浅析Vue中Virtual DOM和Diff原理及实现

<div id="app">
  <div class="home">home</div>
</div>

{
  tag: 'div',
  attrs: {
    id: 'app'
  },
  children: [
    {
      tag: 'div',
      attrs: {
        class: 'home'
      },
      children: [
        {
          tag: undefined,
          attrs: undefined,
          text: 'home',
          children: undefined
        }
      ]
    }
  ]
}

<template>
  <div id="app">
    <div v-for="item in arr">{{ item.name }} : {{ item.id }}</div>
  </div>
  <button id="btn">reRender</button>
</template>

let app = document.getElementById('app')
let data = {
  arr: [   
    { name: 'a', id: 1 },
    { name: 'b', id: 2 },
    { name: 'c', id: 3 },
  ]
}

function render(data) {
  app.innerHtml = ''
  let children = []
  data.forEach(item => {
    let el = document.createElement("div")
    el.innerHtml = `${ item.name } : ${item.id}`
    app.appendChild(el)
  })  
}

// test
render(data.arr) // 首次渲染
let btn = document.getElementById('btn')
btn.onClick = () => {
  data.arr[2].id++ // 修改关联数据
  render(data.arr) // 重新渲染：暴力刷新DOM，没有diff，实际上只用更新最后一个div就行
}

h(tag, attrs, children) // 创建vnode
patch(vnode, newVnode) // 对vnode进行diff后挂载到真实dom上

let app = document.getElementById('app')
let vnode;

function render(data) {
  let newVnode = h('div', { class: 'wrap' }, data.forEach(item => {
      return h('div', {}, `${item.name} : ${item.id}`)
    })
  )
  patch(vnode, newVnode)
  vnode = newVnode
}

render(data.arr) // 首次渲染

let btn = document.getElementById('btn')
btn.onClick = () => {
  data.arr[2].id++ // 修改关联数据
  render(data.arr) // 重新渲染：在patch函数里经过vdom的diff后再挂载到真实dom，这里只更新最后一个div
}

function createElement(vnode) {
  if(vnode.text) return document.createTextNode(vnode) // 文本节点
    
  let { tag, attrs, children } = vnode
  
  let el = document.createElement(tag) // tag
  
  for(let key of attrs){ // attrs
    el.setAttribute(key, attrs[key])
  }
  
  children.forEach(childVnode => { // children
    el.appendChild(createElement(childVnode)) 
  })
  vnode.el = el
  return el
}

function patch(vnode, newVnode) {
  if (isSameNode(vnode, newVnode)) patchVnode(vnode, newVnode)
  else replaceVnode(vnode, newVnode)
}

function replaceVnode(vnode, newVnode) {
  let el = vnode.el // 旧节点
  let parentEl = api.getParentNode(el) // 获取父节点
  api.insertBefore(parentEl, createElement(newVnode), api.getNextSibling(el)) // 插入新节点
  api.removeChild(parentEl, el) // 删除旧节点
}

function isSameNode(vnode, newVnode) {
  return (
    vnode.key == newVnode.key && // key是否相同
    vnode.tag == newVnode.tag && // tag是否相同
    isDef(vnode.data) == isDef(newVnode.data) // 是否都定义了data
    // &&... 其他条件
  )
}

function patchVnode(vnode, newVnode) {
  let el = newVnode.el = vnode.el // 获取当前旧节点对应的dom，并赋值给新节点的el

  // 1.都为文本节点，且文本不一样
  if (vnode.text && newVnode.text && vnode.text != newVnode.text)
    return api.setElText(el, newVnode.text) // 替换文本
  
  let ch = vnode.children
  let newCh = newVnode.children
  if (ch && newCh) return updateChildren(el, ch, newCh) // 2.都有子节点，递归对比
  if (ch) return api.removeChild(el) // 3.vnode有子节点，newVnode无，删除子节点
  return replaceVnode(vnode, newVnode) // 4. newNode有子节点，vnode无，替换即可
}

function updateChildren(el, ch, newCh) {
  // 子节点下标
  let l = 0
  let r = ch.length - 1
  let newL = 0
  let newR = newCh.length - 1

  // 子节点
  let lNode = ch[l]
  let rNode = ch[r]
  let newLNode = newCh[newL]
  let newRNode = newCh[newR]

  while (l <= r && newL <= newR) {
    if (!lNode || !rNode || !newLNode || !newRNode) { // 边界处理
      if (!lNode) lNode = ch[++l]
      if (!rNode) rNode = ch[--r]
      if (!newLNode) newLNode = newCh[++newL]
      if (!newRNode) newRNode = newCh[--newR]
      continue
    }
    
    // 新旧子节点首尾指针对比 l*newL、r*newR、l*newR、r*newL
    if (isSameNode(lNode, newLNode)) {
      patchVnode(lNode, newLNode)
      lNode = ch[++l]
      newLNode = newCh[++newL]
      continue
    }
    if (isSameNode(rNode, newRNode)) {
      patchVnode(rNode, newRNode)
      rNode = ch[--r]
      newRNode = newCh[--newR]
      continue
    }
    if (isSameNode(lNode, newRNode)) {
      patchVnode(lNode, newRNode)
      api.insertBefore(el, lNode.el, api.nextSibling(rNode.el))
      lNode = ch[++l]
      newRNode = newCh[--newR]
      continue
    }
    if (isSameNode(rNode, newLNode)) {
      patchVnode(rNode, newLNode)
      api.insertBefore(el, rNode.el, lNode.el)
      rNode = ch[--r]
      newLNode = newCh[++newL]
      continue
    }

    // 在vnode未知序列区间[l,r]生成key-idx的map表，用newLNode的key在未知序列中找到可复用的位置
    if (!keyIdxMap) keyIdxMap = getKeyIdxMap(ch, l, r) // map

    keyIdx = keyIdxMap.get(newLNode.key)
    if (!keyIdx) {
      api.insertBefore(el, createElement(newLNode), lNode.el)
    }
    else {
      let nodeToMove = ch[keyIdx]
      patchVnode(nodeToMove, newLNode)
      api.insertBefore(el, nodeToMove.el, lNode.el)
    }
    newLNode = newCh[++newL]
  }
}

function getKeyIdxMap(ch, l, r) {
  let map = new Map()
  while (l <= r) map.set(ch[l].key, l++)
  return map
}