Part3-2-2 Vue.js 源码剖析-虚拟 DOM

Posted 2021-08-07 沿着路走到底

什么是虚拟 DOM

虚拟 DOM(Virtual DOM) 是使用 javascript 对象来描述 DOM ，虚拟 DOM 的本质就是 JavaScript 对象，使用 JavaScript 对象来描述 DOM 的结构。应用的各种状态变化首先作用于虚拟 DOM，最终映射 到 DOM 。

Vue.js 中的虚拟 DOM 借鉴了 Snabbdom ，并添加了一些 Vue.js 中的特性，例如：指令和组 件机制。

Vue 1.x 中细粒度监测数据的变化，每一个属性对应一个 watcher ，开销太大 Vue 2.x 中每个组件对应一 个 watcher ，状态变化通知到组件，再引入虚拟 DOM 进行比对和渲染

为什么要使用虚拟 DOM

使用虚拟 DOM ，可以避免用户直接操作 DOM ，开发过程关注在业务代码的实现，不需要关注如

何操作 DOM ，从而提高开发效率。

作为一个中间层可以跨平台，除了 Web 平台外，还支持 SSR 、 Weex 。

关于性能方面，虚拟 DOM 不一定可以提高性能。

在首次渲染的时候肯定不如直接操作 DOM ，因为要维护一层额外的虚拟 DOM ，因此首次渲染的时候会增加开销。

如果后续有频繁操作 DOM 的操作，如果是复杂视图，会提升渲染性能，虚拟 DOM 在更新真实 DOM 之前会通过 Diffff 算法对比新旧两个虚拟 DOM 树的差异，最终把差异更新到真实 DOM。

Vue.js 中的虚拟 DOM

演示 render 中的 h 函数

h 函数就是 createElement()

const vm = new Vue({
  el: '#app',
  render(h) {
    // h(tag, data, children) 
    // return h('h1', this.msg) 
    // return h('h1', { domProps: { innerhtml: this.msg } }) 
    // return h('h1', { attrs: { id: 'title' } }, this.msg) 
    const vnode = h('h1', {
      attrs: { id: 'title' }
    },
      this.msg
    )
    console.log(vnode)
    return vnode
  },
  data: { msg: 'Hello Vue' }
})

createElement

功能

createElement() 函数，用来创建虚拟节点 (VNode) ，我们的 render 函数中的参数 h，

就是 createElement()

render(h) { 
    // 此处的 h 就是 vm.$createElement 
    return h('h1', this.msg)
}

定义

在 vm._render() 中调用了，用户传递的或者编译生成的 render 函数，这个时候传递了 createElement

src/core/instance/render.js

vm._c = (a, b, c, d) => createElement(vm, a, b, c, d, false) 
// normalization is always applied for the public version, used in 
// user-written render functions. 
vm.$createElement = (a, b, c, d) => createElement(vm, a, b, c, d, true)

vm. c 和 vm.$createElement 内部都调用了 createElement ，不同的是最后一个参数。

vm. c 在编译生成的 render 函数内部会调用，

vm.$createElement 在用户传入的 render 函数内部调用。 当用户传入 render 函数的时候，要对用户传入的参数做处理。

src/core/vdom/create-element.js

执行完 createElement 之后创建好了 VNode ，把创建好的 VNode 传递给 vm._update() 继续处理

// wrapper function for providing a more flexible interface
// without getting yelled at by flow
export function createElement (
  context: Component,
  tag: any,
  data: any,
  children: any,
  normalizationType: any,
  alwaysNormalize: boolean
): VNode | Array<VNode> {
  if (Array.isArray(data) || isPrimitive(data)) {
    normalizationType = children
    children = data
    data = undefined
  }
  if (isTrue(alwaysNormalize)) {
    normalizationType = ALWAYS_NORMALIZE
  }
  return _createElement(context, tag, data, children, normalizationType)
}

export function _createElement (
  context: Component,
  tag?: string | Class<Component> | Function | Object,
  data?: VNodeData,
  children?: any,
  normalizationType?: number
): VNode | Array<VNode> {
  if (isDef(data) && isDef((data: any).__ob__)) {
    process.env.NODE_ENV !== 'production' && warn(
      `Avoid using observed data object as vnode data: ${JSON.stringify(data)}\\n` +
      'Always create fresh vnode data objects in each render!',
      context
    )
    return createEmptyVNode()
  }
  // <component v-bind:is="currentTabComponent"></component>
  // object syntax in v-bind
  if (isDef(data) && isDef(data.is)) {
    tag = data.is
  }
  if (!tag) {
    // in case of component :is set to falsy value
    return createEmptyVNode()
  }
  // warn against non-primitive key
  if (process.env.NODE_ENV !== 'production' &&
    isDef(data) && isDef(data.key) && !isPrimitive(data.key)
  ) {
    if (!__WEEX__ || !('@binding' in data.key)) {
      warn(
        'Avoid using non-primitive value as key, ' +
        'use string/number value instead.',
        context
      )
    }
  }
  // support single function children as default scoped slot
  if (Array.isArray(children) &&
    typeof children[0] === 'function'
  ) {
    data = data || {}
    data.scopedSlots = { default: children[0] }
    children.length = 0
  }
  if (normalizationType === ALWAYS_NORMALIZE) {
    // 返回一维数组，处理用户手写的 render
    children = normalizeChildren(children)
  } else if (normalizationType === SIMPLE_NORMALIZE) {
    // 把二维数组，转换成一维数组
    children = simpleNormalizeChildren(children)
  }
  let vnode, ns
  if (typeof tag === 'string') {
    let Ctor
    ns = (context.$vnode && context.$vnode.ns) || config.getTagNamespace(tag)
    // 是否是 html 的保留标签
    if (config.isReservedTag(tag)) {
      // platform built-in elements
      if (process.env.NODE_ENV !== 'production' && isDef(data) && isDef(data.nativeOn)) {
        warn(
          `The .native modifier for v-on is only valid on components but it was used on <${tag}>.`,
          context
        )
      }
      vnode = new VNode(
        config.parsePlatformTagName(tag), data, children,
        undefined, undefined, context
      )
    // 判断是否是 自定义组件
    } else if ((!data || !data.pre) && 
      isDef(Ctor = resolveAsset(context.$options, 'components', tag))) {
      // 查找自定义组件构造函数的声明
      // 根据 Ctor 创建组件的 VNode
      // component
      vnode = createComponent(Ctor, data, context, children, tag)
    } else {
      // unknown or unlisted namespaced elements
      // check at runtime because it may get assigned a namespace when its
      // parent normalizes children
      vnode = new VNode(
        tag, data, children,
        undefined, undefined, context
      )
    }
  } else {
    // direct component options / constructor
    vnode = createComponent(tag, data, context, children)
  }
  if (Array.isArray(vnode)) {
    return vnode
  } else if (isDef(vnode)) {
    if (isDef(ns)) applyNS(vnode, ns)
    if (isDef(data)) registerDeepBindings(data)
    return vnode
  } else {
    return createEmptyVNode()
  }
}

update

功能

内部调用 vm.__patch__() 把虚拟 DOM 转换成真实 DOM

定义

src/core/instance/lifecycle.js

export function lifecycleMixin(Vue: Class<Component>) {
  // _update 方法的作用是把 VNode 渲染成真实的 DOM
  // 首次渲染会调用，数据更新会调用
  Vue.prototype._update = function (vnode: VNode, hydrating?: boolean) {
    const vm: Component = this
    const prevEl = vm.$el
    const prevVnode = vm._vnode
    const restoreActiveInstance = setActiveInstance(vm)
    vm._vnode = vnode
    // Vue.prototype.__patch__ is injected in entry points
    // based on the rendering backend used.
    if (!prevVnode) {
      // initial render
      // 首次渲染
      vm.$el = vm.__patch__(vm.$el, vnode, hydrating, false /* removeOnly */)
    } else {
      // updates
      vm.$el = vm.__patch__(prevVnode, vnode)
    }
    restoreActiveInstance()
    // update __vue__ reference
    if (prevEl) {
      prevEl.__vue__ = null
    }
    if (vm.$el) {
      vm.$el.__vue__ = vm
    }
    // if parent is an HOC, update its $el as well
    if (vm.$vnode && vm.$parent && vm.$vnode === vm.$parent._vnode) {
      vm.$parent.$el = vm.$el
    }
    // updated hook is called by the scheduler to ensure that children are
    // updated in a parent's updated hook.
  }

patch 函数初始化

功能

对比两个 VNode 的差异，把差异更新到真实 DOM 。如果是首次渲染的话，会把真实 DOM 先转换成 VNode

Vue.js 中 patch 函数的初始化

src/platforms/web/runtime/index.js

import { patch } from './patch' 
Vue.prototype.__patch__ = inBrowser ? patch : noop

src/platforms/web/runtime/patch.js

/* @flow */

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'

// the directive module should be applied last, after all
// built-in modules have been applied.
const modules = platformModules.concat(baseModules)

export const patch: Function = createPatchFunction({ nodeOps, modules })

src/core/vdom/patch.js

export function createPatchFunction (backend) {
  let i, j
  const cbs = {}

  // modules 节点的属性/事件/样式的操作
  // nodeOps 节点操作
  const { modules, nodeOps } = backend

  for (i = 0; i < hooks.length; ++i) {
    // cbs['update'] = []
    cbs[hooks[i]] = []
    for (j = 0; j < modules.length; ++j) {
      if (isDef(modules[j][hooks[i]])) {
        // cbs['update'] = [updateAttrs, updateClass, update...]
        cbs[hooks[i]].push(modules[j][hooks[i]])
      }
    }
  }

  function emptyNodeAt (elm) {
    return new VNode(nodeOps.tagName(elm).toLowerCase(), {}, [], undefined, elm)
  }

  function createRmCb (childElm, listeners) {
    function remove () {
      if (--remove.listeners === 0) {
        removeNode(childElm)
      }
    }
    remove.listeners = listeners
    return remove
  }

  function removeNode (el) {
    const parent = nodeOps.parentNode(el)
    // element may have already been removed due to v-html / v-text
    if (isDef(parent)) {
      nodeOps.removeChild(parent, el)
    }
  }

  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

  function createElm (
    vnode,
    insertedVnodeQueue,
    parentElm,
    refElm,
    nested,
    ownerArray,
    index
  ) {
    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)
    }

    vnode.isRootInsert = !nested // for transition enter check
    if (createComponent(vnode, insertedVnodeQueue, parentElm, refElm)) {
      return
    }

    const data = vnode.data
    const children = vnode.children
    const tag = vnode.tag
    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)
    }
  }

  function createComponent (vnode, insertedVnodeQueue, parentElm, refElm) {
    let i = vnode.data
    if (isDef(i)) {
      const isReactivated = isDef(vnode.componentInstance) && i.keepAlive
      if (isDef(i = i.hook) && isDef(i = i.init)) {
        // 调用 init() 方法，创建和挂载组件实例
        // init() 的过程中创建好了组件的真实 DOM,挂载到了 vnode.elm 上
        i(vnode, false /* hydrating */)
      }
      // 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)) {
        // 调用钩子函数（VNode的钩子函数初始化属性/事件/样式等，组件的钩子函数）
        initComponent(vnode, insertedVnodeQueue)
        // 把组件对应的 DOM 插入到父元素中
        insert(parentElm, vnode.elm, refElm)
        if (isTrue(isReactivated)) {
          reactivateComponent(vnode, insertedVnodeQueue, parentElm, refElm)
        }
        return true
      }
    }
  }

  function initComponent (vnode, insertedVnodeQueue) {
    if (isDef(vnode.data.pendingInsert)) {
      insertedVnodeQueue.push.apply(insertedVnodeQueue, vnode.data.pendingInsert)
      vnode.data.pendingInsert = null
    }
    vnode.elm = vnode.componentInstance.$el
    if (isPatchable(vnode)) {
      // 调用钩子函数
      invokeCreateHooks(vnode, insertedVnodeQueue)
      // 设置局部作用于样式
      setScope(vnode)
    } else {
      // empty component root.
      // skip all element-related modules except for ref (#3455)
      registerRef(vnode)
      // make sure to invoke the insert hook
      insertedVnodeQueue.push(vnode)
    }
  }

  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 = vnode
    while (innerNode.componentInstance) {
      innerNode = innerNode.componentInstance._vnode
      if (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 itself
    insert(parentElm, vnode.elm, refElm)
  }

  function insert (parent, elm, ref) {
    if (isDef(parent)) {
      if (isDef(ref)) {
        if (nodeOps.parentNode(ref) === 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) {
    // 调用 VNode 的钩子函数
    for (let i = 0; i < cbs.create.length; ++i) {
      cbs.create[i](emptyNode, vnode)
    }
    i = vnode.data.hook // Reuse variable
    // 调用组件的钩子函数
    if (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 i
    if (isDef(i = vnode.fnScopeId)) {
      nodeOps.setStyleScope(vnode.elm, i)
    } else {
      let ancestor = vnode
      while (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)
    }
  }

  function invokeDestroyHook (vnode) {
    let i, j
    const data = vnode.data
    if (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])
      }
    }
  }

  function removeVnodes (vnodes, startIdx, endIdx) {
    for (; startIdx <= endIdx; ++startIdx) {
      const ch = vnodes[startIdx]
      if (isDef(ch)) {
        if (isDef(ch.tag)) {
          removeAndInvokeRemoveHook(ch)
          invokeDestroyHook(ch)
        } else { // Text node
          removeNode(ch.elm)
        }
      }
    }
  }

  function removeAndInvokeRemoveHook (vnode, rm) {
    if (isDef(rm) || isDef(vnode.data)) {
      let i
      const listeners = cbs.remove.length + 1
      if (isDef(rm)) {
        // we have a recursively passed down rm callback
        // increase the listeners count
        rm.listeners += listeners
      } else {
        // directly removing
        rm = createRmCb(vnode.elm, listeners)
      }
      // recursively invoke hooks on child component root node
      if (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 算法
  // 更新新旧节点的子节点
  function updateChildren (parentElm, oldCh, newCh, insertedVnodeQueue, removeOnly) {
    let oldStartIdx = 0
    let newStartIdx = 0
    let oldEndIdx = oldCh.length - 1
    let oldStartVnode = oldCh[0]
    let oldEndVnode = oldCh[oldEndIdx] 
    let newEndIdx = newCh.length - 1 
    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)
    }
    // diff 算法
    // 当新节点和旧节点都没有遍历完成
    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)) {
        // oldStartVnode 和 newStartVnode 相同(sameVnode)
        // 直接将该 VNode 节点进行 patchVnode
        patchVnode(oldStartVnode, newStartVnode, insertedVnodeQueue, newCh, newStartIdx)
        // 获取下一组开始节点
        oldStartVnode = oldCh[++oldStartIdx]
        newStartVnode = newCh[++newStartIdx]
      } else if (sameVnode(oldEndVnode, newEndVnode)) {
        // 直接将该 VNode 节点进行 patchVnode
        patchVnode(oldEndVnode, newEndVnode, insertedVnodeQueue, newCh, newEndIdx)
        // 获取下一组结束节点
        oldEndVnode = oldCh[--oldEndIdx]
        newEndVnode = newCh[--newEndIdx]
      } else if (sameVnode(oldStartVnode, newEndVnode)) { // Vnode moved right
        // oldStartVnode 和 newEndVnode 相同(sameVnode)
        // 进行 patchVnode，把 oldStartVnode 移动到最后
        patchVnode(oldStartVnode, newEndVnode, insertedVnodeQueue, newCh, newEndIdx)
        canMove && nodeOps.insertBefore(parentElm, oldStartVnode.elm, nodeOps.nextSibling(oldEndVnode.elm))
        // 移动游标，获取下一组节点
        oldStartVnode = oldCh[++oldStartIdx]
        newEndVnode = newCh[--newEndIdx]
      } else if (sameVnode(oldEndVnode, newStartVnode)) { // Vnode moved left
        // oldEndVnode 和 newStartVnode 相同(sameVnode)
        // 进行 patchVnode，把 oldEndVnode 移动到最前面
        patchVnode(oldEndVnode, newStartVnode, insertedVnodeQueue, newCh, newStartIdx)
        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
        // newStartNode 依次和旧的节点比较
        
        // 从新的节点开头获取一个，去老节点中查找相同节点
        // 先找新开始节点的key和老节点相同的索引，如果没找到再通过sameVnode找
        if (isUndef(oldKeyToIdx)) oldKeyToIdx = createKeyToOldIdx(oldCh, oldStartIdx, oldEndIdx)
        idxInOld = isDef(newStartVnode.key)
          ? oldKeyToIdx[newStartVnode.key]
          : findIdxInOld(newStartVnode, oldCh, oldStartIdx, oldEndIdx)
        // 如果没有找到
        if (isUndef(idxInOld)) { // New element
          // 创建节点并插入到最前面
          createElm(newStartVnode, insertedVnodeQueue, parentElm, oldStartVnode.elm, false, newCh, newStartIdx)
        } else {
          // 获取要移动的老节点
          vnodeToMove = oldCh[idxInOld]
          // 如果使用 newStartNode 找到相同的老节点
          if (sameVnode(vnodeToMove, newStartVnode)) {
            // 执行 patchVnode，并且将找到的旧节点移动到最前面
            patchVnode(vnodeToMove, newStartVnode, insertedVnodeQueue, newCh, newStartIdx)
            oldCh[idxInOld] = undefined
            canMove && nodeOps.insertBefore(parentElm, vnodeToMove.elm, oldStartVnode.elm)
          } else {
            // 如果key相同，但是是不同的元素，创建新元素
            // same key but different element. treat as new element
            createElm(newStartVnode, insertedVnodeQueue, parentElm, oldStartVnode.elm, false, newCh, newStartIdx)
          }
        }
        newStartVnode = newCh[++newStartIdx]
      }
    }
    // 当结束时 oldStartIdx > oldEndIdx，旧节点遍历完，但是新节点还没有
    if (oldStartIdx > oldEndIdx) {
      // 说明新节点比老节点多，把剩下的新节点插入到老的节点后面
      refElm = isUndef(newCh[newEndIdx + 1]) ? null : newCh[newEndIdx + 1].elm
      addVnodes(parentElm, refElm, newCh, newStartIdx, newEndIdx, insertedVnodeQueue)
    } else if (newStartIdx > newEndIdx) {
      // 当结束时 newStartIdx > newEndIdx，新节点遍历完，但是旧节点还没有
      removeVnodes(oldCh, oldStartIdx, oldEndIdx)
    }
  }

  function checkDuplicateKeys (children) {
    const seenKeys = {}
    for (let i = 0; i < children.length; i++) {
      const vnode = children[i]
      const key = vnode.key
      if (isDef(key)) {
        if (seenKeys[key]) {
          warn(
            `Duplicate keys detected: '${key}'. This may cause an update error.`,
            vnode.context
          )
        } else {
          seenKeys[key] = true
        }
      }
    }
  }

  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,
    ownerArray,
    index,
    removeOnly
  ) {
    if (oldVnode === vnode) {
      return
    }

    if (isDef(vnode.elm) && isDef(ownerArray)) {
      // clone reused vnode
      vnode = ownerArray[index] = cloneVNode(vnode)
    }

    const elm = vnode.elm = oldVnode.elm

    if (isTrue(oldVnode.isAsyncPlaceholder)) {
      if (isDef(vnode.asyncFactory.resolved)) {
        hydrate(oldVnode.elm, vnode, insertedVnodeQueue)
      } else {
        vnode.isAsyncPlaceholder = true
      }
      return
    }

    // 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.
    // 如果新旧 VNode 都是静态的，那么只需要替换componentInstance
    if (isTrue(vnode.isStatic) &&
      isTrue(oldVnode.isStatic) &&
      vnode.key === oldVnode.key &&
      (isTrue(vnode.isCloned) || isTrue(vnode.isOnce))
    ) {
      vnode.componentInstance = oldVnode.componentInstance
      return
    }

    let i
    const data = vnode.data
    if (isDef(data) && isDef(i = data.hook) && isDef(i = i.prepatch)) {
      i(oldVnode, vnode)
    }

    const oldCh = oldVnode.children
    const ch = vnode.children
    if (isDef(data) && isPatchable(vnode)) {
      // 调用 cbs 中的钩子函数，操作节点的属性/样式/事件....
      for (i = 0; i < cbs.update.length; ++i) cbs.update[i](oldVnode, vnode)
      // 用户的自定义钩子
      if (isDef(i = data.hook) && isDef(i = i.update)) i(oldVnode, vnode)
    }

    // 新节点没有文本
    if (isUndef(vnode.text)) {
      // 老节点和老节点都有有子节点
      // 对子节点进行 diff 操作，调用 updateChildren
      if (isDef(oldCh) && isDef(ch)) {
        if (oldCh !== ch) updateChildren(elm, oldCh, ch, insertedVnodeQueue, removeOnly)
      } else if (isDef(ch)) {
        // 新的有子节点，老的没有子节点
        if (process.env.NODE_ENV !== 'production') {
          checkDuplicateKeys(ch)
        }
        // 先清空老节点 DOM 的文本内容，然后为当前 DOM 节点加入子节点
        if (isDef(oldVnode.text)) nodeOps.setTextContent(elm, '')
        addVnodes(elm, null, ch, 0, ch.length - 1, insertedVnodeQueue)
      } else if (isDef(oldCh)) {
        // 老节点有子节点，新的没有子节点
        // 删除老节点中的子节点
        removeVnodes(oldCh, 0, oldCh.length - 1)
      } else if (isDef(oldVnode.text)) {
        // 老节点有文本，新节点没有文本
        // 清空老节点的文本内容
        nodeOps.setTextContent(elm, '')
      }
    } else if (oldVnode.text !== vnode.text) {
      // 新老节点都有文本节点
      // 修改文本
      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 inserted
    if (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 i
    const { tag, data, children } = vnode
    inVPre = inVPre || (data && data.pre)
    vnode.elm = elm

    if (isTrue(vnode.isComment) && isDef(vnode.asyncFactory)) {
      vnode.isAsyncPlaceholder = true
      return true
    }
    // assert node match
    if (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 children
        if (!elm.hasChildNodes()) {
          createChildren(vnode, children, insertedVnodeQueue)
        } else {
          // v-html and domProps: innerHTML
          if (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 = true
                console.warn('Parent: ', elm)
                console.warn('server innerHTML: ', i)
                console.warn('client innerHTML: ', elm.innerHTML)
              }
              return false
            }
          } else {
            // iterate and compare children lists
            let childrenMatch = true
            let childNode = elm.firstChild
            for (let i = 0; i < children.length; i++) {
              if (!childNode || !hydrate(childNode, children[i], insertedVnodeQueue, inVPre)) {
                childrenMatch = false
                break
              }
              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 = true
                console.warn('Parent: ', elm)
                console.warn('Mismatching childNodes vs. VNodes: ', elm.childNodes, children)
              }
              return false
            }
          }
        }
      }
      if (isDef(data)) {
        let fullInvoke = false
        for (const key in data) {
          if (!isRenderedModule(key)) {
            fullInvoke = true
            invokeCreateHooks(vnode, insertedVnodeQueue)
            break
          }
        }
        if (!fullInvoke && data['class']) {
          // ensure collecting deps for deep class bindings for future updates
          traverse(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)
    }
  }
  // 函数柯里化，让一个函数返回一个函数
  // createPatchFunction({ nodeOps, modules }) 传入平台相关的两个参数

  // core中的createPatchFunction (backend), const { modules, nodeOps } = backend
  // core中方法和平台无关，传入两个参数后，可以在上面的函数中使用这两个参数
  return function patch (oldVnode, vnode, hydrating, removeOnly) {
    // 新的 VNode 不存在
    if (isUndef(vnode)) {
      // 老的 VNode 存在，执行 Destroy 钩子函数
      if (isDef(oldVnode)) invokeDestroyHook(oldVnode)
      return
    }

    let isInitialPatch = false
    const insertedVnodeQueue = []

    // 老的 VNode 不存在
    if (isUndef(oldVnode)) {
      // empty mount (likely as component), create new root element
      isInitialPatch = true
      // 创建新的 VNode
      createElm(vnode, insertedVnodeQueue)
    } else {
      // 新的和老的 VNode 都存在，更新
      const isRealElement = isDef(oldVnode.nodeType)
      // 判断参数1是否是真实 DOM，不是真实 DOM
      if (!isRealElement && sameVnode(oldVnode, vnode)) {
        // 更新操作，diff 算法
        // patch existing root node
        patchVnode(oldVnode, vnode, insertedVnodeQueue, null, null, removeOnly)
      } else {
        // 第一个参数是真实 DOM，创建 VNode
        // 初始化
        if (isRealElement) {
          // mounting to a real element
          // check if this is server-rendered content and if we can perform
          // a successful hydration.
          if (oldVnode.nodeType === 1 && oldVnode.hasAttribute(SSR_ATTR)) {
            oldVnode.removeAttribute(SSR_ATTR)
            hydrating = true
          }
          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 ' +
                '<p>, or missing <tbody>. Bailing hydration and performing ' +
                'full client-side render.'
              )
            }
          }
          // either not server-rendered, or hydration failed.
          // create an empty node and replace it
          oldVnode = emptyNodeAt(oldVnode)
        }

        // replacing existing element
        const oldElm = oldVnode.elm
        const parentElm = nodeOps.parentNode(oldElm)

        // create new node
        // 创建 DOM 节点
        createElm(
          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, recursively
        if (isDef(vnode.parent)) {
          let ancestor = vnode.parent
          const patchable = isPatchable(vnode)
          while (ancestor) {
            for (let i = 0; i < cbs.destroy.length; ++i) {
              cbs.destroy[i](ancestor)
            }
            ancestor.elm = vnode.elm
            if (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.insert
              if (insert.merged) {
                // start at index 1 to avoid re-invoking component mounted hook
                for (let i = 1; i < insert.fns.length; i++) {
                  insert.fns[i]()
                }
              }
            } else {
              registerRef(ancestor)
            }
            ancestor = ancestor.parent
          }
        }

        // destroy old node
        if (isDef(parentElm)) {
          removeVnodes([oldVnode], 0, 0)
        } else if (isDef(oldVnode.tag)) {
          invokeDestroyHook(oldVnode)
        }
      }
    }

    invokeInsertHook(vnode, insertedVnodeQueue, isInitialPatch)
    return vnode.elm
  }
}

patch 函数执行过程

function patch (oldVnode, vnode, hydrating, removeOnly) {
    // 新的 VNode 不存在
    if (isUndef(vnode)) {
      // 老的 VNode 存在，执行 Destroy 钩子函数
      if (isDef(oldVnode)) invokeDestroyHook(oldVnode)
      return
    }

    let isInitialPatch = false
    const insertedVnodeQueue = []

    // 老的 VNode 不存在
    if (isUndef(oldVnode)) {
      // empty mount (likely as component), create new root element
      isInitialPatch = true
      // 创建新的 VNode
      createElm(vnode, insertedVnodeQueue)
    } else {
      // 新的和老的 VNode 都存在，更新
      const isRealElement = isDef(oldVnode.nodeType)
      // 判断参数1是否是真实 DOM，不是真实 DOM
      if (!isRealElement && sameVnode(oldVnode, vnode)) {
        // 更新操作，diff 算法
        // patch existing root node
        patchVnode(oldVnode, vnode, insertedVnodeQueue, null, null, removeOnly)
      } else {
        // 第一个参数是真实 DOM，创建 VNode
        // 初始化
        if (isRealElement) {
          // mounting to a real element
          // check if this is server-rendered content and if we can perform
          // a successful hydration.
          if (oldVnode.nodeType === 1 && oldVnode.hasAttribute(SSR_ATTR)) {
            oldVnode.removeAttribute(SSR_ATTR)
            hydrating = true
          }
          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 ' +
                '<p>, or missing <tbody>. Bailing hydration and performing ' +
                'full client-side render.'
              )
            }
          }
          // either not server-rendered, or hydration failed.
          // create an empty node and replace it
          oldVnode = emptyNodeAt(oldVnode)
        }

        // replacing existing element
        const oldElm = oldVnode.elm
        const parentElm = nodeOps.parentNode(oldElm)

        // create new node
        // 创建 DOM 节点
        createElm(
          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, recursively
        if (isDef(vnode.parent)) {
          let ancestor = vnode.parent
          const patchable = isPatchable(vnode)
          while (ancestor) {
            for (let i = 0; i < cbs.destroy.length; ++i) {
              cbs.destroy[i](ancestor)
            }
            ancestor.elm = vnode.elm
            if (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.insert
              if (insert.merged) {
                // start at index 1 to avoid re-invoking component mounted hook
                for (let i = 1; i < insert.fns.length; i++) {
                  insert.fns[i]()
                }
              }
            } else {
              registerRef(ancestor)
            }
            ancestor = ancestor.parent
          }
        }

        // destroy old node
        if (isDef(parentElm)) {
          removeVnodes([oldVnode], 0, 0)
        } else if (isDef(oldVnode.tag)) {
          invokeDestroyHook(oldVnode)
        }
      }
    }

    invokeInsertHook(vnode, insertedVnodeQueue, isInitialPatch)
    return vnode.elm
  }

createElm

把 VNode 转换成真实 DOM，插入到 DOM 树上

function createElm (
    vnode,
    insertedVnodeQueue,
    parentElm,
    refElm,
    nested,
    ownerArray,
    index
  ) {
    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)
    }

    vnode.isRootInsert = !nested // for transition enter check
    if (createComponent(vnode, insertedVnodeQueue, parentElm, refElm)) {
      return
    }

    const data = vnode.data
    const children = vnode.children
    const tag = vnode.tag
    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)
    }
  }

patchVnode

function patchVnode (
    oldVnode,
    vnode,
    insertedVnodeQueue,
    ownerArray,
    index,
    removeOnly
  ) {
    if (oldVnode === vnode) {
      return
    }

    if (isDef(vnode.elm) && isDef(ownerArray)) {
      // clone reused vnode
      vnode = ownerArray[index] = cloneVNode(vnode)
    }

    const elm = vnode.elm = oldVnode.elm

    if (isTrue(oldVnode.isAsyncPlaceholder)) {
      if (isDef(vnode.asyncFactory.resolved)) {
        hydrate(oldVnode.elm, vnode, insertedVnodeQueue)
      } else {
        vnode.isAsyncPlaceholder = true
      }
      return
    }

    // 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.
    // 如果新旧 VNode 都是静态的，那么只需要替换componentInstance
    if (isTrue(vnode.isStatic) &&
      isTrue(oldVnode.isStatic) &&
      vnode.key === oldVnode.key &&
      (isTrue(vnode.isCloned) || isTrue(vnode.isOnce))
    ) {
      vnode.componentInstance = oldVnode.componentInstance
      return
    }

    let i
    const data = vnode.data
    if (isDef(data) && isDef(i = data.hook) && isDef(i = i.prepatch)) {
      i(oldVnode, vnode)
    }

    const oldCh = oldVnode.children
    const ch = vnode.children
    if (isDef(data) && isPatchable(vnode)) {
      // 调用 cbs 中的钩子函数，操作节点的属性/样式/事件....
      for (i = 0; i < cbs.update.length; ++i) cbs.update[i](oldVnode, vnode)
      // 用户的自定义钩子
      if (isDef(i = data.hook) && isDef(i = i.update)) i(oldVnode, vnode)
    }

    // 新节点没有文本
    if (isUndef(vnode.text)) {
      // 老节点和老节点都有有子节点
      // 对子节点进行 diff 操作，调用 updateChildren
      if (isDef(oldCh) && isDef(ch)) {
        if (oldCh !== ch) updateChildren(elm, oldCh, ch, insertedVnodeQueue, removeOnly)
      } else if (isDef(ch)) {
        // 新的有子节点，老的没有子节点
        if (process.env.NODE_ENV !== 'production') {
          checkDuplicateKeys(ch)
        }
        // 先清空老节点 DOM 的文本内容，然后为当前 DOM 节点加入子节点
        if (isDef(oldVnode.text)) nodeOps.setTextContent(elm, '')
        addVnodes(elm, null, ch, 0, ch.length - 1, insertedVnodeQueue)
      } else if (isDef(oldCh)) {
        // 老节点有子节点，新的没有子节点
        // 删除老节点中的子节点
        removeVnodes(oldCh, 0, oldCh.length - 1)
      } else if (isDef(oldVnode.text)) {
        // 老节点有文本，新节点没有文本
        // 清空老节点的文本内容
        nodeOps.setTextContent(elm, '')
      }
    } else if (oldVnode.text !== vnode.text) {
      // 新老节点都有文本节点
      // 修改文本
      nodeOps.setTextContent(elm, vnode.text)
    }
    if (isDef(data)) {
      if (isDef(i = data.hook) && isDef(i = i.postpatch)) i(oldVnode, vnode)
    }
  }

1