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這篇文章將為大家詳細講解有關Vue如何批量更新dom，小編覺得挺實用的，因此分享給大家做個參考，希望大家閱讀完這篇文章后可以有所收獲。
在一個SFC（single file component，單文件組件）中，我們經(jīng)常會寫這樣的邏輯：
<template>
<div>
<span>{{ a }}</span>
<span>{{ b }}</span>
</div>
</template>
<script type="javascript">
export default {
data() {
return {
a: 0,
b: 0
}
},
created() {
// some logic code
this.a = 1
this.b = 2
}
}
</script>你可能知道，在完成this.a和this.b的賦值操作后，Vue會將this.a和this.b相應的dom更新函數(shù)放到一個微任務中。等待主線程的同步任務執(zhí)行完畢后，該微任務會出隊并執(zhí)行。我們看看Vue的官方文檔"深入響應式原理-聲明響應式property"一節(jié)中，是怎么進行描述的：
可能你還沒有注意到，Vue 在更新 DOM 時是異步執(zhí)行的。只要偵聽到數(shù)據(jù)變化，Vue 將開啟一個隊列，并緩沖在同一事件循環(huán)中發(fā)生的所有數(shù)據(jù)變更。
那么，Vue是怎么實現(xiàn)這一能力的呢？為了回答這個問題，我們需要深入Vue源碼的核心部分——響應式原理。
我們首先看一看在我們對this.a和this.b進行賦值操作以后，發(fā)生了什么。如果使用Vue CLI進行開發(fā)，在main.js文件中，會有一個new Vue()的實例化操作。由于Vue的源碼是使用flow寫的，無形中增加了理解成本。為了方便，我們直接看npm vue包中dist文件夾中的vue.js源碼。搜索‘function Vue'，找到了以下源碼：
function Vue (options) {
if (!(this instanceof Vue)
) {
warn('Vue is a constructor and should be called with the `new` keyword');
}
this._init(options);
}非常簡單的源碼，源碼真的沒有我們想象中那么難！帶著這樣的意外驚喜，我們繼續(xù)找到_init函數(shù)，看看這個函數(shù)做了什么：
Vue.prototype._init = function (options) {
var vm = this;
// a uid
vm._uid = uid$3++;
var startTag, endTag;
/* istanbul ignore if */
if (config.performance && mark) {
startTag = "vue-perf-start:" + (vm._uid);
endTag = "vue-perf-end:" + (vm._uid);
mark(startTag);
}
// a flag to avoid this being observed
vm._isVue = true;
// merge options
if (options && options._isComponent) {
// optimize internal component instantiation
// since dynamic options merging is pretty slow, and none of the
// internal component options needs special treatment.
initInternalComponent(vm, options);
} else {
vm.$options = mergeOptions(
resolveConstructorOptions(vm.constructor),
options || {},
vm
);
}
/* istanbul ignore else */
{
initProxy(vm);
}
// expose real self
vm._self = vm;
initLifecycle(vm);
initEvents(vm);
initRender(vm);
callHook(vm, 'beforeCreate');
initInjections(vm); // resolve injections before data/props
initState(vm);
initProvide(vm); // resolve provide after data/props
callHook(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.$options.el);
}
}我們先不管上面的一堆判斷，直接拉到下面的主邏輯?？梢钥吹剑琠init函數(shù)先后執(zhí)行了initLifeCycle、initEvents、initRender、callHook、initInjections、initState、initProvide以及第二次callHook函數(shù)。從函數(shù)的命名來看，我們可以知道具體的意思。大體來說，這段代碼分為以下兩個部分
在完成初始化生命周期、事件鉤子以及渲染函數(shù)后，進入beforeCreate生命周期（執(zhí)行beforeCreate函數(shù)）
在完成初始化注入值、狀態(tài)以及提供值之后，進入created生命周期（執(zhí)行created函數(shù)）
其中，我們關心的數(shù)據(jù)響應式原理部分在initState函數(shù)中，我們看看這個函數(shù)做了什么：
function initState (vm) {
vm._watchers = [];
var opts = vm.$options;
if (opts.props) { initProps(vm, opts.props); }
if (opts.methods) { initMethods(vm, opts.methods); }
if (opts.data) {
initData(vm);
} else {
observe(vm._data = {}, true /* asRootData */);
}
if (opts.computed) { initComputed(vm, opts.computed); }
if (opts.watch && opts.watch !== nativeWatch) {
initWatch(vm, opts.watch);
}
}這里我們看到了在書寫SFC文件時常常見到的幾個配置項：props、methods、data、computed和watch。我們將注意力集中到opts.data部分，這一部分執(zhí)行了initData函數(shù)：
function initData (vm) {
var data = vm.$options.data;
data = vm._data = typeof data === 'function'
? getData(data, vm)
: data || {};
if (!isPlainObject(data)) {
data = {};
warn(
'data functions should return an object:\n' +
'https://vuejs.org/v2/guide/components.html#data-Must-Be-a-Function',
vm
);
}
// proxy data on instance
var keys = Object.keys(data);
var props = vm.$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 data
observe(data, true /* asRootData */);
}我們在寫data配置項時，會將其定義為函數(shù)，因此這里執(zhí)行了getData函數(shù)：
function getData (data, vm) {
// #7573 disable dep collection when invoking data getters
pushTarget();
try {
return data.call(vm, vm)
} catch (e) {
handleError(e, vm, "data()");
return {}
} finally {
popTarget();
}
}getData函數(shù)做的事情非常簡單，就是在組件實例上下文中執(zhí)行data函數(shù)。注意，在執(zhí)行data函數(shù)前后，分別執(zhí)行了pushTarget函數(shù)和popTarget函數(shù)，這兩個函數(shù)我們后面再講。
執(zhí)行getData函數(shù)后，我們回到initData函數(shù)，后面有一個循環(huán)的錯誤判斷，暫時不用管。于是我們來到了observe函數(shù)：
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函數(shù)為data對象創(chuàng)建了一個觀察者（ob），也就是實例化Observer，實例化Observer具體做了什么呢？我們繼續(xù)看源碼：
var Observer = function Observer (value) {
this.value = 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);
}
}正常情況下，因為我們定義的data函數(shù)返回的都是一個對象，所以這里我們先不管對數(shù)組的處理。那么就是繼續(xù)執(zhí)行walk函數(shù)：
Observer.prototype.walk = function walk (obj) {
var keys = Object.keys(obj);
for (var i = 0; i < keys.length; i++) {
defineReactive$$1(obj, keys[i]);
}
}對于data函數(shù)返回的對象，即組件實例的data對象中的每個可枚舉屬性，執(zhí)行defineReactive$$1函數(shù)：
function defineReactive$$1 (
obj,
key,
val,
customSetter,
shallow
) {
var dep = new Dep();
var property = Object.getOwnPropertyDescriptor(obj, key);
if (property && property.configurable === false) {
return
}
// cater for pre-defined getter/setters
var getter = property && property.get;
var setter = property && property.set;
if ((!getter || setter) && arguments.length === 2) {
val = obj[key];
}
var childOb = !shallow && observe(val);
Object.defineProperty(obj, key, {
enumerable: true,
configurable: true,
get: function reactiveGetter () {
var value = getter ? getter.call(obj) : val;
if (Dep.target) {
dep.depend();
if (childOb) {
childOb.dep.depend();
if (Array.isArray(value)) {
dependArray(value);
}
}
}
return value
},
set: function reactiveSetter (newVal) {
var value = getter ? getter.call(obj) : val;
/* eslint-disable no-self-compare */
if (newVal === value || (newVal !== newVal && value !== value)) {
return
}
/* eslint-enable no-self-compare */
if (customSetter) {
customSetter();
}
// #7981: for accessor properties without setter
if (getter && !setter) { return }
if (setter) {
setter.call(obj, newVal);
} else {
val = newVal;
}
childOb = !shallow && observe(newVal);
dep.notify();
}
});
}在defineReactive$$1函數(shù)中，首先實例化一個依賴收集器。然后使用Object.defineProperty重新定義對象屬性的getter（即上面的get函數(shù)）和setter（即上面的set函數(shù)）。
getter和setter某種意義上可以理解為回調(diào)函數(shù)，當讀取對象某個屬性的值時，會觸發(fā)get函數(shù)（即getter）；當設置對象某個屬性的值時，會觸發(fā)set函數(shù)（即setter）。我們回到最開始的例子：
<template>
<div>
<span>{{ a }}</span>
<span>{{ b }}</span>
</div>
</template>
<script type="javascript">
export default {
data() {
return {
a: 0,
b: 0
}
},
created() {
// some logic code
this.a = 1
this.b = 2
}
}
</script>這里有設置this對象的屬性a和屬性b的值，因此會觸發(fā)setter。我們把上面set函數(shù)代碼單獨拿出來：
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 setter
if (getter && !setter) { return }
if (setter) {
setter.call(obj, newVal);
} else {
val = newVal;
}
childOb = !shallow && observe(newVal);
dep.notify();
}setter先執(zhí)行了getter：
function reactiveGetter () {
var value = getter ? getter.call(obj) : val;
if (Dep.target) {
dep.depend();
if (childOb) {
childOb.dep.depend();
if (Array.isArray(value)) {
dependArray(value);
}
}
}
return value
}getter先檢測Dep.target是否存在。在前面執(zhí)行getData函數(shù)的時候，Dep.target的初始值為null，它在什么時候被賦值了呢？我們前面講getData函數(shù)的時候，有看到一個pushTarget函數(shù)和popTarget函數(shù)，這兩個函數(shù)的源碼如下：
Dep.target = null;
var targetStack = [];
function pushTarget (target) {
targetStack.push(target);
Dep.target = target;
}
function popTarget () {
targetStack.pop();
Dep.target = targetStack[targetStack.length - 1];
}想要正常執(zhí)行getter，就需要先執(zhí)行pushTarget函數(shù)。我們找找pushTarget函數(shù)在哪里執(zhí)行的。在vue.js中搜索pushTarget，我們找到了5個地方，除去定義的地方，執(zhí)行的地方有4個。
第一個執(zhí)行pushTarget函數(shù)的地方。這是一個處理錯誤的函數(shù)，正常邏輯不會觸發(fā)：
function handleError (err, vm, info) {
// Deactivate deps tracking while processing error handler to avoid possible infinite rendering.
// See: https://github.com/vuejs/vuex/issues/1505
pushTarget();
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();
}
}第二個執(zhí)行pushTarget的地方。這是調(diào)用對應的鉤子函數(shù)。在執(zhí)行到對應的鉤子函數(shù)時會觸發(fā)。不過，我們現(xiàn)在的操作介于beforeCreate鉤子和created鉤子之間，還沒有觸發(fā)：
function callHook (vm, hook) {
// #7573 disable dep collection when invoking lifecycle hooks
pushTarget();
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();
}第三個執(zhí)行pushTarget的地方。這是實例化watcher時執(zhí)行的函數(shù)。檢查前面的代碼，我們似乎也沒有看到new Watcher的操作：
Watcher.prototype.get = function get () {
pushTarget(this);
var value;
var vm = this.vm;
try {
value = 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 watching
if (this.deep) {
traverse(value);
}
popTarget();
this.cleanupDeps();
}
return value
}第四個執(zhí)行pushTarget的地方，這就是前面的getData函數(shù)。但是getData函數(shù)的執(zhí)行位于defineReactive$$1函數(shù)之前。在執(zhí)行完getData函數(shù)以后，Dep.target已經(jīng)被重置為null了。
function getData (data, vm) {
// #7573 disable dep collection when invoking data getters
pushTarget();
try {
return data.call(vm, vm)
} catch (e) {
handleError(e, vm, "data()");
return {}
} finally {
popTarget();
}
}看起來，直接觸發(fā)setter并不能讓getter中的邏輯正常執(zhí)行。并且，我們還發(fā)現(xiàn)，由于setter中也有Dep.target的判斷，所以如果我們找不到Dep.target的來源，setter的邏輯也無法繼續(xù)往下走。
那么，到底Dep.target的值是從哪里來的呢？不用著急，我們回到_init函數(shù)的操作繼續(xù)往下看：
Vue.prototype._init = function (options) {
var vm = this;
// a uid
vm._uid = uid$3++;
var startTag, endTag;
/* istanbul ignore if */
if (config.performance && mark) {
startTag = "vue-perf-start:" + (vm._uid);
endTag = "vue-perf-end:" + (vm._uid);
mark(startTag);
}
// a flag to avoid this being observed
vm._isVue = true;
// merge options
if (options && options._isComponent) {
// optimize internal component instantiation
// since dynamic options merging is pretty slow, and none of the
// internal component options needs special treatment.
initInternalComponent(vm, options);
} else {
vm.$options = mergeOptions(
resolveConstructorOptions(vm.constructor),
options || {},
vm
);
}
/* istanbul ignore else */
{
initProxy(vm);
}
// expose real self
vm._self = vm;
initLifecycle(vm);
initEvents(vm);
initRender(vm);
callHook(vm, 'beforeCreate');
initInjections(vm); // resolve injections before data/props
initState(vm);
initProvide(vm); // resolve provide after data/props
callHook(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.$options.el);
}
}我們發(fā)現(xiàn)，在_init函數(shù)的最后，執(zhí)行了vm.$mount函數(shù)，這個函數(shù)做了什么呢？
Vue.prototype.$mount = function (
el,
hydrating
) {
el = el && inBrowser ? query(el) : undefined;
return mountComponent(this, el, hydrating)
}我們繼續(xù)進入mountComponent函數(shù)看看：
function mountComponent (
vm,
el,
hydrating
) {
vm.$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 $forceUpdate (e.g. inside child
// component's mounted hook), which relies on vm._watcher being already defined
new Watcher(vm, updateComponent, noop, {
before: function before () {
if (vm._isMounted && !vm._isDestroyed) {
callHook(vm, 'beforeUpdate');
}
}
}, true /* isRenderWatcher */);
hydrating = false;
// manually mounted instance, call mounted on self
// mounted is called for render-created child components in its inserted hook
if (vm.$vnode == null) {
vm._isMounted = true;
callHook(vm, 'mounted');
}
return vm
}我們驚喜地發(fā)現(xiàn)，這里有一個new Watcher的操作！真是山重水復疑無路，柳暗花明又一村！這里實例化的watcher是一個用來更新dom的watcher。他會依次讀取SFC文件中的template部分中的所有值。這也就意味著會觸發(fā)對應的getter。
由于new Watcher會執(zhí)行watcher.get函數(shù)，該函數(shù)執(zhí)行pushTarget函數(shù)，于是Dep.target被賦值。getter內(nèi)部的邏輯順利執(zhí)行。
至此，我們終于到了Vue的響應式原理的核心。我們再次回到getter，看一看有了Dep.target以后，getter做了什么：
function reactiveGetter () {
var value = getter ? getter.call(obj) : val;
if (Dep.target) {
dep.depend();
if (childOb) {
childOb.dep.depend();
if (Array.isArray(value)) {
dependArray(value);
}
}
}
return value
}同樣地，我們先不關注提高代碼健壯性的細節(jié)處理，直接看主線?？梢钥吹剑擠ep.target存在時，執(zhí)行了dep.depend函數(shù)。這個函數(shù)做了什么呢？我們看看代碼：
Dep.prototype.depend = function depend () {
if (Dep.target) {
Dep.target.addDep(this);
}
}做的事情也非常簡單。就是執(zhí)行了Dep.target.addDep函數(shù)。但是Dep.target其實是一個watcher，所以我們要回到Watcher的代碼：
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);
}
}
}同樣地，我們先忽略一些次要的邏輯處理，把注意力集中到dep.addSub函數(shù)上：
Dep.prototype.addSub = function addSub (sub) {
this.subs.push(sub);
}也是非常簡單的邏輯，把watcher作為一個訂閱者推入數(shù)組中緩存。至此，getter的整個邏輯走完。此后執(zhí)行popTarget函數(shù)，Dep.target被重置為null
我們再次回到業(yè)務代碼：
<template>
<div>
<span>{{ a }}</span>
<span>{{ b }}</span>
</div>
</template>
<script type="javascript">
export default {
data() {
return {
a: 0,
b: 0
}
},
created() {
// some logic code
this.a = 1
this.b = 2
}
}
</script>在created生命周期中，我們觸發(fā)了兩次setter，setter執(zhí)行的邏輯如下：
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 setter
if (getter && !setter) { return }
if (setter) {
setter.call(obj, newVal);
} else {
val = newVal;
}
childOb = !shallow && observe(newVal);
dep.notify();
}這里，我們只需要關注setter最后執(zhí)行的函數(shù)：dep.notify()。我們看看這個函數(shù)做了什么：
Dep.prototype.notify = function notify () {
// stabilize the subscriber list first
var 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
// order
subs.sort(function (a, b) { return a.id - b.id; });
}
for (var i = 0, l = subs.length; i < l; i++) {
subs[i].update();
}
}This.subs的每一項元素均為一個watcher。在上面getter章節(jié)中，我們只收集到了一個watcher。因為觸發(fā)了兩次setter，所以subs[0].update()，即watcher.update()函數(shù)會執(zhí)行兩次。我們看看這個函數(shù)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);
}
}按照慣例，我們直接跳入queueWatcher函數(shù)：
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 flush
if (!waiting) {
waiting = true;
if (!config.async) {
flushSchedulerQueue();
return
}
nextTick(flushSchedulerQueue);
}
}
}由于id相同，所以watcher的回調(diào)函數(shù)只會被推入到queue一次。這里我們再次看到了一個熟悉的面孔：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-line
if (!cb && typeof Promise !== 'undefined') {
return new Promise(function (resolve) {
_resolve = resolve;
})
}
}nextTick函數(shù)將回調(diào)函數(shù)再次包裹一層后，執(zhí)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.x
MutationObserver.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 = 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函數(shù)是微任務的平穩(wěn)降級。他將根據(jù)所在環(huán)境的支持程度，依次調(diào)用Promise、MutationObserver、setImmediate和setTimeout。并在對應的微任務或者模擬微任務隊列中執(zhí)行回調(diào)函數(shù)。
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 watchers
for (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 state
var activatedQueue = activatedChildren.slice();
var updatedQueue = queue.slice();
resetSchedulerState();
// call component updated and activated hooks
callActivatedHooks(activatedQueue);
callUpdatedHooks(updatedQueue);
// devtool hook
/* istanbul ignore if */
if (devtools && config.devtools) {
devtools.emit('flush');
}
}回調(diào)函數(shù)的核心邏輯是執(zhí)行watcher.run函數(shù)：
Watcher.prototype.run = function run () {
if (this.active) {
var value = 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 value
var 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);
}
}
}
}執(zhí)行this.cb函數(shù)，即watcher的回調(diào)函數(shù)。至此，所有的邏輯走完。
我們再次回到業(yè)務場景：
<template>
<div>
<span>{{ a }}</span>
<span>{{ b }}</span>
</div>
</template>
<script type="javascript">
export default {
data() {
return {
a: 0,
b: 0
}
},
created() {
// some logic code
this.a = 1
this.b = 2
}
}
</script>雖然我們觸發(fā)了兩次setter，但是對應的渲染函數(shù)在微任務中卻只執(zhí)行了一次。也就是說，在dep.notify函數(shù)發(fā)出通知以后，Vue將對應的watcher進行了去重、排隊操作并最終執(zhí)行回調(diào)。
可以看出，兩次賦值操作實際上觸發(fā)的是同一個渲染函數(shù)，這個渲染函數(shù)更新了多個dom。這就是所謂的批量更新dom。
關于“Vue如何批量更新dom”這篇文章就分享到這里了，希望以上內(nèi)容可以對大家有一定的幫助，使各位可以學到更多知識，如果覺得文章不錯，請把它分享出去讓更多的人看到。
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