性能优化
约 2133 字大约 7 分钟
2026-02-11
Vue2 在设计时就考虑了性能问题，通过多种机制来优化运行时性能。本文将深入探讨Vue2性能优化的实现原理和内部机制。
性能优化概述
Vue2 的性能优化主要包括：
- 静态节点提升
- 事件缓存
- 虚拟DOM Diff算法优化
- 异步更新队列
- 组件级别优化
- 内存管理优化
静态节点提升
Vue2 编译器会识别模板中的静态节点并将其提升，避免重复创建：
// src/compiler/optimizer.js
function markStatic (node: ASTNode) {
node.static = isStatic(node)
if (node.type === 1) {
// 不对组件槽内容进行静态标记
if (
!isPlatformReservedTag(node.tag) &&
node.tag !== 'slot' &&
node.attrsMap['inline-template'] == null
) {
return
}
for (let i = 0, l = node.children.length; i < l; i++) {
const child = node.children[i]
markStatic(child)
if (!child.static) {
node.static = false
}
}
if (node.ifConditions) {
for (let i = 1, l = node.ifConditions.length; i < l; i++) {
const block = node.ifConditions[i].block
markStatic(block)
if (!block.static) {
node.static = false
}
}
}
}
}
function markStaticRoots (node: ASTElement, isInFor: boolean) {
if (node.type === 1) {
if (node.static || node.once) {
node.staticInFor = isInFor
}
// 为了成为静态根节点，节点应该有子节点，
// 而不只是静态文本。否则提升的成本会超过收益
if (node.static && node.children.length && !(
node.children.length === 1 &&
node.children[0].type === 3
)) {
node.staticRoot = true
return
} else {
node.staticRoot = false
}
if (node.children) {
for (let i = 0, l = node.children.length; i < l; i++) {
markStaticRoots(node.children[i], isInFor || !!node.for)
}
}
if (node.ifConditions) {
for (let i = 1, l = node.ifConditions.length; i < l; i++) {
markStaticRoots(node.ifConditions[i].block, isInFor)
}
}
}
}
// src/compiler/codegen/index.js
function genStatic (el: ASTElement, state: CodegenState): string {
el.staticProcessed = true
// 一些元素（模板）在 v-pre 节点内需要有不同的行为
const originalPreState = state.pre
if (el.pre) {
state.pre = el.pre
}
state.staticRenderFns.push(`with(this){return ${genElement(el, state)}}`)
state.pre = originalPreState
return `_m(${
state.staticRenderFns.length - 1
}${
el.staticInFor ? ',true' : ''
})`
}事件缓存
Vue2 会缓存事件处理函数以避免重复创建：
// src/compiler/codegen/events.js
export function genHandlers (
events: ASTElementHandlers,
isNative: boolean,
warn: Function
): string {
let res = isNative ? 'nativeOn:{' : 'on:{'
for (const name in events) {
res += `"${name}":${genHandler(name, events[name])},`
}
return res.slice(0, -1) + '}'
}
function genHandler (name: string, handler: ASTElementHandler | Array<ASTElementHandler>): string {
if (!handler) {
return 'function(){}'
}
if (Array.isArray(handler)) {
return `[${handler.map(handler => genHandler(name, handler)).join(',')}]`
}
const isMethodPath = simplePathRE.test(handler.value)
const isFunctionExpression = fnExpRE.test(handler.value)
const isFunctionInvocation = simplePathRE.test(handler.value.replace(leftParenRE, ''))
if (!handler.modifiers) {
if (isMethodPath || isFunctionExpression) {
return handler.value
}
/* istanbul ignore if */
if (__WEEX__ && handler.params) {
return genWeexHandler(handler.params, handler.value)
}
return `function($event){${
isFunctionInvocation ? `return ${handler.value}` : handler.value
}}` // inline statement
} else {
// 复杂的带修饰符的事件处理
let code = ''
let genModifierCode = ''
const keys = Object.keys(handler.modifiers)
if (keys.length) {
genModifierCode += genKeys(handler)
}
// 确保按键修饰符在其他修饰符之后
if (handler.modifiers.kbd) {
genModifierCode += genKeyFilter(handler)
}
// 进入过渡模式
if (handler.modifiers.enter) {
genModifierCode += genEnterFilter(handler)
}
// 生成事件处理代码
let handlerCode = isMethodPath
? `return ${handler.value}($event)`
: isFunctionExpression
? `return (${handler.value})($event)`
: isFunctionInvocation
? `return ${handler.value}`
: handler.value
/* istanbul ignore if */
if (__WEEX__ && handler.params) {
handlerCode = genWeexHandler(handler.params, handler.value)
}
return `function($event){${genModifierCode}${handlerCode}}`
}
}
// 事件缓存的实现
export function genData (el: ASTElement, state: CodegenState): string {
let data = '{'
// directives first.
// directives may mutate the el's other properties before they are generated.
const dirs = genDirectives(el, state)
if (dirs) data += dirs + ','
// key
if (el.key) {
data += `key:${el.key},`
}
// ref
if (el.ref) {
data += `ref:${el.ref},`
}
if (el.refInFor) {
data += `refInFor:true,`
}
// pre
if (el.pre) {
data += `pre:true,`
}
// record original tag name for components using "is" attribute
if (el.component) {
data += `tag:"${el.tag}",`
}
// module data generation functions
for (let i = 0; i < state.dataGenFns.length; i++) {
data += state.dataGenFns[i](el, state)
}
// attributes
if (el.attrs) {
data += `attrs:${genProps(el.attrs)},`
}
// ... 其他属性
// 事件处理
if (el.events) {
data += `on:${genHandlers(el.events, false, state.warn)},`
}
if (el.nativeEvents) {
data += `nativeOn:${genHandlers(el.nativeEvents, true, state.warn)},`
}
// ...
return data.replace(/,$/, '') + '}'
}虚拟DOM Diff算法优化
Vue2 的Diff算法采用了多种优化策略：
// src/core/vdom/patch.js
function updateChildren (parentElm, oldCh, newCh, insertedVnodeQueue, removeOnly) {
let oldStartIdx = 0
let newStartIdx = 0
let oldEndIdx = oldCh.length - 1
let newEndIdx = newCh.length - 1
let oldStartVnode = oldCh[0]
let oldEndVnode = oldCh[oldEndIdx]
let newStartVnode = newCh[0]
let newEndVnode = newCh[newEndIdx]
let oldKeyToIdx, idxInOld, vnodeToMove, refElm
// removeOnly is a special flag used only by <transition-group>
// to ensure removed elements stay in correct relative positions
// during leaving transitions
const canMove = !removeOnly
if (process.env.NODE_ENV !== 'production') {
checkDuplicateKeys(newCh)
}
while (oldStartIdx <= oldEndIdx && newStartIdx <= newEndIdx) {
if (isUndef(oldStartVnode)) {
oldStartVnode = oldCh[++oldStartIdx] // Vnode has been moved left
} else if (isUndef(oldEndVnode)) {
oldEndVnode = oldCh[--oldEndIdx]
} else if (sameVnode(oldStartVnode, newStartVnode)) {
patchVnode(oldStartVnode, newStartVnode, insertedVnodeQueue, newCh, newStartIdx)
oldStartVnode = oldCh[++oldStartIdx]
newStartVnode = newCh[++newStartIdx]
} else if (sameVnode(oldEndVnode, newEndVnode)) {
patchVnode(oldEndVnode, newEndVnode, insertedVnodeQueue, newCh, newEndIdx)
oldEndVnode = oldCh[--oldEndIdx]
newEndVnode = newCh[--newEndIdx]
} else if (sameVnode(oldStartVnode, newEndVnode)) { // Vnode moved right
patchVnode(oldStartVnode, newEndVnode, insertedVnodeQueue, newCh, newEndIdx)
canMove && nodeOps.insertBefore(parentElm, oldStartVnode.elm, nodeOps.nextSibling(oldEndVnode.elm))
oldStartVnode = oldCh[++oldStartIdx]
newEndVnode = newCh[--newEndIdx]
} else if (sameVnode(oldEndVnode, newStartVnode)) { // Vnode moved left
patchVnode(oldEndVnode, newStartVnode, insertedVnodeQueue, newCh, newStartIdx)
canMove && nodeOps.insertBefore(parentElm, oldEndVnode.elm, oldStartVnode.elm)
oldEndVnode = oldCh[--oldEndIdx]
newStartVnode = newCh[++newStartIdx]
} else {
if (isUndef(oldKeyToIdx)) oldKeyToIdx = createKeyToOldIdx(oldCh, oldStartIdx, oldEndIdx)
idxInOld = isDef(newStartVnode.key)
? oldKeyToIdx[newStartVnode.key]
: findIdxInOld(newStartVnode, oldCh, oldStartIdx, oldEndIdx)
if (isUndef(idxInOld)) { // New element
createElm(newStartVnode, insertedVnodeQueue, parentElm, oldStartVnode.elm, false, newCh, newStartIdx)
} else {
vnodeToMove = oldCh[idxInOld]
if (sameVnode(vnodeToMove, newStartVnode)) {
patchVnode(vnodeToMove, newStartVnode, insertedVnodeQueue, newCh, newStartIdx)
oldCh[idxInOld] = undefined
canMove && nodeOps.insertBefore(parentElm, vnodeToMove.elm, oldStartVnode.elm)
} else {
// same key but different element. treat as new element
createElm(newStartVnode, insertedVnodeQueue, parentElm, oldStartVnode.elm, false, newCh, newStartIdx)
}
}
newStartVnode = newCh[++newStartIdx]
}
}
if (oldStartIdx > oldEndIdx) {
refElm = isUndef(newCh[newEndIdx + 1]) ? null : newCh[newEndIdx + 1].elm
addVnodes(parentElm, refElm, newCh, newStartIdx, newEndIdx, insertedVnodeQueue)
} else if (newStartIdx > newEndIdx) {
removeVnodes(oldCh, oldStartIdx, oldEndIdx)
}
}
// sameVnode 函数优化了节点比较
function sameVnode (a, b) {
return (
a.key === b.key && (
(
a.tag === b.tag &&
a.isComment === b.isComment &&
isDef(a.data) === isDef(b.data) &&
sameInputType(a, b)
) || (
isTrue(a.isAsyncPlaceholder) &&
a.asyncFactory === b.asyncFactory &&
isUndef(b.asyncResolved)
)
)
)
}异步更新队列
Vue2 使用异步更新队列来批量处理DOM更新，减少重排重绘：
// src/core/observer/scheduler.js
const queue: Array<Watcher> = []
let has: { [key: number]: ?true } = {}
let circular: { [key: number]: number } = {}
let waiting = false
let flushing = false
let index = 0
/**
* Push a watcher into the watcher queue.
* Jobs with duplicate IDs will be skipped unless it's
* pushed when the queue is being flushed.
*/
export function queueWatcher (watcher: Watcher) {
const id = watcher.id
if (has[id] == null) {
has[id] = true
if (!flushing) {
queue.push(watcher)
} else {
// 如果正在刷新队列，则根据id排序插入
let 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
nextTick(flushSchedulerQueue)
}
}
}
function flushSchedulerQueue () {
currentFlushTimestamp = getNow()
waiting = flushing = true
let 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((a, b) => 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 (process.env.NODE_ENV !== 'production' && 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
const activatedQueue = activatedChildren.slice()
const 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')
}
}组件级别优化
Vue2 提供了多种组件级别的性能优化：
keep-alive 组件
// src/core/components/keep-alive.js
export default {
name: 'keep-alive',
abstract: true,
props: {
include: patternTypes,
exclude: patternTypes,
max: [String, Number]
},
methods: {
cacheVNode() {
const { cache, keys, vnodeToCache } = this
if (vnodeToCache) {
const { id, tag, componentInstance, componentOptions } = vnodeToCache
cache[id] = {
name: getComponentName(componentOptions),
id,
tag,
componentInstance,
}
keys.push(id)
// prune oldest entry
if (this.max && keys.length > parseInt(this.max)) {
pruneCacheEntry(cache, keys[0], this._vnode, this.keys)
}
this.vnodeToCache = null
}
}
},
created() {
this.cache = Object.create(null)
this.keys = []
},
destroyed() {
for (const key in this.cache) {
pruneCacheEntry(this.cache, key, this._vnode)
}
},
mounted() {
this.cacheVNode()
this.$watch('include', val => {
pruneCache(this.cache, this.keys, name => matches(val, name))
})
this.$watch('exclude', val => {
pruneCache(this.cache, this.keys, name => !matches(val, name))
})
},
updated() {
this.cacheVNode()
},
render() {
const slot = this.$slots.default
const vnode: VNode = getFirstComponentChild(slot)
const componentOptions: ?VNodeComponentOptions = vnode && vnode.componentOptions
if (componentOptions) {
// check pattern
const name: ?string = getComponentName(componentOptions)
const { include, exclude } = this
if (
// not included
(include && (!name || !matches(include, name))) ||
// excluded
(exclude && name && matches(exclude, name))
) {
return vnode
}
const { cache, keys } = this
const key: ?string = vnode.key == null
// same constructor may get registered as different local components
// so cid alone is not enough (#3269)
? componentOptions.Ctor.cid + (componentOptions.tag ? `::${componentOptions.tag}` : '')
: vnode.key
if (cache[key]) {
vnode.componentInstance = cache[key].componentInstance
// make current key freshest
remove(keys, key)
keys.push(key)
} else {
// delay setting the cache until update()
this.vnodeToCache = vnode
}
vnode.data.keepAlive = true
}
return vnode || (slot && slot[0])
}
}计算属性优化
Vue2 的计算属性具有缓存机制，只有依赖变化时才会重新计算：
// src/core/observer/watcher.js
export default class Watcher {
// ...
evaluate () {
this.value = this.get()
this.dirty = false
}
depend () {
let i = this.deps.length
while (i--) {
this.deps[i].depend()
}
}
}
// src/core/observer/computed.js
function createComputedGetter (key) {
return function computedGetter () {
const watcher = this._computedWatchers && this._computedWatchers[key]
if (watcher) {
if (watcher.dirty) {
watcher.evaluate()
}
if (Dep.target) {
watcher.depend()
}
return watcher.value
}
}
}内存管理优化
Vue2 在组件销毁时会清理相关的观察者和依赖：
// src/core/instance/lifecycle.js
Vue.prototype.$destroy = function () {
const vm = this
if (vm._isBeingDestroyed) {
return
}
callHook(vm, 'beforeDestroy')
vm._isBeingDestroyed = true
// remove self from parent
const parent = vm.$parent
if (parent && !parent._isBeingDestroyed && !vm.$options.abstract) {
remove(parent.$children, vm)
}
// teardown watchers
if (vm._watcher) {
vm._watcher.teardown()
}
let i = vm._watchers.length
while (i--) {
vm._watchers[i].teardown()
}
// remove reference from data ob
// frozen object may not have observer.
if (vm._data.__ob__) {
vm._data.__ob__.vmCount--
}
// call the last hook...
vm._isDestroyed = true
// invoke destroy hooks on current rendered tree
vm.__patch__(vm._vnode, null)
// fire destroyed hook
callHook(vm, 'destroyed')
// turn off all instance listeners.
vm.$off()
// remove __vue__ reference
if (vm.$el) {
vm.$el.__vue__ = null
}
// release circular reference (#6759)
if (vm.$vnode) {
vm.$vnode.parent = null
}
}性能监控
Vue2 提供了性能监控功能：
// 在 _init 方法中
if (process.env.NODE_ENV !== 'production' && config.performance && mark) {
vm._name = formatComponentName(vm, false)
mark(endTag)
measure(`vue ${vm._name} init`, startTag, endTag)
}
// 在 mountComponent 方法中
if (process.env.NODE_ENV !== 'production' && config.performance && mark) {
updateComponent = () => {
const name = vm._name
const id = vm._uid
const startTag = `vue-perf-start:${id}`
const endTag = `vue-perf-end:${id}`
mark(startTag)
const 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 = () => {
vm._update(vm._render(), hydrating)
}
}总结
Vue2 通过多种机制来优化性能：
- 编译时优化: 静态节点提升、事件缓存
- 运行时优化: 异步更新队列、高效的Diff算法
- 组件优化: keep-alive、计算属性缓存
- 内存管理: 组件销毁时清理观察者和依赖
- 性能监控: 提供性能测量工具
这些优化措施共同作用，使得Vue2在大多数场景下都能提供良好的性能表现。