React笔记-事件分发

事件分发

之前讲述了事件如何绑定在document上，那么具体事件触发的时候是如何分发到具体的监听者呢？我们接着上次注册的事件代理看。当我点击update counter按钮时，触发注册的click事件代理。

function dispatchInteractiveEvent(topLevelType, nativeEvent) {interactiveUpdates(dispatchEvent, topLevelType, nativeEvent);
}
function interactiveUpdates(fn, a, b) {return _interactiveUpdatesImpl(fn, a, b);
}
var _interactiveUpdatesImpl = function (fn, a, b) {return fn(a, b);
};

topLevelType为click，nativeEvent为真实dom事件对象。看似很多，其实就做了一件事: 执行dispatchEvent(topLevelType, nativeEvent)。其实不然，_interactiveUpdatesImpl在后面被重新赋值为interactiveUpdates$1，完成了一次自我蜕变。

function setBatchingImplementation(batchedUpdatesImpl, interactiveUpdatesImpl, flushInteractiveUpdatesImpl) {_batchedUpdatesImpl = batchedUpdatesImpl;_interactiveUpdatesImpl = interactiveUpdatesImpl;_flushInteractiveUpdatesImpl = flushInteractiveUpdatesImpl;
}function interactiveUpdates$1(fn, a, b) {if (!isBatchingUpdates && !isRendering && lowestPriorityPendingInteractiveExpirationTime !== NoWork) {performWork(lowestPriorityPendingInteractiveExpirationTime, false);lowestPriorityPendingInteractiveExpirationTime = NoWork;}var previousIsBatchingUpdates = isBatchingUpdates;isBatchingUpdates = true;try {return scheduler.unstable_runWithPriority(scheduler.unstable_UserBlockingPriority, function () {return fn(a, b);});} finally {isBatchingUpdates = previousIsBatchingUpdates;if (!isBatchingUpdates && !isRendering) {performSyncWork();}}
}setBatchingImplementation(batchedUpdates$1, interactiveUpdates$1, flushInteractiveUpdates$1);

如果有任何等待的交互更新，条件满足的情况下会先同步更新，然后设置isBatchingUpdates，进行scheduler调度。最后同步更新。scheduler的各类优先级如下:

unstable_ImmediatePriority: 1
unstable_UserBlockingPriority: 2
unstable_NormalPriority: 3
unstable_LowPriority: 4
unstable_IdlePriority: 5

进入scheduler调度，根据优先级计算时间，开始执行传入的回调函数。然后调用dispatchEvent，最后更新immediate work。flushImmediateWork里的调用关系很复杂，最终会调用requestAnimationFrame进行更新，这里不进行过多讨论。

function unstable_runWithPriority(priorityLevel, eventHandler) {switch (priorityLevel) {case ImmediatePriority:case UserBlockingPriority:case NormalPriority:case LowPriority:case IdlePriority:break;default:priorityLevel = NormalPriority;}var previousPriorityLevel = currentPriorityLevel;var previousEventStartTime = currentEventStartTime;currentPriorityLevel = priorityLevel;currentEventStartTime = exports.unstable_now();try {return eventHandler();} finally {currentPriorityLevel = previousPriorityLevel;currentEventStartTime = previousEventStartTime;flushImmediateWork();}
}

下面看看dispatchEvent的具体执行过程。

function dispatchEvent(topLevelType, nativeEvent) {if (!_enabled) {return;}// 获取事件触发的原始节点var nativeEventTarget = getEventTarget(nativeEvent);// 获取原始节点最近的fiber对象（通过缓存在dom上的internalInstanceKey属性来寻找），如果没找到会往父节点继续寻找。var targetInst = getClosestInstanceFromNode(nativeEventTarget);if (targetInst !== null && typeof targetInst.tag === 'number' && !isFiberMounted(targetInst)) {targetInst = null;}// 创建对象，包含事件名称，原始事件，目标fiber对象和ancestor(空数组)；如果缓存池有则直接取出并根据参数初始化属性。var bookKeeping = getTopLevelCallbackBookKeeping(topLevelType, nativeEvent, targetInst);try {// 批处理事件batchedUpdates(handleTopLevel, bookKeeping);} finally {// 释放bookKeeping对象内存，并放入对象池缓存releaseTopLevelCallbackBookKeeping(bookKeeping);}
}

接着看batchedUpdates，其实就是设置isBatching变量然后调用handleTopLevel(bookkeeping)。

function batchedUpdates(fn, bookkeeping) {if (isBatching) {return fn(bookkeeping);}isBatching = true;try {// _batchedUpdatesImpl其实指向batchedUpdates$1函数，具体细节这里不再赘述return _batchedUpdatesImpl(fn, bookkeeping);} finally {isBatching = false;var controlledComponentsHavePendingUpdates = needsStateRestore();if (controlledComponentsHavePendingUpdates) {_flushInteractiveUpdatesImpl();restoreStateIfNeeded();}}
}

所以将原始节点对应最近的fiber缓存在bookKeeping.ancestors中。

function handleTopLevel(bookKeeping) {var targetInst = bookKeeping.targetInst;var ancestor = targetInst;do {if (!ancestor) {bookKeeping.ancestors.push(ancestor);break;}var root = findRootContainerNode(ancestor);if (!root) {break;}bookKeeping.ancestors.push(ancestor);ancestor = getClosestInstanceFromNode(root);} while (ancestor);for (var i = 0; i < bookKeeping.ancestors.length; i++) {targetInst = bookKeeping.ancestors[i];runExtractedEventsInBatch(bookKeeping.topLevelType, targetInst, bookKeeping.nativeEvent, getEventTarget(bookKeeping.nativeEvent));}
}

runExtractedEventsInBatch中调用了两个方法: extractEvents和runEventsInBatch。前者构造合成事件，后者批处理合成事件。

function runExtractedEventsInBatch(topLevelType, targetInst, nativeEvent, nativeEventTarget) {var events = extractEvents(topLevelType, targetInst, nativeEvent, nativeEventTarget);runEventsInBatch(events);
}

事件合成

 function extractEvents(topLevelType, targetInst, nativeEvent, nativeEventTarget) {var events = null;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;
}

plugins是所有合成事件集合的数组，EventPluginHub初始化的时候完成注入。遍历所有plugins，调用其extractEvents方法，返回构造的合成事件。accumulateInto函数则把合成事件放入events。本例click事件合适的plugin是SimpleEventPlugin，其他plugin得到的extractedEvents都不满足if (extractedEvents)条件。

EventPluginHubInjection.injectEventPluginsByName({SimpleEventPlugin: SimpleEventPlugin,EnterLeaveEventPlugin: EnterLeaveEventPlugin,ChangeEventPlugin: ChangeEventPlugin,SelectEventPlugin: SelectEventPlugin,BeforeInputEventPlugin: BeforeInputEventPlugin,
});

接下来看看构造合成事件的具体过程，这里针对SimpleEventPlugin，其他plugin就不一一分析了，来看下其extractEvents:

extractEvents: function(topLevelType, targetInst, nativeEvent, nativeEventTarget) {var dispatchConfig = topLevelEventsToDispatchConfig[topLevelType];if (!dispatchConfig) {return null;}var EventConstructor = void 0;switch (topLevelType) {...case TOP_CLICK:...EventConstructor = SyntheticMouseEvent;break;...    }var event = EventConstructor.getPooled(dispatchConfig, targetInst, nativeEvent, nativeEventTarget);accumulateTwoPhaseDispatches(event);return event;}

topLevelEventsToDispatchConfig是一个map对象，存储着各类事件对应的配置信息。这里获取到click的配置信息，然后根据topLevelType选择对应的合成构造函数，这里为SyntheticMouseEvent。接着从SyntheticMouseEvent合成事件对象池中获取合成事件。调用EventConstructor.getPooled，最终调用的是getPooledEvent。

注意: SyntheticEvent.extend方法中明确写有addEventPoolingTo(Class)；所以，SyntheticMouseEvent有eventPool、getPooled和release属性。后面会详细介绍SyntheticEvent.extend

function addEventPoolingTo(EventConstructor) {EventConstructor.eventPool = [];EventConstructor.getPooled = getPooledEvent;EventConstructor.release = releasePooledEvent;
}

首次触发事件，对象池为空，所以这里需要新创建。如果不为空，则取出一个并初始化。

function getPooledEvent(dispatchConfig, targetInst, nativeEvent, nativeInst) {var EventConstructor = this;if (EventConstructor.eventPool.length) {var instance = EventConstructor.eventPool.pop();EventConstructor.call(instance, dispatchConfig, targetInst, nativeEvent, nativeInst);return instance;}return new EventConstructor(dispatchConfig, targetInst, nativeEvent, nativeInst);
}

合成事件的属性是由React主动生成的，一些属性和原生事件的属性名完全一致，使其完全符合W3C标准，因此在事件层面上具有跨浏览器兼容性。如果要访问原生对象，通过nativeEvent属性即可获取。这里SyntheticMouseEvent由SyntheticUIEvent扩展而来，而SyntheticUIEvent由SyntheticEvent扩展而来。

var SyntheticMouseEvent = SyntheticUIEvent.extend({...
});var SyntheticUIEvent = SyntheticEvent.extend({...
});SyntheticEvent.extend = function (Interface) {var Super = this;// 原型继承var E = function () {};E.prototype = Super.prototype;var prototype = new E();// 构造继承function Class() {return Super.apply(this, arguments);}_assign(prototype, Class.prototype);Class.prototype = prototype;Class.prototype.constructor = Class;Class.Interface = _assign({}, Super.Interface, Interface);Class.extend = Super.extend;addEventPoolingTo(Class);return Class;
};

当被new创建时，会调用父类SyntheticEvent进行构造。主要是将原生事件上的属性挂载到合成事件上，还配置了一些额外属性。

function SyntheticEvent(dispatchConfig, targetInst, nativeEvent, nativeEventTarget) {this.dispatchConfig = dispatchConfig;this._targetInst = targetInst;this.nativeEvent = nativeEvent;...
}

合成事件构造完成后，调用accumulateTwoPhaseDispatches。

function accumulateTwoPhaseDispatches(events) {forEachAccumulated(events, accumulateTwoPhaseDispatchesSingle);
}// 循环处理所有的合成事件
function forEachAccumulated(arr, cb, scope) {if (Array.isArray(arr)) {arr.forEach(cb, scope);} else if (arr) {cb.call(scope, arr);}
}// 检测事件是否具有捕获阶段和冒泡阶段
function accumulateTwoPhaseDispatchesSingle(event) {if (event && event.dispatchConfig.phasedRegistrationNames) {traverseTwoPhase(event._targetInst, accumulateDirectionalDispatches, event);}
}function traverseTwoPhase(inst, fn, arg) {var path = [];// 循环遍历当前元素及父元素，缓存至pathwhile (inst) {path.push(inst);inst = getParent(inst);}var i = void 0;// 捕获阶段for (i = path.length; i-- > 0;) {fn(path[i], 'captured', arg);}// 冒泡阶段for (i = 0; i < path.length; i++) {fn(path[i], 'bubbled', arg);}
}function accumulateDirectionalDispatches(inst, phase, event) {// 获取当前阶段对应的事件处理函数var listener = listenerAtPhase(inst, event, phase);// 将相关listener和目标fiber挂载到event对应的属性上if (listener) {event._dispatchListeners = accumulateInto(event._dispatchListeners, listener);event._dispatchInstances = accumulateInto(event._dispatchInstances, inst);}
}

事件执行（批处理合成事件）

首先将events合并到事件队列，之前没有处理完毕的队列也一同合并。如果新的事件队列为空，则退出。反之开始循环处理事件队列中每一个event。forEachAccumulated前面有提到过，这里不再赘述。

function runEventsInBatch(events) {if (events !== null) {eventQueue = accumulateInto(eventQueue, events);}var processingEventQueue = eventQueue;eventQueue = null;if (!processingEventQueue) {return;}forEachAccumulated(processingEventQueue, executeDispatchesAndReleaseTopLevel);rethrowCaughtError();
}

接下来看看事件处理，executeDispatchesAndRelease方法将事件执行和事件清理分开。

var executeDispatchesAndReleaseTopLevel = function (e) {return executeDispatchesAndRelease(e);
};var executeDispatchesAndRelease = function (event) {if (event) {// 执行事件executeDispatchesInOrder(event);if (!event.isPersistent()) {// 事件清理，将合成事件放入对象池event.constructor.release(event);}}
};

提取事件的处理函数和对应的fiber，调用executeDispatch。

function executeDispatchesInOrder(event) {var dispatchListeners = event._dispatchListeners;var dispatchInstances = event._dispatchInstances;if (Array.isArray(dispatchListeners)) {for (var i = 0; i < dispatchListeners.length; i++) {if (event.isPropagationStopped()) {break;}executeDispatch(event, dispatchListeners[i], dispatchInstances[i]);}} else if (dispatchListeners) {executeDispatch(event, dispatchListeners, dispatchInstances);}event._dispatchListeners = null;event._dispatchInstances = null;
}

获取真实dom挂载到event对象上，然后开始执行事件。

function executeDispatch(event, listener, inst) {var type = event.type || 'unknown-event';// 获取真实domevent.currentTarget = getNodeFromInstance(inst);invokeGuardedCallbackAndCatchFirstError(type, listener, undefined, event);event.currentTarget = null;
}

invokeGuardedCallbackAndCatchFirstError下面调用的方法很多，最终会来到invokeGuardedCallbackImpl，关键就在func.apply(context, funcArgs);这里的func就是listener（本例中是handleClick），而funcArgs就是合成事件对象。至此，事件执行完毕。

var invokeGuardedCallbackImpl = function (name, func, context, a, b, c, d, e, f) {var funcArgs = Array.prototype.slice.call(arguments, 3);try {func.apply(context, funcArgs);} catch (error) {this.onError(error);}
};

事件清理

事件执行完之后，剩下就是一些清理操作。event.constructor.release(event)相当于releasePooledEvent(event)。由于click对应的是SyntheticMouseEvent，所以会放入SyntheticMouseEvent.eventPool中。EVENT_POOL_SIZE固定为10。

function releasePooledEvent(event) {var EventConstructor = this;event.destructor();if (EventConstructor.eventPool.length < EVENT_POOL_SIZE) {EventConstructor.eventPool.push(event);}
}

这里做了两件事，第一手动释放event属性上的内存（将属性置为null），第二将event放入对象池。至此，清理工作完毕。

destructor: function () {...this.dispatchConfig = null;this._targetInst = null;this.nativeEvent = null;this.isDefaultPrevented = functionThatReturnsFalse;this.isPropagationStopped = functionThatReturnsFalse;this._dispatchListeners = null;this._dispatchInstances = null;...
}    

event清理完后，还会清理bookKeeping，同样也会放入对象池进行缓存。同样CALLBACK_BOOKKEEPING_POOL_SIZE也固定为10。

// callbackBookkeepingPool是react-dom中的全局变量
function releaseTopLevelCallbackBookKeeping(instance) {instance.topLevelType = null;instance.nativeEvent = null;instance.targetInst = null;instance.ancestors.length = 0;if (callbackBookkeepingPool.length < CALLBACK_BOOKKEEPING_POOL_SIZE) {callbackBookkeepingPool.push(instance);}
}

总结

最后执行performSyncWork。如果执行的事件内调用了this.setState，会进行reconciliation和commit。由于事件流的执行是批处理过程，同步调用this.setState不会立马更新，需等待所有事件执行完成，即scheduler调度完后才开始performSyncWork，最终才能拿到新的state。如果是setTimeout或者是在dom上另外addEventListener的回调函数中调用this.setState则会立马更新。因为执行回调函数的时候不经过React事件流。

更好的阅读体验在我的github，欢迎?提issue。