... trapBubbledEvent: function (topLevelType, handlerBaseName, element) { ... var handler = ReactEventListener.dispatchEvent.bind(null, topLevelType); return EventListener.listen(element, handlerBaseName, handler); }, trapCapturedEvent: function (topLevelType, handlerBaseName, element) { var handler = ReactEventListener.dispatchEvent.bind(null, topLevelType); return EventListener.capture(element, handlerBaseName, handler); } dispatchEvent: function (topLevelType, nativeEvent) { ... ReactUpdates.batchedUpdates(handleTopLevelImpl, bookKeeping); ... } } function handleTopLevelImpl(bookKeeping) { ... ReactEventListener._handleTopLevel(bookKeeping.topLevelType, targetInst, bookKeeping.nativeEvent, getEventTarget(bookKeeping.nativeEvent)); ... }</code></pre> <p>代码解析:</p> <ul> <li>事件的注册、触发，具体是在 ReactEventListener 中实现的</li> <li>事件的注册有两个方法: 支持冒泡(trapBubbledEvent)、trapCapturedEvent</li> <li>document不管注册的是什么事件，具有统一的回调函数 handleTopLevelImpl</li> <li>document的回调函数中不包含任何的事物处理，只起到事件分发的作用</li> </ul> <h3>2.2 回调函数存储</h3> <p>函数的存储，在 ReactReconcileTransaction 事务的close阶段执行:</p> <pre> <code class="language-javascript">transaction.getReactMountReady().enqueue(putListener, { inst: inst, registrationName: registrationName, listener: listener }); function putListener() { var listenerToPut = this; EventPluginHub.putListener(listenerToPut.inst, listenerToPut.registrationName, listenerToPut.listener); }</code></pre> <p>事件的存储由 EventPluginHub 来进行管理，来看看其中的具体实现:</p> <pre> <code class="language-javascript">// var listenerBank = {}; var getDictionaryKey = function (inst) { return '.' + inst._rootNodeID; } var EventPluginHub = { putListener: function (inst, registrationName, listener) { ... var key = getDictionaryKey(inst); var bankForRegistrationName = listenerBank[registrationName] || (listenerBank[registrationName] = {}); bankForRegistrationName[key] = listener; ... } }</code></pre> <p>react中的所有事件的回调函数均存储在 listenerBank 对象里面，根据事件类型、component对象的_rootNodeID为两个key，来存储对应的回调函数。</p> <h2>3. 事件的执行</h2> <p>事件注册完之后，就可以依据事件委托进行事件的执行。由事件注册可以知道，几乎所有的事件均委托到document上，而document上事件的回调函数只有一个: ReactEventListener.dispatchEvent，然后进行相关的分发:</p> <pre> <code class="language-javascript">var ReactEventListener = { dispatchEvent: function (topLevelType, nativeEvent) { ... ReactUpdates.batchedUpdates(handleTopLevelImpl, bookKeeping); ... } } function handleTopLevelImpl(bookKeeping) { var nativeEventTarget = getEventTarget(bookKeeping.nativeEvent); var targetInst = ReactDOMComponentTree.getClosestInstanceFromNode(nativeEventTarget); // 初始化时用ReactEventEmitterMixin注入进来的 ReactEventListener._handleTopLevel(..., nativeEventTarget, targetInst); } // ReactEventEmitterMixin.js var ReactEventEmitterMixin = { handleTopLevel: function (...) { var events = EventPluginHub.extractEvents(...); runEventQueueInBatch(events); } } function runEventQueueInBatch(events) { EventPluginHub.enqueueEvents(events); EventPluginHub.processEventQueue(false); }</code></pre> <p>代码解析:</p> <ul> <li>handleTopLevelImpl: 根据原生的事件对象，找到事件触发的dom元素以及该dom对应的compoennt对象</li> <li>ReactEventEmitterMixin: 一方面生成合成的事件对象，另一方面批量执行定义的回调函数</li> <li>runEventQueueInBatch: 进行批量更新</li> </ul> <h3>3.1 合成事件的生成过程</h3> <p>react中的事件对象不是原生的事件对象，而是经过处理后的对象，下面从源码层面解析是如何生成的:</p> <pre> <code class="language-javascript">// EventPluginHub.js var EventPluginHub = { extractEvents: function (...) { var events; var plugins = EventPluginRegistry.plugins; for (var i = 0; i < plugins.length; i++) { var possiblePlugin = plugins[i]; if (possiblePlugin) { var extractedEvents = possiblePlugin.extractEvents(topLevelType, targetInst, nativeEvent, nativeEventTarget); if (extractedEvents) { events = accumulateInto(events, extractedEvents); } } } return events; } }</code></pre> <p>EventPluginHub不仅存储事件的回调函数，而且还管理其中不同的plugins，这些plugins是在系统启动过程中注入(injection)过来的:</p> <pre> <code class="language-javascript">// react-dom模块的入口文件ReactDOM.js: var ReactDefaultInjection = require('./ReactDefaultInjection'); ReactDefaultInjection.inject(); ... // ReactDefaultInjection.js module.exports = { inject: inject }; function inject() { ... ReactInjection.EventPluginHub.injectEventPluginsByName({ SimpleEventPlugin: SimpleEventPlugin, EnterLeaveEventPlugin: EnterLeaveEventPlugin, ChangeEventPlugin: ChangeEventPlugin, SelectEventPlugin: SelectEventPlugin, BeforeInputEventPlugin: BeforeInputEventPlugin }); ... }</code></pre> <p>从上面代码可以看到，默认情况下，react注入了五种事件plugin，针对不同的事件，得到不同的合成事件，以最常见的 SimpleEventPlugin 为例进行分析:</p> <pre> <code class="language-javascript">var SimpleEventPlugin = { extractEvents: function (topLevelType, ...) { var EventConstructor; switch (topLevelType) { EventConstructor = one of [ SyntheticEvent, SyntheticKeyboardEvent, SyntheticFocusEvent, SyntheticMouseEvent, SyntheticDragEvent, SyntheticTouchEvent, SyntheticAnimationEvent, SyntheticTransitionEvent, SyntheticUIEvent, SyntheticWheelEvent, SyntheticClipboardEvent]; } var event = EventConstructor.getPooled(dispatchConfig, targetInst, nativeEvent, nativeEventTarget); EventPropagators.accumulateTwoPhaseDispatches(event); return event; } }</code></pre> <p>代码解析:</p> <ul> <li>针对不同的事件类型，会生成不同的合成事件</li> <li>EventPropagators.accumulateTwoPhaseDispatches: 用于从EventPluginHub中获取回调函数，后面小节会具体分析获取过程</li> </ul> <p>以其中的最基本的 SyntheticEvent 为例进行分析:</p> <pre> <code class="language-javascript">function SyntheticEvent(dispatchConfig, targetInst, nativeEvent, nativeEventTarget) { ... this.dispatchConfig = dispatchConfig; this._targetInst = targetInst; this.nativeEvent = nativeEvent; var Interface = this.constructor.Interface; for (var propName in Interface) { var normalize = Interface[propName]; if (normalize) { this[propName] = normalize(nativeEvent); } else { if (propName === 'target') { this.target = nativeEventTarget; } else { this[propName] = nativeEvent[propName]; } } } ... } _assign(SyntheticEvent.prototype, { preventDefault: function () { ... }, stopPropagation: function () { ... }, ... }); var EventInterface = { type: null, target: null, // currentTarget is set when dispatching; no use in copying it here currentTarget: emptyFunction.thatReturnsNull, eventPhase: null, bubbles: null, cancelable: null, timeStamp: function (event) { return event.timeStamp || Date.now(); }, defaultPrevented: null, isTrusted: null }; SyntheticEvent.Interface = EventInterface; // 实现继承关系 SyntheticEvent.augmentClass = function (Class, Interface) { ... }</code></pre> <h3>3.2 获取具体的回调函数</h3> <p>上述合成事件对象在生成的过程中，会从 EventPluginHub 处获取相关的回调函数，具体实现如下:</p> <pre> <code class="language-javascript">// EventPropagators.js function accumulateTwoPhaseDispatches(events) { forEachAccumulated(events, accumulateTwoPhaseDispatchesSingle); } function accumulateTwoPhaseDispatchesSingle(event) { if (event && event.dispatchConfig.phasedRegistrationNames) { EventPluginUtils.traverseTwoPhase(event._targetInst, accumulateDirectionalDispatches, event); } } function accumulateDirectionalDispatches(inst, phase, event) { var listener = listenerAtPhase(inst, event, phase); if (listener) { event._dispatchListeners = accumulateInto(event._dispatchListeners, listener); event._dispatchInstances = accumulateInto(event._dispatchInstances, inst); } } var getListener = EventPluginHub.getListener; function listenerAtPhase(inst, event, propagationPhase) { var registrationName = event.dispatchConfig.phasedRegistrationNames[propagationPhase]; return getListener(inst, registrationName); } // EventPluginHub.js getListener: function (inst, registrationName) { var bankForRegistrationName = listenerBank[registrationName]; var key = getDictionaryKey(inst); return bankForRegistrationName && bankForRegistrationName[key]; },</code></pre> <h3>3.3 批量执行事件的具体回调函数</h3> <p>react会进行批量处理具体的回调函数，回调函数的执行为了两步，第一步是将所有的合成事件放到事件队列里面，第二步是逐个执行:</p> <pre> <code class="language-javascript">var eventQueue = null; var EventPluginHub = { enqueueEvents: function (events) { if (events) { eventQueue = accumulateInto(eventQueue, events); } }, processEventQueue: function (simulated) { var processingEventQueue = eventQueue; ... forEachAccumulated(processingEventQueue, executeDispatchesAndReleaseSimulated); ... }, } var executeDispatchesAndReleaseSimulated = function (e) { return executeDispatchesAndRelease(e, true); }; var executeDispatchesAndRelease = function (event, simulated) { if (event) { EventPluginUtils.executeDispatchesInOrder(event, simulated); if (!event.isPersistent()) { event.constructor.release(event); } } }; // EventPluginUtils.js function executeDispatchesInOrder(event, simulated) { var dispatchListeners = event._dispatchListeners; var dispatchInstances = event._dispatchInstances; ... executeDispatch(event, simulated, dispatchListeners, dispatchInstances); ... event._dispatchListeners = null; event._dispatchInstances = null; }</code></pre> <h2>4. 可能存在的问题</h2> <h3>4.1 合成事件与原生事件混用</h3> <p>在开发过程中，有时候需要使用到原生事件，例如存在如下的业务场景: 点击input框展示日历，点击文档其他部分，日历消失，代码如下:</p> <pre> <code class="language-javascript">// js部分 var React = require('react'); var ReactDOM = require('react-dom'); class App extends React.Component { constructor(props) { super(props); this.state = { showCalender: false }; } componentDidMount() { document.addEventListener('click', () => { this.setState({showCalender: false}); console.log('it is document') }, false); } render() { return (<div> <input type="text" onClick={(e) => { this.setState({showCalender: true}); console.log('it is button') e.stopPropagation(); }} /> <Calendar isShow={this.state.showCalender}></Calendar> </div>); } }</code></pre> <p>上述代码: 在点击input的时候，state状态变成true，展示日历，同时阻止冒泡，但是document上的click事件仍然触发了？到底是什么原因造成的呢？</p> <p>原因解读: 因为react的事件基本都是委托到document上的，并没有真正绑定到input元素上，所以在react中执行stopPropagation并没有什么用处，document上的事件依然会触发。</p> <p>解决办法:</p> <p>4.1.1 input的onClick事件也使用原生事件</p> <pre> <code class="language-javascript">class App extends React.Component { constructor(props) { super(props); this.state = { showCalender: false }; } componentDidMount() { document.addEventListener('click', () => { this.setState({showCalender: false}); console.log('it is document') }, false); this.refs.myBtn.addEventListener('click', (e) => { this.setState({showCalender: true}); e.stopPropagation(); }, false); } render() { return (<div> <input type="text" ref="myBtn" /> <Calendar isShow={this.state.showCalender}></Calendar> </div>); } }</code></pre> <p>4.1.2 在document中进行判断，排除目标元素</p> <pre> <code class="language-javascript">class App extends React.Component { constructor(props) { super(props); this.state = { showCalender: false }; } componentDidMount() { document.addEventListener('click', (e) => { var tar = document.getElementById('myInput'); if (tar.contains(e.target)) return; console.log('document!!!'); this.setState({showCalender: false}); }, false); } render() { return (<div> <input id="myInput" type="text" onClick={(e) => { this.setState({showCalender: true}); console.log('it is button') // e.stopPropagation(); }} /> <Calendar isShow={this.state.showCalender}></Calendar> </div>); } }</code></pre> <h2>5. 小结</h2> <p>React在设计事件机制的时候，利用冒泡原理充分提高事件绑定的效率，使用 EventPluginHub 对回调函数、事件插件进行管理，然后通过一个统一的入口函数实现事件的分发，整个设计思考跟jQuery的事件实现上存在相似的地方，非常值得学习借鉴。</p> <p> </p> <p>来自：http://zhenhua-lee.github.io/react/react-event.html</p> <p> </p>