Vue批量更新dom的实现步骤 Vue批量更新dom的实现步骤

场景介绍
深入响应式
触发getter
寻找Dep.target
getter
setter
总结

场景介绍
在一个SFC（single file component，单文件组件）中，我们经常会写这样的逻辑：

你可能知道，在完成this.a和this.b的赋值操作后，Vue会将this.a和this.b相应的dom更新函数放到一个微任务中。等待主线程的同步任务执行完毕后，该微任务会出队并执行。我们看看Vue的官方文档"深入响应式原理-声明响应式property"一节中，是怎么进行描述的：
可能你还没有注意到，Vue 在更新 DOM 时是异步执行的。只要侦听到数据变化，Vue 将开启一个队列，并缓冲在同一事件循环中发生的所有数据变更。
那么，Vue是怎么实现这一能力的呢？为了回答这个问题，我们需要深入Vue源码的核心部分——响应式原理。

深入响应式
我们首先看一看在我们对this.a和this.b进行赋值操作以后，发生了什么。如果使用Vue CLI进行开发，在main.js文件中，会有一个new Vue()的实例化操作。由于Vue的源码是使用flow写的，无形中增加了理解成本。为了方便，我们直接看npm vue包中dist文件夹中的vue.js源码。搜索‘function Vue'，找到了以下源码：

function Vue (options) {if (!(this instanceof Vue)) {warn('Vue is a constructor and should be called with the `new` keyword'); }this._init(options); }

非常简单的源码，源码真的没有我们想象中那么难！带着这样的意外惊喜，我们继续找到_init函数，看看这个函数做了什么：

Vue.prototype._init = function (options) {var vm = this; // a uidvm._uid = uid$3++; var startTag, endTag; /* istanbul ignore if */if (config.performance && mark) {startTag = "vue-perf-start:" + (vm._uid); endTag = "vue-perf-end:" + (vm._uid); mark(startTag); }// a flag to avoid this being observedvm._isVue = true; // merge optionsif (options && options._isComponent) {// optimize internal component instantiation// since dynamic options merging is pretty slow, and none of the// internal component options needs special treatment.initInternalComponent(vm, options); } else {vm.$options = mergeOptions(resolveConstructorOptions(vm.constructor),options || {},vm); }/* istanbul ignore else */{initProxy(vm); }// expose real selfvm._self = vm; initLifecycle(vm); initEvents(vm); initRender(vm); callHook(vm, 'beforeCreate'); initInjections(vm); // resolve injections before data/propsinitState(vm); initProvide(vm); // resolve provide after data/propscallHook(vm, 'created'); /* istanbul ignore if */if (config.performance && mark) {vm._name = formatComponentName(vm, false); mark(endTag); measure(("vue " + (vm._name) + " init"), startTag, endTag); }if (vm.$options.el) {vm.$mount(vm.$options.el); }}

我们先不管上面的一堆判断，直接拉到下面的主逻辑。可以看到，_init函数先后执行了initLifeCycle、initEvents、initRender、callHook、initInjections、initState、initProvide以及第二次callHook函数。从函数的命名来看，我们可以知道具体的意思。大体来说，这段代码分为以下两个部分

在完成初始化生命周期、事件钩子以及渲染函数后，进入beforeCreate生命周期（执行beforeCreate函数）
在完成初始化注入值、状态以及提供值之后，进入created生命周期（执行created函数）

其中，我们关心的数据响应式原理部分在initState函数中，我们看看这个函数做了什么：

function initState (vm) {vm._watchers = []; var opts = vm.$options; if (opts.props) { initProps(vm, opts.props); }if (opts.methods) { initMethods(vm, opts.methods); }if (opts.data) {initData(vm); } else {observe(vm._data = https://www.it610.com/article/{}, true /* asRootData */); }if (opts.computed) { initComputed(vm, opts.computed); }if (opts.watch && opts.watch !== nativeWatch) {initWatch(vm, opts.watch); }}

【Vue批量更新dom的实现步骤】这里我们看到了在书写SFC文件时常常见到的几个配置项：props、methods、data、computed和watch。我们将注意力集中到opts.data部分，这一部分执行了initData函数：

function initData (vm) {var data = https://www.it610.com/article/vm.$options.data; data = vm._data = typeof data ==='function'? getData(data, vm): data || {}; if (!isPlainObject(data)) {data = https://www.it610.com/article/{}; warn('data functions should return an object:\n' +'https://vuejs.org/v2/guide/components.html#data-Must-Be-a-Function',vm); }// proxy data on instancevar keys = Object.keys(data); var props = vm.$options.props; var methods = vm.$options.methods; var i = keys.length; while (i--) {var key = keys[i]; {if (methods && hasOwn(methods, key)) {warn(("Method \"" + key + "\" has already been defined as a data property."),vm); }}if (props && hasOwn(props, key)) {warn("The data property \"" + key + "\" is already declared as a prop. " +"Use prop default value instead.",vm); } else if (!isReserved(key)) {proxy(vm, "_data", key); }}// observe dataobserve(data, true /* asRootData */); }

我们在写data配置项时，会将其定义为函数，因此这里执行了getData函数：

function getData (data, vm) {// #7573 disable dep collection when invoking data getterspushTarget(); try {return data.call(vm, vm)} catch (e) {handleError(e, vm, "data()"); return {}} finally {popTarget(); }}

getData函数做的事情非常简单，就是在组件实例上下文中执行data函数。注意，在执行data函数前后，分别执行了pushTarget函数和popTarget函数，这两个函数我们后面再讲。

执行getData函数后，我们回到initData函数，后面有一个循环的错误判断，暂时不用管。于是我们来到了observe函数：

function observe (value, asRootData) {if (!isObject(value) || value instanceof VNode) {return}var ob; if (hasOwn(value, '__ob__') && value.__ob__ instanceof Observer) {ob = value.__ob__; } else if (shouldObserve &&!isServerRendering() &&(Array.isArray(value) || isPlainObject(value)) &&Object.isExtensible(value) &&!value._isVue) {ob = new Observer(value); }if (asRootData && ob) {ob.vmCount++; }return ob}

observe函数为data对象创建了一个观察者（ob），也就是实例化Observer，实例化Observer具体做了什么呢？我们继续看源码：

var Observer = function Observer (value) {this.value = https://www.it610.com/article/value; this.dep = new Dep(); this.vmCount = 0; def(value,'__ob__', this); if (Array.isArray(value)) {if (hasProto) {protoAugment(value, arrayMethods); } else {copyAugment(value, arrayMethods, arrayKeys); }this.observeArray(value); } else {this.walk(value); }}

正常情况下，因为我们定义的data函数返回的都是一个对象，所以这里我们先不管对数组的处理。那么就是继续执行walk函数：

Observer.prototype.walk = function walk (obj) {var keys = Object.keys(obj); for (var i = 0; i < keys.length; i++) {defineReactive$$1(obj, keys[i]); }}

对于data函数返回的对象，即组件实例的data对象中的每个可枚举属性，执行defineReactive$$1函数：

function defineReactive$$1 (obj,key,val,customSetter,shallow) {var dep = new Dep(); var property = Object.getOwnPropertyDescriptor(obj, key); if (property && property.configurable === false) {return}// cater for pre-defined getter/settersvar getter = property && property.get; var setter = property && property.set; if ((!getter || setter) && arguments.length === 2) {val = obj[key]; }var childOb = !shallow && observe(val); Object.defineProperty(obj, key, {enumerable: true,configurable: true,get: function reactiveGetter () {var value = https://www.it610.com/article/getter ? getter.call(obj) : val; if (Dep.target) {dep.depend(); if (childOb) {childOb.dep.depend(); if (Array.isArray(value)) {dependArray(value); }}}return value},set: function reactiveSetter (newVal) {var value = getter ? getter.call(obj) : val; /* eslint-disable no-self-compare */if (newVal === value || (newVal !== newVal && value !== value)) {return}/* eslint-enable no-self-compare */if (customSetter) {customSetter(); }// #7981: for accessor properties without setterif (getter && !setter) { return }if (setter) {setter.call(obj, newVal); } else {val = newVal; }childOb = !shallow && observe(newVal); dep.notify(); }}); }

在defineReactive$$1函数中，首先实例化一个依赖收集器。然后使用Object.defineProperty重新定义对象属性的getter（即上面的get函数）和setter（即上面的set函数）。

触发getter
getter和setter某种意义上可以理解为回调函数，当读取对象某个属性的值时，会触发get函数（即getter）；当设置对象某个属性的值时，会触发set函数（即setter）。我们回到最开始的例子：

这里有设置this对象的属性a和属性b的值，因此会触发setter。我们把上面set函数代码单独拿出来：

function reactiveSetter (newVal) {var value = https://www.it610.com/article/getter ? getter.call(obj) : val; /* eslint-disable no-self-compare */if (newVal === value || (newVal !== newVal && value !== value)) {return}/* eslint-enable no-self-compare */if (customSetter) {customSetter(); }// #7981: for accessor properties without setterif (getter && !setter) { return }if (setter) {setter.call(obj, newVal); } else {val = newVal; }childOb = !shallow && observe(newVal); dep.notify(); }

setter先执行了getter：

function reactiveGetter () {var value = https://www.it610.com/article/getter ? getter.call(obj) : val; if (Dep.target) {dep.depend(); if (childOb) {childOb.dep.depend(); if (Array.isArray(value)) {dependArray(value); }}}return value}

getter先检测Dep.target是否存在。在前面执行getData函数的时候，Dep.target的初始值为null，它在什么时候被赋值了呢？我们前面讲getData函数的时候，有看到一个pushTarget函数和popTarget函数，这两个函数的源码如下：

Dep.target = null; var targetStack = []; function pushTarget (target) {targetStack.push(target); Dep.target = target; }function popTarget () {targetStack.pop(); Dep.target = targetStack[targetStack.length - 1]; }

想要正常执行getter，就需要先执行pushTarget函数。我们找找pushTarget函数在哪里执行的。在vue.js中搜索pushTarget，我们找到了5个地方，除去定义的地方，执行的地方有4个。
第一个执行pushTarget函数的地方。这是一个处理错误的函数，正常逻辑不会触发：

function handleError (err, vm, info) {// Deactivate deps tracking while processing error handler to avoid possible infinite rendering.// See: https://github.com/vuejs/vuex/issues/1505pushTarget(); try {if (vm) {var cur = vm; while ((cur = cur.$parent)) {var hooks = cur.$options.errorCaptured; if (hooks) {for (var i = 0; i < hooks.length; i++) {try {var capture = hooks[i].call(cur, err, vm, info) === false; if (capture) { return }} catch (e) {globalHandleError(e, cur, 'errorCaptured hook'); }}}}}globalHandleError(err, vm, info); } finally {popTarget(); }}

第二个执行pushTarget的地方。这是调用对应的钩子函数。在执行到对应的钩子函数时会触发。不过，我们现在的操作介于beforeCreate钩子和created钩子之间，还没有触发：

function callHook (vm, hook) {// #7573 disable dep collection when invoking lifecycle hookspushTarget(); var handlers = vm.$options[hook]; var info = hook + " hook"; if (handlers) {for (var i = 0, j = handlers.length; i < j; i++) {invokeWithErrorHandling(handlers[i], vm, null, vm, info); }}if (vm._hasHookEvent) {vm.$emit('hook:' + hook); }popTarget(); }

第三个执行pushTarget的地方。这是实例化watcher时执行的函数。检查前面的代码，我们似乎也没有看到new Watcher的操作：

Watcher.prototype.get = function get () {pushTarget(this); var value; var vm = this.vm; try {value = https://www.it610.com/article/this.getter.call(vm, vm); } catch (e) {if (this.user) {handleError(e, vm, ("getter for watcher \"" + (this.expression) + "\"")); } else {throw e}} finally {// "touch" every property so they are all tracked as// dependencies for deep watchingif (this.deep) {traverse(value); }popTarget(); this.cleanupDeps(); }return value}

第四个执行pushTarget的地方，这就是前面的getData函数。但是getData函数的执行位于defineReactive$$1函数之前。在执行完getData函数以后，Dep.target已经被重置为null了。

function getData (data, vm) {// #7573 disable dep collection when invoking data getterspushTarget(); try {return data.call(vm, vm)} catch (e) {handleError(e, vm, "data()"); return {}} finally {popTarget(); }}

看起来，直接触发setter并不能让getter中的逻辑正常执行。并且，我们还发现，由于setter中也有Dep.target的判断，所以如果我们找不到Dep.target的来源，setter的逻辑也无法继续往下走。

寻找Dep.target
那么，到底Dep.target的值是从哪里来的呢？不用着急，我们回到_init函数的操作继续往下看：

Vue.prototype._init = function (options) {var vm = this; // a uidvm._uid = uid$3++; var startTag, endTag; /* istanbul ignore if */if (config.performance && mark) {startTag = "vue-perf-start:" + (vm._uid); endTag = "vue-perf-end:" + (vm._uid); mark(startTag); }// a flag to avoid this being observedvm._isVue = true; // merge optionsif (options && options._isComponent) {// optimize internal component instantiation// since dynamic options merging is pretty slow, and none of the// internal component options needs special treatment.initInternalComponent(vm, options); } else {vm.$options = mergeOptions(resolveConstructorOptions(vm.constructor),options || {},vm); }/* istanbul ignore else */{initProxy(vm); }// expose real selfvm._self = vm; initLifecycle(vm); initEvents(vm); initRender(vm); callHook(vm, 'beforeCreate'); initInjections(vm); // resolve injections before data/propsinitState(vm); initProvide(vm); // resolve provide after data/propscallHook(vm, 'created'); /* istanbul ignore if */if (config.performance && mark) {vm._name = formatComponentName(vm, false); mark(endTag); measure(("vue " + (vm._name) + " init"), startTag, endTag); }if (vm.$options.el) {vm.$mount(vm.$options.el); }}

我们发现，在_init函数的最后，执行了vm.$mount函数，这个函数做了什么呢？

Vue.prototype.$mount = function (el,hydrating) {el = el && inBrowser ? query(el) : undefined; return mountComponent(this, el, hydrating)}

我们继续进入mountComponent函数看看：

function mountComponent (vm,el,hydrating) {vm.$el = el; if (!vm.$options.render) {vm.$options.render = createEmptyVNode; {/* istanbul ignore if */if ((vm.$options.template && vm.$options.template.charAt(0) !== '#') ||vm.$options.el || el) {warn('You are using the runtime-only build of Vue where the template ' +'compiler is not available. Either pre-compile the templates into ' +'render functions, or use the compiler-included build.',vm); } else {warn('Failed to mount component: template or render function not defined.',vm); }}}callHook(vm, 'beforeMount'); var updateComponent; /* istanbul ignore if */if (config.performance && mark) {updateComponent = function () {var name = vm._name; var id = vm._uid; var startTag = "vue-perf-start:" + id; var endTag = "vue-perf-end:" + id; mark(startTag); var vnode = vm._render(); mark(endTag); measure(("vue " + name + " render"), startTag, endTag); mark(startTag); vm._update(vnode, hydrating); mark(endTag); measure(("vue " + name + " patch"), startTag, endTag); }; } else {updateComponent = function () {vm._update(vm._render(), hydrating); }; }// we set this to vm._watcher inside the watcher's constructor// since the watcher's initial patch may call $forceUpdate (e.g. inside child// component's mounted hook), which relies on vm._watcher being already definednew Watcher(vm, updateComponent, noop, {before: function before () {if (vm._isMounted && !vm._isDestroyed) {callHook(vm, 'beforeUpdate'); }}}, true /* isRenderWatcher */); hydrating = false; // manually mounted instance, call mounted on self// mounted is called for render-created child components in its inserted hookif (vm.$vnode == null) {vm._isMounted = true; callHook(vm, 'mounted'); }return vm}

我们惊喜地发现，这里有一个new Watcher的操作！真是山重水复疑无路，柳暗花明又一村！这里实例化的watcher是一个用来更新dom的watcher。他会依次读取SFC文件中的template部分中的所有值。这也就意味着会触发对应的getter。
由于new Watcher会执行watcher.get函数，该函数执行pushTarget函数，于是Dep.target被赋值。getter内部的逻辑顺利执行。

getter
至此，我们终于到了Vue的响应式原理的核心。我们再次回到getter，看一看有了Dep.target以后，getter做了什么：

function reactiveGetter () {var value = https://www.it610.com/article/getter ? getter.call(obj) : val; if (Dep.target) {dep.depend(); if (childOb) {childOb.dep.depend(); if (Array.isArray(value)) {dependArray(value); }}}return value}

同样地，我们先不关注提高代码健壮性的细节处理，直接看主线。可以看到，当Dep.target存在时，执行了dep.depend函数。这个函数做了什么呢？我们看看代码：

Dep.prototype.depend = function depend () {if (Dep.target) {Dep.target.addDep(this); }}

做的事情也非常简单。就是执行了Dep.target.addDep函数。但是Dep.target其实是一个watcher，所以我们要回到Watcher的代码：

Watcher.prototype.addDep = function addDep (dep) {var id = dep.id; if (!this.newDepIds.has(id)) {this.newDepIds.add(id); this.newDeps.push(dep); if (!this.depIds.has(id)) {dep.addSub(this); }}}

同样地，我们先忽略一些次要的逻辑处理，把注意力集中到dep.addSub函数上：

Dep.prototype.addSub = function addSub (sub) {this.subs.push(sub); }

也是非常简单的逻辑，把watcher作为一个订阅者推入数组中缓存。至此，getter的整个逻辑走完。此后执行popTarget函数，Dep.target被重置为null

setter
我们再次回到业务代码：

在created生命周期中，我们触发了两次setter，setter执行的逻辑如下：

function reactiveSetter (newVal) {var value = https://www.it610.com/article/getter ? getter.call(obj) : val; /* eslint-disable no-self-compare */if (newVal === value || (newVal !== newVal && value !== value)) {return}/* eslint-enable no-self-compare */if (customSetter) {customSetter(); }// #7981: for accessor properties without setterif (getter && !setter) { return }if (setter) {setter.call(obj, newVal); } else {val = newVal; }childOb = !shallow && observe(newVal); dep.notify(); }

这里，我们只需要关注setter最后执行的函数：dep.notify()。我们看看这个函数做了什么：

Dep.prototype.notify = function notify () {// stabilize the subscriber list firstvar subs = this.subs.slice(); if (!config.async) {// subs aren't sorted in scheduler if not running async// we need to sort them now to make sure they fire in correct// ordersubs.sort(function (a, b) { return a.id - b.id; }); }for (var i = 0, l = subs.length; i < l; i++) {subs[i].update(); }}

This.subs的每一项元素均为一个watcher。在上面getter章节中，我们只收集到了一个watcher。因为触发了两次setter，所以subs[0].update()，即watcher.update()函数会执行两次。我们看看这个函数做了什么：

Watcher.prototype.update = function update () {/* istanbul ignore else */if (this.lazy) {this.dirty = true; } else if (this.sync) {this.run(); } else {queueWatcher(this); }}

按照惯例，我们直接跳入queueWatcher函数：

function queueWatcher (watcher) {var id = watcher.id; if (has[id] == null) {has[id] = true; if (!flushing) {queue.push(watcher); } else {// if already flushing, splice the watcher based on its id// if already past its id, it will be run next immediately.var i = queue.length - 1; while (i > index && queue[i].id > watcher.id) {i--; }queue.splice(i + 1, 0, watcher); }// queue the flushif (!waiting) {waiting = true; if (!config.async) {flushSchedulerQueue(); return}nextTick(flushSchedulerQueue); }}}

由于id相同，所以watcher的回调函数只会被推入到queue一次。这里我们再次看到了一个熟悉的面孔：nextTick。

function nextTick (cb, ctx) {var _resolve; callbacks.push(function () {if (cb) {try {cb.call(ctx); } catch (e) {handleError(e, ctx, 'nextTick'); }} else if (_resolve) {_resolve(ctx); }}); if (!pending) {pending = true; timerFunc(); }// $flow-disable-lineif (!cb && typeof Promise !== 'undefined') {return new Promise(function (resolve) {_resolve = resolve; })}}

nextTick函数将回调函数再次包裹一层后，执行timerFunc()

var timerFunc; // The nextTick behavior leverages the microtask queue, which can be accessed// via either native Promise.then or MutationObserver.// MutationObserver has wider support, however it is seriously bugged in// UIWebView in iOS >= 9.3.3 when triggered in touch event handlers. It// completely stops working after triggering a few times... so, if native// Promise is available, we will use it:/* istanbul ignore next, $flow-disable-line */if (typeof Promise !== 'undefined' && isNative(Promise)) {var p = Promise.resolve(); timerFunc = function () {p.then(flushCallbacks); // In problematic UIWebViews, Promise.then doesn't completely break, but// it can get stuck in a weird state where callbacks are pushed into the// microtask queue but the queue isn't being flushed, until the browser// needs to do some other work, e.g. handle a timer. Therefore we can// "force" the microtask queue to be flushed by adding an empty timer.if (isIOS) { setTimeout(noop); }}; isUsingMicroTask = true; } else if (!isIE && typeof MutationObserver !== 'undefined' && (isNative(MutationObserver) ||// PhantomJS and iOS 7.xMutationObserver.toString() === '[object MutationObserverConstructor]')) {// Use MutationObserver where native Promise is not available,// e.g. PhantomJS, iOS7, Android 4.4// (#6466 MutationObserver is unreliable in IE11)var counter = 1; var observer = new MutationObserver(flushCallbacks); var textNode = document.createTextNode(String(counter)); observer.observe(textNode, {characterData: true}); timerFunc = function () {counter = (counter + 1) % 2; textNode.data = https://www.it610.com/article/String(counter); }; isUsingMicroTask = true; } else if (typeof setImmediate !=='undefined' && isNative(setImmediate)) {// Fallback to setImmediate.// Technically it leverages the (macro) task queue,// but it is still a better choice than setTimeout.timerFunc = function () {setImmediate(flushCallbacks); }; } else {// Fallback to setTimeout.timerFunc = function () {setTimeout(flushCallbacks, 0); }; }

timerFunc函数是微任务的平稳降级。他将根据所在环境的支持程度，依次调用Promise、MutationObserver、setImmediate和setTimeout。并在对应的微任务或者模拟微任务队列中执行回调函数。

function flushSchedulerQueue () {currentFlushTimestamp = getNow(); flushing = true; var watcher, id; // Sort queue before flush.// This ensures that:// 1. Components are updated from parent to child. (because parent is always//created before the child)// 2. A component's user watchers are run before its render watcher (because//user watchers are created before the render watcher)// 3. If a component is destroyed during a parent component's watcher run,//its watchers can be skipped.queue.sort(function (a, b) { return a.id - b.id; }); // do not cache length because more watchers might be pushed// as we run existing watchersfor (index = 0; index < queue.length; index++) {watcher = queue[index]; if (watcher.before) {watcher.before(); }id = watcher.id; has[id] = null; watcher.run(); // in dev build, check and stop circular updates.if (has[id] != null) {circular[id] = (circular[id] || 0) + 1; if (circular[id] > MAX_UPDATE_COUNT) {warn('You may have an infinite update loop ' + (watcher.user? ("in watcher with expression \"" + (watcher.expression) + "\""): "in a component render function."),watcher.vm); break}}}// keep copies of post queues before resetting statevar activatedQueue = activatedChildren.slice(); var updatedQueue = queue.slice(); resetSchedulerState(); // call component updated and activated hookscallActivatedHooks(activatedQueue); callUpdatedHooks(updatedQueue); // devtool hook/* istanbul ignore if */if (devtools && config.devtools) {devtools.emit('flush'); }}

回调函数的核心逻辑是执行watcher.run函数：

Watcher.prototype.run = function run () {if (this.active) {var value = https://www.it610.com/article/this.get(); if (value !== this.value ||// Deep watchers and watchers on Object/Arrays should fire even// when the value is the same, because the value may// have mutated.isObject(value) ||this.deep) {// set new valuevar oldValue = this.value; this.value = value; if (this.user) {try {this.cb.call(this.vm, value, oldValue); } catch (e) {handleError(e, this.vm, ("callback for watcher \"" + (this.expression) + "\"")); }} else {this.cb.call(this.vm, value, oldValue); }}}}

执行this.cb函数，即watcher的回调函数。至此，所有的逻辑走完。

总结
我们再次回到业务场景：

虽然我们触发了两次setter，但是对应的渲染函数在微任务中却只执行了一次。也就是说，在dep.notify函数发出通知以后，Vue将对应的watcher进行了去重、排队操作并最终执行回调。

可以看出，两次赋值操作实际上触发的是同一个渲染函数，这个渲染函数更新了多个dom。这就是所谓的批量更新dom。
到此这篇关于Vue批量更新dom的实现步骤的文章就介绍到这了,更多相关Vue批量更新dom 内容请搜索脚本之家以前的文章或继续浏览下面的相关文章希望大家以后多多支持脚本之家！