浅谈React Event实现原理

 // 循环 eventQueue调用
 var executeDispatchesAndReleaseTopLevel = function (e) {
 return executeDispatchesAndRelease(e, false);
 };
 /* 从event._dispatchListener 取出 dispatchlistener，然后dispatch事件，
 * 循环_dispatchListeners，调用executeDispatch
 */
 var executeDispatchesAndRelease = function (event, simulated) {
 if (event) {
 // 在这里dispatch事件
 EventPluginUtils.executeDispatchesInOrder(event, simulated);
 // 释放事件
 if (!event.isPersistent()) {
 event.constructor.release(event);
 }
 }
 };

 enqueueEvents: function (events) {
 if (events) {
 eventQueue = accumulateInto(eventQueue, events);
 }
 },

 /**
 * Dispatches all synthetic events on the event queue.
 *
 * @internal
 */
 processEventQueue: function (simulated) {
 // Set `eventQueue` to null before processing it so that we can tell if more
 // events get enqueued while processing.
 var processingEventQueue = eventQueue;
 eventQueue = null;
 if (simulated) {
 forEachAccumulated(processingEventQueue, executeDispatchesAndReleaseSimulated);
 } else {
 forEachAccumulated(processingEventQueue, executeDispatchesAndReleaseTopLevel);
 }
 // This would be a good time to rethrow if any of the event fexers threw.
 ReactErrorUtils.rethrowCaughtError();
 },
/**
 * Standard/simple iteration through an event's collected dispatches.
 */
function executeDispatchesInOrder(event, simulated) {
 var dispatchListeners = event._dispatchListeners;
 var dispatchInstances = event._dispatchInstances;

 if (Array.isArray(dispatchListeners)) {
 for (var i = 0; i < dispatchListeners.length; i++) {
 // 由这里可以看出，合成事件的stopPropagation只能阻止react合成事件的冒泡，
 // 因为event._dispatchListeners 只记录了由jsx绑定的绑定的事件，对于原生绑定的是没有记录的
 if (event.isPropagationStopped()) {
 break;
 }
 // Listeners and Instances are two parallel arrays that are always in sync.
 executeDispatch(event, simulated, dispatchListeners[i], dispatchInstances[i]);
 }
 } else if (dispatchListeners) {
 executeDispatch(event, simulated, dispatchListeners, dispatchInstances);
 }
 event._dispatchListeners = null;
 event._dispatchInstances = null;
}
function executeDispatch(event, simulated, listener, inst) {
 var type = event.type || 'unknown-event';
 // 注意这里将事件对应的dom元素绑定到了currentTarget上
 event.currentTarget = EventPluginUtils.getNodeFromInstance(inst);
 if (simulated) {
 ReactErrorUtils.invokeGuardedCallbackWithCatch(type, listener, event);
 } else {
 // 一般都是非模拟的情况，执行invokeGuardedCallback
 ReactErrorUtils.invokeGuardedCallback(type, listener, event);
 }
 event.currentTarget = null;
}

var fakeNode = document.createElement('react');
ReactErrorUtils.invokeGuardedCallback = function(name, func, a, b) {
 var boundFunc = func.bind(null, a, b);
 var evtType = `react-${name}`;
 fakeNode.addEventListener(evtType, boundFunc, false);
 var evt = document.createEvent('Event');
 evt.initEvent(evtType, false, false);
 fakeNode.dispatchEvent(evt);
 fakeNode.removeEventListener(evtType, boundFunc, false);
 };