浅谈React Event实现原理

// 事件绑定
function handleClick(e) {
  e.preventDefault();
  console.log('The link was clicked.');
}

 return (
  <a href="#" rel="external nofollow" onClick={handleClick}>
   Click me
  </a>
 );

_updateDOMProperties: function(lastProps, nextProps, transaction) {
...
if (registrationNameModules.hasOwnProperty(propKey)) {
    if (nextProp) {
     // 如果传入的是事件，去注册事件
     enqueuePutListener(this, propKey, nextProp, transaction);
    } else if (lastProp) {
     deleteListener(this, propKey);
    }
   } 
...
}

// 注册事件
function enqueuePutListener(inst, registrationName, listener, transaction) {
 var containerInfo = inst._nativeContainerInfo;
 var doc = containerInfo._ownerDocument;
  ...
  // 去doc上注册
 listenTo(registrationName, doc);
  // 事务结束之后 putListener
 transaction.getReactMountReady().enqueue(putListener, {
  inst: inst,
  registrationName: registrationName,
  listener: listener,
 });
}

//registrationName:需要绑定的事件
//当前component所属的document，即事件需要绑定的位置
listenTo: function (registrationName, contentDocumentHandle) {
  var mountAt = contentDocumentHandle;
  //获取当前document上已经绑定的事件
  var isListening = getListeningForDocument(mountAt);
  ...
   if (...) {
   //冒泡处理 
   ReactBrowserEventEmitter.ReactEventListener.trapBubbledEvent(...);
   } else if (...) {
    //捕捉处理
    ReactBrowserEventEmitter.ReactEventListener.trapCapturedEvent(...);
   }
   ...
 },

function enqueuePutListener(inst, registrationName, listener, transaction) {
 var containerInfo = inst._nativeContainerInfo;
 var doc = containerInfo._ownerDocument;
 if (!doc) {
  // Server rendering.
  return;
 }
 listenTo(registrationName, doc);
 transaction.getReactMountReady().enqueue(putListener, {
  inst: inst,
  registrationName: registrationName,
  listener: listener,
 });
}

var listenerBank = {};
var getDictionaryKey = function (inst) {
//inst为组建的实例化对象
//_rootNodeID为组件的唯一标识
 return '.' + inst._rootNodeID;
}
var EventPluginHub = {
//inst为组建的实例化对象
//registrationName为事件名称
//listner为我们写的回调函数，也就是列子中的this.autoFocus
 putListener: function (inst, registrationName, listener) {
  ...
  var key = getDictionaryKey(inst);
  var bankForRegistrationName = listenerBank[registrationName] || (listenerBank[registrationName] = {});
  bankForRegistrationName[key] = listener;
  ...
 }
}

dispatchEvent: function (topLevelType, nativeEvent) {
  if (!ReactEventListener._enabled) {
   return;
  }
  // 这里得到TopLevelCallbackBookKeeping的实例对象，本例中第一次触发dispatchEvent时
  // bookKeeping instanceof TopLevelCallbackBookKeeping
  // bookKeeping = TopLevelCallbackBookKeeping {topLevelType: "topClick", nativeEvent: "click", ancestors: Array(0)}
  var bookKeeping = TopLevelCallbackBookKeeping.getPooled(topLevelType, nativeEvent);
  try {
   // Event queue being processed in the same cycle allows
   // `preventDefault`.
   // 接着执行handleTopLevelImpl(bookKeeping)
   ReactUpdates.batchedUpdates(handleTopLevelImpl, bookKeeping);
  } finally {
   // 回收
   TopLevelCallbackBookKeeping.release(bookKeeping);
  }
 }

function handleTopLevelImpl(bookKeeping) {
 var nativeEventTarget = getEventTarget(bookKeeping.nativeEvent);
 // 获取当前事件的虚拟dom元素
 var targetInst = ReactDOMComponentTree.getClosestInstanceFromNode(nativeEventTarget);

 var ancestor = targetInst;
 do {
  bookKeeping.ancestors.push(ancestor);
  ancestor = ancestor && findParent(ancestor);
 } while (ancestor);

 for (var i = 0; i < bookKeeping.ancestors.length; i++) {
  targetInst = bookKeeping.ancestors[i];
  // 这里的_handleTopLevel 对应的就是ReactEventEmitterMixin.js里的handleTopLevel
  ReactEventListener._handleTopLevel(bookKeeping.topLevelType, targetInst, bookKeeping.nativeEvent, getEventTarget(bookKeeping.nativeEvent));
 }
}

// 这里的findParent曾经给我带来误导，我以为去找当前元素所有的父节点，但其实不是的，
// 我们知道一般情况下，我们的组件最后会被包裹在<div id='root'></div>的标签里
// 一般是没有组件再去嵌套它的，所以通常返回null
/**
 * Find the deepest React component completely containing the root of the
 * passed-in instance (for use when entire React trees are nested within each
 * other). If React trees are not nested, returns null.
 */
function findParent(inst) {
 while (inst._hostParent) {
  inst = inst._hostParent;
 }
 var rootNode = ReactDOMComponentTree.getNodeFromInstance(inst);
 var container = rootNode.parentNode;
 return ReactDOMComponentTree.getClosestInstanceFromNode(container);
}

//这就是核心的处理了
handleTopLevel: function (topLevelType, targetInst, nativeEvent, nativeEventTarget) {
  //返回合成事件
  //这里进入了EventPluginHub，调用事件插件方法，返回合成事件，并执行队列里的dispatchListener
  var events = EventPluginHub.extractEvents(topLevelType, targetInst, nativeEvent, nativeEventTarget);
  //执行合成事件
  runEventQueueInBatch(events);
 }

 // 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);
  };