模板tamplate经过parse，optimize，generate等一些列操作之后，把AST转为render function code进而生成虚拟VNode,模板编译阶段基本已经完成了，那么这一章，我们来探讨一下Vue中的一个算法策略–dom diff 首先来介绍下什么叫dom diff

什么是虚拟dom

我们经过前面的章节学习已经知道，要知道渲染真实DOM的开销是很大的，比如有时候我们修改了某个数据，如果直接渲染到真实dom上会引起整个dom树的重绘和重排，有没有可能我们只更新我们修改的那一小块dom而不要更新整个dom呢？

为了解决这个问题，我们的解决方案是–根据真实DOM生成一颗virtual DOM，当virtual DOM某个节点的数据改变后会生成一个新的Vnode，然后Vnode和oldVnode作对比，发现有不一样的地方就直接修改在真实的DOM上，然后使oldVnode的值为Vnode。这也就是我们所说的一个虚拟dom diff的过程

图示

传统的Diff算法所耗费的时间复杂度为O(n^3),那么这个O(n^3)是怎么算出来的？

1. 传统diff算法时间复杂度为n(第一次Old与新的所有节点对比)----O(n)
2. 传统diff算法时间复杂度为n(第二次Old树的所有节点与新的所有节点对比)----O(n^2)
3. 新树的生成，节点可变编辑，时间复杂度为n(遍历当前树)----O(n^3)

第一次对比 (1:n)

第二次对比 (1:n)

第n次对比 (n:n)

到这里那么n个节点与n个节点暴力对比就对比完了，那么就开启第三轮可编辑树节点遍历，更改之后的树由vdom(old)到vdom(new)

故而传统diff算法O(n^3)是这么算出来的,但是这不是我们今天研究的重点。

现代diff算法

现代diff算法策略说的是，同层级比较，广度优先

那么这里的话我们要深入源码了，在深入源码之前我们在心中应该形成这样一个概念，整个diff的流程是什么？我们再对比着源码解读

diff算法流程图

深入源码

我们在Vue初始化的时候调用lifecycleMixin函数的时候，会给Vue的原型上挂载_update方法

_update

Vue.prototype._update = function (vnode: VNode, hydrating?: boolean) {const vm: Component = thisif (vm._isMounted) {//会调用声明周期中的beforeUpdate回调函数callHook(vm, 'beforeUpdate')}const prevEl = vm.$elconst prevVnode = vm._vnodeconst prevActiveInstance = activeInstanceactiveInstance = vmvm._vnode = vnode// Vue.prototype.__patch__ is injected in entry points// based on the rendering backend used.//若组件本身的vnode未生成,直接用传入的vnode生成domif (!prevVnode) {// initial rendervm.$el = vm.__patch__(vm.$el, vnode, hydrating, false /* removeOnly */,vm.$options._parentElm,vm.$options._refElm)// no need for the ref nodes after initial patch// this prevents keeping a detached DOM tree in memory (#5851)vm.$options._parentElm = vm.$options._refElm = null} else {//对新旧vnode进行diff// updatesvm.$el = vm.__patch__(prevVnode, vnode)}activeInstance = prevActiveInstance// update __vue__ referenceif (prevEl) {prevEl.__vue__ = null}if (vm.$el) {vm.$el.__vue__ = vm}// if parent is an HOC, update its $el as wellif (vm.$vnode && vm.$parent && vm.$vnode === vm.$parent._vnode) {vm.$parent.$el = vm.$el}

我们在这里可以看到vm.$el = vm.__patch__方法，追根溯源_patch_的定义：

Vue.prototype.__patch__ = inBrowser ? patch : noop

可见这里是一个浏览器环境的鉴别，如果在浏览器环境中，我们会执行patch，不在的话会执行noop，这是一个util里面的一个方法，用来跨平台的，我们这里暂时不考虑，接着我们去看patch的具体实现./patch文件，

import * as nodeOps from 'web/runtime/node-ops'
import { createPatchFunction } from 'core/vdom/patch'
import baseModules from 'core/vdom/modules/index'
import platformModules from 'web/runtime/modules/index'
const modules = platformModules.concat(baseModules)export const patch: Function = createPatchFunction({ nodeOps, modules })

createPatchFunction函数

参考 Vue面试题详细解答

/** * 创建patch方法 */
export function createPatchFunction (backend) {let i, jconst cbs = {}const { modules, nodeOps } = backendfor (i = 0; i < hooks.length; ++i) {cbs[hooks[i]] = []for (j = 0; j < modules.length; ++j) {if (isDef(modules[j][hooks[i]])) {cbs[hooks[i]].push(modules[j][hooks[i]])}}}function emptyNodeAt (elm) {return new VNode(nodeOps.tagName(elm).toLowerCase(), {}, [], undefined, elm)}/**   * 创建一个回调方法, 用于删除节点   *    *    */function createRmCb (childElm, listeners) {function remove () {if (--remove.listeners === 0) {removeNode(childElm)}}remove.listeners = listenersreturn remove}function removeNode (el) {const parent = nodeOps.parentNode(el)// element may have already been removed due to v-html / v-textif (isDef(parent)) {nodeOps.removeChild(parent, el)}}/**   * 通过vnode的tag判断是否是原生dom标签或者组件标签   * 用于创建真实DOM节点时, 预先判断tag的合法性   */function isUnknownElement (vnode, inVPre) {return (!inVPre &&!vnode.ns &&!(config.ignoredElements.length &&config.ignoredElements.some(ignore => {return isRegExp(ignore)? ignore.test(vnode.tag): ignore === vnode.tag})) &&config.isUnknownElement(vnode.tag))}let creatingElmInVPre = 0// 创建一个节点function createElm (vnode,insertedVnodeQueue,parentElm,refElm,nested,ownerArray,index) {// 节点已经被渲染, 需要使用一个克隆节点if (isDef(vnode.elm) && isDef(ownerArray)) {// This vnode was used in a previous render!// now it's used as a new node, overwriting its elm would cause// potential patch errors down the road when it's used as an insertion// reference node. Instead, we clone the node on-demand before creating// associated DOM element for it.vnode = ownerArray[index] = cloneVNode(vnode)}// 创建组件节点 详见本文件中的createComponent方法vnode.isRootInsert = !nested // for transition enter checkif (createComponent(vnode, insertedVnodeQueue, parentElm, refElm)) {return}const data = vnode.dataconst children = vnode.childrenconst tag = vnode.tag/**     * 如果要创建的节点有tag属性, 这里做一下校验     * 如果该节点上面有v-pre指令, 直接给flag加1     * 如果没有v-pre需要调用isUnknownElement判断标签是否合法, 然后给出警告     */if (isDef(tag)) {if (process.env.NODE_ENV !== 'production') {if (data && data.pre) {creatingElmInVPre++}if (isUnknownElement(vnode, creatingElmInVPre)) {warn('Unknown custom element: <' + tag + '> - did you ' +'register the component correctly? For recursive components, ' +'make sure to provide the "name" option.',vnode.context)}}vnode.elm = vnode.ns? nodeOps.createElementNS(vnode.ns, tag): nodeOps.createElement(tag, vnode)setScope(vnode)/* istanbul ignore if */if (__WEEX__) {// in Weex, the default insertion order is parent-first.// List items can be optimized to use children-first insertion// with append="tree".const appendAsTree = isDef(data) && isTrue(data.appendAsTree)if (!appendAsTree) {if (isDef(data)) {invokeCreateHooks(vnode, insertedVnodeQueue)}insert(parentElm, vnode.elm, refElm)}createChildren(vnode, children, insertedVnodeQueue)if (appendAsTree) {if (isDef(data)) {invokeCreateHooks(vnode, insertedVnodeQueue)}insert(parentElm, vnode.elm, refElm)}} else {createChildren(vnode, children, insertedVnodeQueue)if (isDef(data)) {invokeCreateHooks(vnode, insertedVnodeQueue)}insert(parentElm, vnode.elm, refElm)}if (process.env.NODE_ENV !== 'production' && data && data.pre) {creatingElmInVPre--}} else if (isTrue(vnode.isComment)) {vnode.elm = nodeOps.createComment(vnode.text)insert(parentElm, vnode.elm, refElm)} else {vnode.elm = nodeOps.createTextNode(vnode.text)insert(parentElm, vnode.elm, refElm)}}/**   * 创建组件   * 如果组件实例已经存在, 只需要初始化组件并重新激活组件即可   */function createComponent (vnode, insertedVnodeQueue, parentElm, refElm) {let i = vnode.dataif (isDef(i)) {const isReactivated = isDef(vnode.componentInstance) && i.keepAliveif (isDef(i = i.hook) && isDef(i = i.init)) {i(vnode, false /* hydrating */, parentElm, refElm)}// after calling the init hook, if the vnode is a child component// it should've created a child instance and mounted it. the child// component also has set the placeholder vnode's elm.// in that case we can just return the element and be done.if (isDef(vnode.componentInstance)) {initComponent(vnode, insertedVnodeQueue)if (isTrue(isReactivated)) {reactivateComponent(vnode, insertedVnodeQueue, parentElm, refElm)}return true}}}/**   * 初始化组件   * 主要的操作是已插入的vnode队列, 触发create钩子, 设置style的scope, 注册ref   */function initComponent (vnode, insertedVnodeQueue) {if (isDef(vnode.data.pendingInsert)) {insertedVnodeQueue.push.apply(insertedVnodeQueue, vnode.data.pendingInsert)vnode.data.pendingInsert = null}vnode.elm = vnode.componentInstance.$elif (isPatchable(vnode)) {invokeCreateHooks(vnode, insertedVnodeQueue)setScope(vnode)} else {// empty component root.// skip all element-related modules except for ref (#3455)registerRef(vnode)// make sure to invoke the insert hookinsertedVnodeQueue.push(vnode)}}/**   * 激活组件   */function reactivateComponent (vnode, insertedVnodeQueue, parentElm, refElm) {let i// hack for #4339: a reactivated component with inner transition// does not trigger because the inner node's created hooks are not called// again. It's not ideal to involve module-specific logic in here but// there doesn't seem to be a better way to do it.let innerNode = vnodewhile (innerNode.componentInstance) {innerNode = innerNode.componentInstance._vnodeif (isDef(i = innerNode.data) && isDef(i = i.transition)) {for (i = 0; i < cbs.activate.length; ++i) {cbs.activate[i](emptyNode, innerNode)}insertedVnodeQueue.push(innerNode)break}}// unlike a newly created component,// a reactivated keep-alive component doesn't insert itselfinsert(parentElm, vnode.elm, refElm)}/**   * 插入节点, 有父节点的插入到前面, 没有的插入到后面   */function insert (parent, elm, ref) {if (isDef(parent)) {if (isDef(ref)) {if (ref.parentNode === parent) {nodeOps.insertBefore(parent, elm, ref)}} else {nodeOps.appendChild(parent, elm)}}}function createChildren (vnode, children, insertedVnodeQueue) {if (Array.isArray(children)) {if (process.env.NODE_ENV !== 'production') {checkDuplicateKeys(children)}for (let i = 0; i < children.length; ++i) {createElm(children[i], insertedVnodeQueue, vnode.elm, null, true, children, i)}} else if (isPrimitive(vnode.text)) {nodeOps.appendChild(vnode.elm, nodeOps.createTextNode(String(vnode.text)))}}function isPatchable (vnode) {while (vnode.componentInstance) {vnode = vnode.componentInstance._vnode}return isDef(vnode.tag)}function invokeCreateHooks (vnode, insertedVnodeQueue) {for (let i = 0; i < cbs.create.length; ++i) {cbs.create[i](emptyNode, vnode)}i = vnode.data.hook // Reuse variableif (isDef(i)) {if (isDef(i.create)) i.create(emptyNode, vnode)if (isDef(i.insert)) insertedVnodeQueue.push(vnode)}}// set scope id attribute for scoped CSS.// this is implemented as a special case to avoid the overhead// of going through the normal attribute patching process.function setScope (vnode) {let iif (isDef(i = vnode.fnScopeId)) {nodeOps.setStyleScope(vnode.elm, i)} else {let ancestor = vnodewhile (ancestor) {if (isDef(i = ancestor.context) && isDef(i = i.$options._scopeId)) {nodeOps.setStyleScope(vnode.elm, i)}ancestor = ancestor.parent}}// for slot content they should also get the scopeId from the host instance.if (isDef(i = activeInstance) &&i !== vnode.context &&i !== vnode.fnContext &&isDef(i = i.$options._scopeId)) {nodeOps.setStyleScope(vnode.elm, i)}}function addVnodes (parentElm, refElm, vnodes, startIdx, endIdx, insertedVnodeQueue) {for (; startIdx <= endIdx; ++startIdx) {createElm(vnodes[startIdx], insertedVnodeQueue, parentElm, refElm, false, vnodes, startIdx)}}// 递归调用销毁钩子function invokeDestroyHook (vnode) {let i, jconst data = vnode.dataif (isDef(data)) {if (isDef(i = data.hook) && isDef(i = i.destroy)) i(vnode)for (i = 0; i < cbs.destroy.length; ++i) cbs.destroy[i](vnode)}if (isDef(i = vnode.children)) {for (j = 0; j < vnode.children.length; ++j) {invokeDestroyHook(vnode.children[j])}}}/**   * 删除多个节点   * 文本节点可以直接删除, 其他节点需要触发两个钩子   */function removeVnodes (parentElm, vnodes, startIdx, endIdx) {for (; startIdx <= endIdx; ++startIdx) {const ch = vnodes[startIdx]if (isDef(ch)) {if (isDef(ch.tag)) {removeAndInvokeRemoveHook(ch)invokeDestroyHook(ch)} else { // Text noderemoveNode(ch.elm)}}}}function removeAndInvokeRemoveHook (vnode, rm) {if (isDef(rm) || isDef(vnode.data)) {let iconst listeners = cbs.remove.length + 1if (isDef(rm)) {// we have a recursively passed down rm callback// increase the listeners countrm.listeners += listeners} else {// directly removingrm = createRmCb(vnode.elm, listeners)}// recursively invoke hooks on child component root nodeif (isDef(i = vnode.componentInstance) && isDef(i = i._vnode) && isDef(i.data)) {removeAndInvokeRemoveHook(i, rm)}for (i = 0; i < cbs.remove.length; ++i) {cbs.remove[i](vnode, rm)}if (isDef(i = vnode.data.hook) && isDef(i = i.remove)) {i(vnode, rm)} else {rm()}} else {removeNode(vnode.elm)}}// diff操作核心算法function updateChildren (parentElm, oldCh, newCh, insertedVnodeQueue, removeOnly) {// 记录新旧节点列表的首尾元素 用于比较let oldStartIdx = 0let newStartIdx = 0let oldEndIdx = oldCh.length - 1let oldStartVnode = oldCh[0]let oldEndVnode = oldCh[oldEndIdx]let newEndIdx = newCh.length - 1let newStartVnode = newCh[0]let newEndVnode = newCh[newEndIdx]let oldKeyToIdx, idxInOld, vnodeToMove, refElm// removeOnly is a special flag used only by // to ensure removed elements stay in correct relative positions// during leaving transitions// 在transition中 不能移动节点const canMove = !removeOnly// 检查是否有重复的keyif (process.env.NODE_ENV !== 'production') {checkDuplicateKeys(newCh)}// 一共分四种情况讨论, 旧列表第一个与新列表第一个对比, 旧列表最后一个与新列表最后一个对比// 然后新列表第一个和旧列表最后一个对比, 新列表最后一个和旧列表第一个对比// 之所以要交叉头尾对比, 是为了防止最差的情况出现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)oldStartVnode = oldCh[++oldStartIdx]newStartVnode = newCh[++newStartIdx]} else if (sameVnode(oldEndVnode, newEndVnode)) {patchVnode(oldEndVnode, newEndVnode, insertedVnodeQueue)oldEndVnode = oldCh[--oldEndIdx]newEndVnode = newCh[--newEndIdx]} else if (sameVnode(oldStartVnode, newEndVnode)) { // Vnode moved rightpatchVnode(oldStartVnode, newEndVnode, insertedVnodeQueue)canMove && nodeOps.insertBefore(parentElm, oldStartVnode.elm, nodeOps.nextSibling(oldEndVnode.elm))oldStartVnode = oldCh[++oldStartIdx]newEndVnode = newCh[--newEndIdx]} else if (sameVnode(oldEndVnode, newStartVnode)) { // Vnode moved leftpatchVnode(oldEndVnode, newStartVnode, insertedVnodeQueue)canMove && nodeOps.insertBefore(parentElm, oldEndVnode.elm, oldStartVnode.elm)oldEndVnode = oldCh[--oldEndIdx]newStartVnode = newCh[++newStartIdx]} else {// 以上四种情况都不满足时, 使用新列表第一个vdom的key去旧列表查找// 如果可以找到key相同的元素, 直接进行patch然后进入下一次循环// 找不到则插入一个新节点if (isUndef(oldKeyToIdx)) oldKeyToIdx = createKeyToOldIdx(oldCh, oldStartIdx, oldEndIdx)idxInOld = isDef(newStartVnode.key)? oldKeyToIdx[newStartVnode.key]: findIdxInOld(newStartVnode, oldCh, oldStartIdx, oldEndIdx)if (isUndef(idxInOld)) { // New elementcreateElm(newStartVnode, insertedVnodeQueue, parentElm, oldStartVnode.elm, false, newCh, newStartIdx)} else {vnodeToMove = oldCh[idxInOld]if (sameVnode(vnodeToMove, newStartVnode)) {patchVnode(vnodeToMove, newStartVnode, insertedVnodeQueue)oldCh[idxInOld] = undefinedcanMove && nodeOps.insertBefore(parentElm, vnodeToMove.elm, oldStartVnode.elm)} else {// same key but different element. treat as new elementcreateElm(newStartVnode, insertedVnodeQueue, parentElm, oldStartVnode.elm, false, newCh, newStartIdx)}}newStartVnode = newCh[++newStartIdx]}}// 新旧列表其中之一全部循环完成后, 开始清理剩余的节点// 如果旧列表全部遍历完成, 新列表还有剩余, 直接创建这些新节点// 反之, 如果新列表全部遍历, 旧列表还有剩余, 直接删除这些旧节点if (oldStartIdx > oldEndIdx) {refElm = isUndef(newCh[newEndIdx + 1]) ? null : newCh[newEndIdx + 1].elmaddVnodes(parentElm, refElm, newCh, newStartIdx, newEndIdx, insertedVnodeQueue)} else if (newStartIdx > newEndIdx) {removeVnodes(parentElm, oldCh, oldStartIdx, oldEndIdx)}}/**   * 检查是否有重复的key   * 一个很简单的遍历查找重复值的操作   * 其实这个seenKeys我觉得改成数组会更好, 写成object又给每个key的value置为true蛮奇怪的   */function checkDuplicateKeys (children) {const seenKeys = {}for (let i = 0; i < children.length; i++) {const vnode = children[i]const key = vnode.keyif (isDef(key)) {if (seenKeys[key]) {warn(`Duplicate keys detected: '${key}'. This may cause an update error.`,vnode.context)} else {seenKeys[key] = true}}}}/**   * 在旧的子节点列表寻找相似节点(只查找第一个)   */function findIdxInOld (node, oldCh, start, end) {for (let i = start; i < end; i++) {const c = oldCh[i]if (isDef(c) && sameVnode(node, c)) return i}}function patchVnode (oldVnode, vnode, insertedVnodeQueue, removeOnly) {// 如果oldVnode跟vnode完全一致，那么不需要做任何事情if (oldVnode === vnode) {return}const elm = vnode.elm = oldVnode.elmif (isTrue(oldVnode.isAsyncPlaceholder)) {if (isDef(vnode.asyncFactory.resolved)) {hydrate(oldVnode.elm, vnode, insertedVnodeQueue)} else {vnode.isAsyncPlaceholder = true}return}// 如果oldVnode跟vnode都是静态节点，且具有相同的key// 当vnode是克隆节点或是v-once指令控制的节点时// 只需要把oldVnode.elm和oldVnode.child都复制到vnode上，也不用再有其他操作// reuse element for static trees.// note we only do this if the vnode is cloned -// if the new node is not cloned it means the render functions have been// reset by the hot-reload-api and we need to do a proper re-render.if (isTrue(vnode.isStatic) &&isTrue(oldVnode.isStatic) &&vnode.key === oldVnode.key &&(isTrue(vnode.isCloned) || isTrue(vnode.isOnce))) {vnode.componentInstance = oldVnode.componentInstancereturn}let iconst data = vnode.dataif (isDef(data) && isDef(i = data.hook) && isDef(i = i.prepatch)) {i(oldVnode, vnode)}const oldCh = oldVnode.childrenconst ch = vnode.childrenif (isDef(data) && isPatchable(vnode)) {for (i = 0; i < cbs.update.length; ++i) cbs.update[i](oldVnode, vnode)if (isDef(i = data.hook) && isDef(i = i.update)) i(oldVnode, vnode)}// 如果vnode不是文本节点或注释节点if (isUndef(vnode.text)) {// 如果oldVnode和vnode都有子节点，且2方的子节点不完全一致，就执行updateChildrenif (isDef(oldCh) && isDef(ch)) {if (oldCh !== ch) updateChildren(elm, oldCh, ch, insertedVnodeQueue, removeOnly)} else if (isDef(ch)) {// 如果只有vnode有子节点，那就创建这些子节点if (isDef(oldVnode.text)) nodeOps.setTextContent(elm, '')addVnodes(elm, null, ch, 0, ch.length - 1, insertedVnodeQueue)// 如果只有oldVnode有子节点，那就把这些节点都删除} else if (isDef(oldCh)) {removeVnodes(elm, oldCh, 0, oldCh.length - 1)// 如果oldVnode和vnode都没有子节点，但是oldVnode是文本节点或注释节点，就把vnode.elm的文本设置为空字符串} else if (isDef(oldVnode.text)) {nodeOps.setTextContent(elm, '')}// 如果vnode是文本节点或注释节点，但是vnode.text != oldVnode.text时，只需要更新vnode.elm的文本内容即可} else if (oldVnode.text !== vnode.text) {nodeOps.setTextContent(elm, vnode.text)}if (isDef(data)) {if (isDef(i = data.hook) && isDef(i = i.postpatch)) i(oldVnode, vnode)}}function invokeInsertHook (vnode, queue, initial) {// delay insert hooks for component root nodes, invoke them after the// element is really insertedif (isTrue(initial) && isDef(vnode.parent)) {vnode.parent.data.pendingInsert = queue} else {for (let i = 0; i < queue.length; ++i) {queue[i].data.hook.insert(queue[i])}}}let hydrationBailed = false// list of modules that can skip create hook during hydration because they// are already rendered on the client or has no need for initialization// Note: style is excluded because it relies on initial clone for future// deep updates (#7063).const isRenderedModule = makeMap('attrs,class,staticClass,staticStyle,key')// Note: this is a browser-only function so we can assume elms are DOM nodes.function hydrate (elm, vnode, insertedVnodeQueue, inVPre) {let iconst { tag, data, children } = vnodeinVPre = inVPre || (data && data.pre)vnode.elm = elmif (isTrue(vnode.isComment) && isDef(vnode.asyncFactory)) {vnode.isAsyncPlaceholder = truereturn true}// assert node matchif (process.env.NODE_ENV !== 'production') {if (!assertNodeMatch(elm, vnode, inVPre)) {return false}}if (isDef(data)) {if (isDef(i = data.hook) && isDef(i = i.init)) i(vnode, true /* hydrating */)if (isDef(i = vnode.componentInstance)) {// child component. it should have hydrated its own tree.initComponent(vnode, insertedVnodeQueue)return true}}if (isDef(tag)) {if (isDef(children)) {// empty element, allow client to pick up and populate childrenif (!elm.hasChildNodes()) {createChildren(vnode, children, insertedVnodeQueue)} else {// v-html and domProps: innerHTMLif (isDef(i = data) && isDef(i = i.domProps) && isDef(i = i.innerHTML)) {if (i !== elm.innerHTML) {/* istanbul ignore if */if (process.env.NODE_ENV !== 'production' &&typeof console !== 'undefined' &&!hydrationBailed) {hydrationBailed = trueconsole.warn('Parent: ', elm)console.warn('server innerHTML: ', i)console.warn('client innerHTML: ', elm.innerHTML)}return false}} else {// iterate and compare children listslet childrenMatch = truelet childNode = elm.firstChildfor (let i = 0; i < children.length; i++) {if (!childNode || !hydrate(childNode, children[i], insertedVnodeQueue, inVPre)) {childrenMatch = falsebreak}childNode = childNode.nextSibling}// if childNode is not null, it means the actual childNodes list is// longer than the virtual children list.if (!childrenMatch || childNode) {/* istanbul ignore if */if (process.env.NODE_ENV !== 'production' &&typeof console !== 'undefined' &&!hydrationBailed) {hydrationBailed = trueconsole.warn('Parent: ', elm)console.warn('Mismatching childNodes vs. VNodes: ', elm.childNodes, children)}return false}}}}if (isDef(data)) {let fullInvoke = falsefor (const key in data) {if (!isRenderedModule(key)) {fullInvoke = trueinvokeCreateHooks(vnode, insertedVnodeQueue)break}}if (!fullInvoke && data['class']) {// ensure collecting deps for deep class bindings for future updatestraverse(data['class'])}}} else if (elm.data !== vnode.text) {elm.data = vnode.text}return true}function assertNodeMatch (node, vnode, inVPre) {if (isDef(vnode.tag)) {return vnode.tag.indexOf('vue-component') === 0 || (!isUnknownElement(vnode, inVPre) &&vnode.tag.toLowerCase() === (node.tagName && node.tagName.toLowerCase()))} else {return node.nodeType === (vnode.isComment ? 8 : 3)}}/**   * 这里返回一个patch函数供后续对vnode进行patch操作   * 这里的patch操作是指, 将oldVnode对应的真实DOM更改为vnode对应的真实DOM, 所需要的最低性能开销的操作(或者说是较低)   * 参数中的oldVnode是更新前的旧节点, vnode是将要更新的新节点, hydrating是一个flag标识是否与原生DOM混合, removeOnly是在过渡动画中使用   */return function patch (oldVnode, vnode, hydrating, removeOnly, parentElm, refElm) {// 这里很简单, 如果新节点不存在, 旧节点也不存在, 无需任何操作, 如果新节点不存在,但旧节点存在, 说明需要删除旧节点, 调用一个销毁钩子if (isUndef(vnode)) {if (isDef(oldVnode)) invokeDestroyHook(oldVnode)return}// 用于标识是否初始化这个节点let isInitialPatch = falseconst insertedVnodeQueue = []// 旧节点不存在 说明需要创建一个新节点if (isUndef(oldVnode)) {// empty mount (likely as component), create new root elementisInitialPatch = truecreateElm(vnode, insertedVnodeQueue, parentElm, refElm)} else {// 走到这里 说明新旧节点都存在, 这时比较复杂, 分几种情况处理 // 先通过nodeType判断是否是真正的节点, 真正的节点nodeType取值范围是1~12// vue里常用的基本只有三种 1代表是dom元素节点 3是文本节点 8是注释节点const isRealElement = isDef(oldVnode.nodeType)if (!isRealElement && sameVnode(oldVnode, vnode)) {// patch existing root node// 常规情况下, 新旧节点是相似节点, 对新旧节点做详细的对比操作patchVnode(oldVnode, vnode, insertedVnodeQueue, removeOnly)} else {if (isRealElement) {// 当新旧节点不是相似节点, 旧节点是一个真实节点时// mounting to a real element// check if this is server-rendered content and if we can perform// a successful hydration.// 服务端渲染特殊处理if (oldVnode.nodeType === 1 && oldVnode.hasAttribute(SSR_ATTR)) {oldVnode.removeAttribute(SSR_ATTR)hydrating = true}// 需要用hydrate函数将虚拟DOM和真实DOM进行映射if (isTrue(hydrating)) {if (hydrate(oldVnode, vnode, insertedVnodeQueue)) {invokeInsertHook(vnode, insertedVnodeQueue, true)return oldVnode} else if (process.env.NODE_ENV !== 'production') {warn('The client-side rendered virtual DOM tree is not matching ' +'server-rendered content. This is likely caused by incorrect ' +'HTML markup, for example nesting block-level elements inside ' +'
, or missing . Bailing hydration and performing ' +'full client-side render.')}}// either not server-rendered, or hydration failed.// create an empty node and replace itoldVnode = emptyNodeAt(oldVnode)}// replacing existing elementconst oldElm = oldVnode.elmconst parentElm = nodeOps.parentNode(oldElm)// create new nodecreateElm(vnode,insertedVnodeQueue,// extremely rare edge case: do not insert if old element is in a// leaving transition. Only happens when combining transition +// keep-alive + HOCs. (#4590)oldElm._leaveCb ? null : parentElm,nodeOps.nextSibling(oldElm))// update parent placeholder node element, recursivelyif (isDef(vnode.parent)) {let ancestor = vnode.parentconst patchable = isPatchable(vnode)while (ancestor) {for (let i = 0; i < cbs.destroy.length; ++i) {cbs.destroy[i](ancestor)}ancestor.elm = vnode.elmif (patchable) {for (let i = 0; i < cbs.create.length; ++i) {cbs.create[i](emptyNode, ancestor)}// #6513// invoke insert hooks that may have been merged by create hooks.// e.g. for directives that uses the "inserted" hook.const insert = ancestor.data.hook.insertif (insert.merged) {// start at index 1 to avoid re-invoking component mounted hookfor (let i = 1; i < insert.fns.length; i++) {insert.fns[i]()}}} else {registerRef(ancestor)}ancestor = ancestor.parent}}// destroy old nodeif (isDef(parentElm)) {removeVnodes(parentElm, [oldVnode], 0, 0)} else if (isDef(oldVnode.tag)) {invokeDestroyHook(oldVnode)}}}invokeInsertHook(vnode, insertedVnodeQueue, isInitialPatch)return vnode.elm}
}

可以看到patch接收的参数

1. oldVnode：旧的虚拟节点
2. vnode：新的虚拟节点
3. hydrating：是否映射
4. removeOnly：标识
5. parentElm：父节点
6. refElm：被插入之后的占位符

那么核心diff代码在于 sameVnode、createElm、patchVNode我们依次展开来说

sameVnode

顾名思义可以看判断两个节点是不是同一个节点

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.asyncFactory.error))))
}
/** * 节点 key 必须相同 * tag、注释、data是否存在、input类型是否相同 * 如果isAsyncPlaceholder是true，则需要asyncFactory属性相同 */

createElm

// 创建一个节点function createElm (vnode,insertedVnodeQueue,parentElm,refElm,nested,ownerArray,index) {// 节点已经被渲染, 需要使用一个克隆节点if (isDef(vnode.elm) && isDef(ownerArray)) {// This vnode was used in a previous render!// now it's used as a new node, overwriting its elm would cause// potential patch errors down the road when it's used as an insertion// reference node. Instead, we clone the node on-demand before creating// associated DOM element for it.vnode = ownerArray[index] = cloneVNode(vnode)}// 创建组件节点 详见本文件中的createComponent方法vnode.isRootInsert = !nested // for transition enter checkif (createComponent(vnode, insertedVnodeQueue, parentElm, refElm)) {return}const data = vnode.dataconst children = vnode.childrenconst tag = vnode.tag/**     * 如果要创建的节点有tag属性, 这里做一下校验     * 如果该节点上面有v-pre指令, 直接给flag加1     * 如果没有v-pre需要调用isUnknownElement判断标签是否合法, 然后给出警告     */if (isDef(tag)) {if (process.env.NODE_ENV !== 'production') {if (data && data.pre) {creatingElmInVPre++}if (isUnknownElement(vnode, creatingElmInVPre)) {warn('Unknown custom element: <' + tag + '> - did you ' +'register the component correctly? For recursive components, ' +'make sure to provide the "name" option.',vnode.context)}}vnode.elm = vnode.ns? nodeOps.createElementNS(vnode.ns, tag): nodeOps.createElement(tag, vnode)setScope(vnode)/* istanbul ignore if */if (__WEEX__) {// in Weex, the default insertion order is parent-first.// List items can be optimized to use children-first insertion// with append="tree".const appendAsTree = isDef(data) && isTrue(data.appendAsTree)if (!appendAsTree) {if (isDef(data)) {invokeCreateHooks(vnode, insertedVnodeQueue)}insert(parentElm, vnode.elm, refElm)}createChildren(vnode, children, insertedVnodeQueue)if (appendAsTree) {if (isDef(data)) {invokeCreateHooks(vnode, insertedVnodeQueue)}insert(parentElm, vnode.elm, refElm)}} else {createChildren(vnode, children, insertedVnodeQueue)if (isDef(data)) {invokeCreateHooks(vnode, insertedVnodeQueue)}insert(parentElm, vnode.elm, refElm)}if (process.env.NODE_ENV !== 'production' && data && data.pre) {creatingElmInVPre--}} else if (isTrue(vnode.isComment)) {vnode.elm = nodeOps.createComment(vnode.text)insert(parentElm, vnode.elm, refElm)} else {vnode.elm = nodeOps.createTextNode(vnode.text)insert(parentElm, vnode.elm, refElm)}}

此段代码就是创建真实dom的目的，下一章会谈到。

patchVnode

  function patchVnode (oldVnode, vnode, insertedVnodeQueue, removeOnly) {// 如果oldVnode跟vnode完全一致，那么不需要做任何事情if (oldVnode === vnode) {return}const elm = vnode.elm = oldVnode.elmif (isTrue(oldVnode.isAsyncPlaceholder)) {if (isDef(vnode.asyncFactory.resolved)) {hydrate(oldVnode.elm, vnode, insertedVnodeQueue)} else {vnode.isAsyncPlaceholder = true}return}// 如果oldVnode跟vnode都是静态节点，且具有相同的key// 当vnode是克隆节点或是v-once指令控制的节点时// 只需要把oldVnode.elm和oldVnode.child都复制到vnode上，也不用再有其他操作// reuse element for static trees.// note we only do this if the vnode is cloned -// if the new node is not cloned it means the render functions have been// reset by the hot-reload-api and we need to do a proper re-render.if (isTrue(vnode.isStatic) &&isTrue(oldVnode.isStatic) &&vnode.key === oldVnode.key &&(isTrue(vnode.isCloned) || isTrue(vnode.isOnce))) {vnode.componentInstance = oldVnode.componentInstancereturn}let iconst data = vnode.dataif (isDef(data) && isDef(i = data.hook) && isDef(i = i.prepatch)) {i(oldVnode, vnode)}const oldCh = oldVnode.childrenconst ch = vnode.childrenif (isDef(data) && isPatchable(vnode)) {for (i = 0; i < cbs.update.length; ++i) cbs.update[i](oldVnode, vnode)if (isDef(i = data.hook) && isDef(i = i.update)) i(oldVnode, vnode)}// 如果vnode不是文本节点或注释节点if (isUndef(vnode.text)) {// 如果oldVnode和vnode都有子节点，且2方的子节点不完全一致，就执行updateChildrenif (isDef(oldCh) && isDef(ch)) {if (oldCh !== ch) updateChildren(elm, oldCh, ch, insertedVnodeQueue, removeOnly)} else if (isDef(ch)) {// 如果只有vnode有子节点，那就创建这些子节点if (isDef(oldVnode.text)) nodeOps.setTextContent(elm, '')addVnodes(elm, null, ch, 0, ch.length - 1, insertedVnodeQueue)// 如果只有oldVnode有子节点，那就把这些节点都删除} else if (isDef(oldCh)) {removeVnodes(elm, oldCh, 0, oldCh.length - 1)// 如果oldVnode和vnode都没有子节点，但是oldVnode是文本节点或注释节点，就把vnode.elm的文本设置为空字符串} else if (isDef(oldVnode.text)) {nodeOps.setTextContent(elm, '')}// 如果vnode是文本节点或注释节点，但是vnode.text != oldVnode.text时，只需要更新vnode.elm的文本内容即可} else if (oldVnode.text !== vnode.text) {nodeOps.setTextContent(elm, vnode.text)}if (isDef(data)) {if (isDef(i = data.hook) && isDef(i = i.postpatch)) i(oldVnode, vnode)}}

具体代码功能已经解释的很清楚了，这里的addVnodes和removeVnodes就是新增与移除虚拟节点，核心代码我们主要关注一个updateChildren

updateChildren

// diff操作核心算法function updateChildren (parentElm, oldCh, newCh, insertedVnodeQueue, removeOnly) {// 记录新旧节点列表的首尾元素 用于比较let oldStartIdx = 0 // 旧列表起点位置let newStartIdx = 0 // 新列表起点位置let oldEndIdx = oldCh.length - 1 // 旧列表终点位置let oldStartVnode = oldCh[0] // 旧列表起点值let oldEndVnode = oldCh[oldEndIdx] // 旧列表终点值let newEndIdx = newCh.length - 1 // 新列表终点位置let newStartVnode = newCh[0] // 新列表起点值let newEndVnode = newCh[newEndIdx] // 新列表终点值let oldKeyToIdx, idxInOld, vnodeToMove, refElm// removeOnly is a special flag used only by // to ensure removed elements stay in correct relative positions// during leaving transitions// 在transition中 不能移动节点const canMove = !removeOnly// 检查是否有重复的keyif (process.env.NODE_ENV !== 'production') {checkDuplicateKeys(newCh)}// 一共分四种情况讨论, 旧列表第一个与新列表第一个对比, 旧列表最后一个与新列表最后一个对比// 然后新列表第一个和旧列表最后一个对比, 新列表最后一个和旧列表第一个对比// 之所以要交叉头尾对比, 是为了防止最差的情况出现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)oldStartVnode = oldCh[++oldStartIdx]newStartVnode = newCh[++newStartIdx]} else if (sameVnode(oldEndVnode, newEndVnode)) {patchVnode(oldEndVnode, newEndVnode, insertedVnodeQueue)oldEndVnode = oldCh[--oldEndIdx]newEndVnode = newCh[--newEndIdx]} else if (sameVnode(oldStartVnode, newEndVnode)) { // Vnode moved rightpatchVnode(oldStartVnode, newEndVnode, insertedVnodeQueue)canMove && nodeOps.insertBefore(parentElm, oldStartVnode.elm, nodeOps.nextSibling(oldEndVnode.elm))oldStartVnode = oldCh[++oldStartIdx]newEndVnode = newCh[--newEndIdx]} else if (sameVnode(oldEndVnode, newStartVnode)) { // Vnode moved leftpatchVnode(oldEndVnode, newStartVnode, insertedVnodeQueue)canMove && nodeOps.insertBefore(parentElm, oldEndVnode.elm, oldStartVnode.elm)oldEndVnode = oldCh[--oldEndIdx]newStartVnode = newCh[++newStartIdx]} else {// 以上四种情况都不满足时, 使用新列表第一个vdom的key去旧列表查找// 如果可以找到key相同的元素, 直接进行patch然后进入下一次循环// 找不到则插入一个新节点if (isUndef(oldKeyToIdx)) oldKeyToIdx = createKeyToOldIdx(oldCh, oldStartIdx, oldEndIdx)idxInOld = isDef(newStartVnode.key)? oldKeyToIdx[newStartVnode.key]: findIdxInOld(newStartVnode, oldCh, oldStartIdx, oldEndIdx)if (isUndef(idxInOld)) { // New elementcreateElm(newStartVnode, insertedVnodeQueue, parentElm, oldStartVnode.elm, false, newCh, newStartIdx)} else {vnodeToMove = oldCh[idxInOld]if (sameVnode(vnodeToMove, newStartVnode)) {patchVnode(vnodeToMove, newStartVnode, insertedVnodeQueue)oldCh[idxInOld] = undefinedcanMove && nodeOps.insertBefore(parentElm, vnodeToMove.elm, oldStartVnode.elm)} else {// same key but different element. treat as new elementcreateElm(newStartVnode, insertedVnodeQueue, parentElm, oldStartVnode.elm, false, newCh, newStartIdx)}}newStartVnode = newCh[++newStartIdx]}}// 新旧列表其中之一全部循环完成后, 开始清理剩余的节点// 如果旧列表全部遍历完成, 新列表还有剩余, 直接创建这些新节点// 反之, 如果新列表全部遍历, 旧列表还有剩余, 直接删除这些旧节点if (oldStartIdx > oldEndIdx) {refElm = isUndef(newCh[newEndIdx + 1]) ? null : newCh[newEndIdx + 1].elmaddVnodes(parentElm, refElm, newCh, newStartIdx, newEndIdx, insertedVnodeQueue)} else if (newStartIdx > newEndIdx) {removeVnodes(parentElm, oldCh, oldStartIdx, oldEndIdx)}}

这里利用了while循环与双指针对比新旧虚拟dom

定义指针变量

  let oldStartIdx = 0 // 旧列表起点位置let newStartIdx = 0 // 新列表起点位置let oldEndIdx = oldCh.length - 1 // 旧列表终点位置let oldStartVnode = oldCh[0] // 旧列表起点值let oldEndVnode = oldCh[oldEndIdx] // 旧列表终点值let newEndIdx = newCh.length - 1 // 新列表终点位置let newStartVnode = newCh[0] // 新列表起点值let newEndVnode = newCh[newEndIdx] // 新列表终点值

定义循环

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)oldStartVnode = oldCh[++oldStartIdx]newStartVnode = newCh[++newStartIdx]} else if (sameVnode(oldEndVnode, newEndVnode)) {patchVnode(oldEndVnode, newEndVnode, insertedVnodeQueue)oldEndVnode = oldCh[--oldEndIdx]newEndVnode = newCh[--newEndIdx]} else if (sameVnode(oldStartVnode, newEndVnode)) { // Vnode moved rightpatchVnode(oldStartVnode, newEndVnode, insertedVnodeQueue)canMove && nodeOps.insertBefore(parentElm, oldStartVnode.elm, nodeOps.nextSibling(oldEndVnode.elm))oldStartVnode = oldCh[++oldStartIdx]newEndVnode = newCh[--newEndIdx]} else if (sameVnode(oldEndVnode, newStartVnode)) { // Vnode moved leftpatchVnode(oldEndVnode, newStartVnode, insertedVnodeQueue)canMove && nodeOps.insertBefore(parentElm, oldEndVnode.elm, oldStartVnode.elm)oldEndVnode = oldCh[--oldEndIdx]newStartVnode = newCh[++newStartIdx]} else {// 以上四种情况都不满足时, 使用新列表第一个vdom的key去旧列表查找// 如果可以找到key相同的元素, 直接进行patch然后进入下一次循环// 找不到则插入一个新节点if (isUndef(oldKeyToIdx)) oldKeyToIdx = createKeyToOldIdx(oldCh, oldStartIdx, oldEndIdx)idxInOld = isDef(newStartVnode.key)? oldKeyToIdx[newStartVnode.key]: findIdxInOld(newStartVnode, oldCh, oldStartIdx, oldEndIdx)if (isUndef(idxInOld)) { // New elementcreateElm(newStartVnode, insertedVnodeQueue, parentElm, oldStartVnode.elm, false, newCh, newStartIdx)} else {vnodeToMove = oldCh[idxInOld]if (sameVnode(vnodeToMove, newStartVnode)) {patchVnode(vnodeToMove, newStartVnode, insertedVnodeQueue)oldCh[idxInOld] = undefinedcanMove && nodeOps.insertBefore(parentElm, vnodeToMove.elm, oldStartVnode.elm)} else {// same key but different element. treat as new elementcreateElm(newStartVnode, insertedVnodeQueue, parentElm, oldStartVnode.elm, false, newCh, newStartIdx)}}newStartVnode = newCh[++newStartIdx]}}

检测oldStartVnode、oldEndVnode

if (isUndef(oldStartVnode)) {oldStartVnode = oldCh[++oldStartIdx]
} else if (isUndef(oldEndVnode)) {oldEndVnode = oldCh[--oldEndIdx]
}

如果oldStartVnode不存在，oldCh起始点向后移动。如果oldEndVnode不存在，oldCh终止点向前移动。

oldStartVnode 和 newStartVnode 是相同节点

else if (sameVnode(oldStartVnode, newStartVnode)) {patchVnode(oldStartVnode, newStartVnode, insertedVnodeQueue)oldStartVnode = oldCh[++oldStartIdx]newStartVnode = newCh[++newStartIdx]
}

如果oldStartVnode 和 newStartVnode 是相同节点，则patchVnode，同时彼此向后移动一位

oldEndVnode 和 newEndVnode 是相同节点

else if (sameVnode(oldEndVnode, newEndVnode)) {patchVnode(oldEndVnode, newEndVnode, insertedVnodeQueue)oldEndVnode = oldCh[--oldEndIdx]newEndVnode = newCh[--newEndIdx]
}

如果oldEndVnode 和 newEndVnode 是相同节点，则patchVnode，同时彼此向前移动一位

oldStartVnode 和 newEndVnode 是相同节点

else if (sameVnode(oldStartVnode, newEndVnode)) { // Vnode moved rightpatchVnode(oldStartVnode, newEndVnode, insertedVnodeQueue)canMove && nodeOps.insertBefore(parentElm, oldStartVnode.elm, nodeOps.nextSibling(oldEndVnode.elm))oldStartVnode = oldCh[++oldStartIdx]newEndVnode = newCh[--newEndIdx]
}

如果oldStartVnode 和 newEndVnode 是相同节点，则先 patchVnode，然后把oldStartVnode移到oldCh最后的位置即可，然后oldStartIdx向后移动一位，newEndIdx向前移动一位

oldEndVnode 和 newStartVnode 是相同节点

else if (sameVnode(oldEndVnode, newStartVnode)) { // Vnode moved leftpatchVnode(oldEndVnode, newStartVnode, insertedVnodeQueue)canMove && nodeOps.insertBefore(parentElm, oldEndVnode.elm, oldStartVnode.elm)oldEndVnode = oldCh[--oldEndIdx]newStartVnode = newCh[++newStartIdx]
}

如果oldEndVnode 和 newStartVnode 是相同节点，则先 patchVnode，然后把oldEndVnode移到oldCh最前的位置即可，然后newStartIdx向后移动一位，oldEndIdx向前移动一位

key不相同执行createElm方法

if (isUndef(oldKeyToIdx)) oldKeyToIdx = createKeyToOldIdx(oldCh, oldStartIdx, oldEndIdx)
idxInOld = isDef(newStartVnode.key)? oldKeyToIdx[newStartVnode.key]: findIdxInOld(newStartVnode, oldCh, oldStartIdx, oldEndIdx)
if (isUndef(idxInOld)) { // New elementcreateElm(newStartVnode, insertedVnodeQueue, parentElm, oldStartVnode.elm)
}

如果以上条件都不匹配，则查找oldVnode中与vnode具有相同key的节点，并将查找的结果赋值给elmToMove。如果找不到相同key的节点，则表示是新创建的节点

key相同，就认为是同一节点

vnodeToMove = oldCh[idxInOld]
if (sameVnode(vnodeToMove, newStartVnode)) {patchVnode(vnodeToMove, newStartVnode, insertedVnodeQueue)oldCh[idxInOld] = undefinedcanMove && nodeOps.insertBefore(parentElm, vnodeToMove.elm, oldStartVnode.elm)
} else {// same key but different element. treat as new elementcreateElm(newStartVnode, insertedVnodeQueue, parentElm, oldStartVnode.elm)
}
newStartVnode = newCh[++newStartIdx]

若为同一类型就调用patchVnode，就将对应下标处的oldVnode设置为undefined，把vnodeToMove插入到oldCh之前，newStartIdx继续向后移动。如果两个 vnode 不相同，视为新元素，执行 createElm创建。

如果老dom的开始索引大于结束索引，新dom数组大于老dom数组，表示新增会调用addVnodes方法

if (oldStartIdx > oldEndIdx) {refElm = isUndef(newCh[newEndIdx + 1]) ? null : newCh[newEndIdx + 1].elmaddVnodes(parentElm, refElm, newCh, newStartIdx, newEndIdx, insertedVnodeQueue)
}

如果老dom的开始索引小于结束索引，新dom数组小于老dom数组，表示新增会调用removeVnodes方法

else if (newStartIdx > newEndIdx) {removeVnodes(parentElm, oldCh, oldStartIdx, oldEndIdx)
}

总结

因为现代diff算法策略是同层级比较，广度优先，故而现代算法复杂度为O(n) 这一章我们讲述了传统diff算法复杂度,O(n^3)到现代的O(n)的实现的一个思路，下一章就开始讲解对比过后的vdom如何映射成真实dom