深入理解Vue2.x的虚拟DOM diff原理

前言

经常看到讲解Vue2的虚拟Dom diff原理的，但很多都是在原代码的基础上添加些注释等等，这里从0行代码开始实现一个Vue2的虚拟DOM

实现VNode

src/core/vdom/Vnode.js

export class VNode{
 constructor (
 tag, //标签名
 children,//孩子[VNode,VNode],
 text, //文本节点
 elm //对应的真实dom对象
 ){
 this.tag = tag;
 this.children = children
 this.text = text;
 this.elm = elm;
 }
}
export function createTextNode(val){
 //为什么这里默认把elm置为undefined，不直接根据tag 用document.createElement(tagName)把elm赋值？而要等后面createElm时候再赋值呢？
 return new VNode(undefined,undefined,String(val),undefined)
}
export function createCommentNode(tag,children){
 if(children){
 for(var i=0;i<children.length;i++){
 var child = children[i];
 if(typeof child == 'string'){
 children[i] = createTextNode(child)
 }
 }
 }
 return new VNode(tag,children,undefined,null)
}

定义一个Vnode类， 创建节点分为两类，一类为text节点，一类非text节点

src/main.js

import {VNode,createCommentNode} from './core/vdom/vnode'
var newVonde = createCommentNode('ul',[createCommentNode('li',['item 1']),createCommentNode('li',['item 2']),createCommentNode('li',['item 3'])])

在main.js就可以根据Vnode 生成对应的Vnode对象，上述代码对应的dom表示

<ul>

<li>item1</li>
<li>item2</li>
<li>item3</li>
</ul>

先实现不用diff把Vnode渲染到页面中来

为什么先来实现不用diff渲染Vnode的部分，这里也是为了统计渲染的时间，来表明一个道理。并不是diff就比非diff要开，虚拟DOM并不是任何时候性能都比非虚拟DOM 要快

先来实现一个工具函数，不熟悉的人可以手工敲下代码 熟悉下

// 真实的dom操作
// src/core/vdom/node-ops.js

export function createElement (tagName) {
 return document.createElement(tagName)
}

export function createTextNode (text) {
 return document.createTextNode(text)
}

export function createComment (text) {
 return document.createComment(text)
}

export function insertBefore (parentNode, newNode, referenceNode) {
 parentNode.insertBefore(newNode, referenceNode)
}

export function removeChild (node, child) {
 node.removeChild(child)
}

export function appendChild (node, child) {
 node.appendChild(child)
}

export function parentNode (node) {
 return node.parentNode
}

export function nextSibling (node) {
 return node.nextSibling
}

export function tagName (node) {
 return node.tagName
}

export function setTextContent (node, text) {
 node.textContent = text
}

export function setAttribute (node, key, val) {
 node.setAttribute(key, val)
}

src/main.js

import {VNode,createCommentNode} from './core/vdom/vnode'
import patch from './core/vdom/patch'


var container = document.getElementById("app");
var oldVnode = new VNode(container.tagName,[],undefined,container);
var newVonde = createCommentNode('ul',[createCommentNode('li',['item 1']),createCommentNode('li',['item 2']),createCommentNode('li',['item 3'])])


console.time('start');
patch(oldVnode,newVonde); //渲染页面
console.timeEnd('start');

这里我们要实现一个patch方法，把Vnode渲染到页面中

src/core/vdom/patch.js

import * as nodeOps from './node-ops'
import VNode from './vnode'


export default function patch(oldVnode,vnode){
 let isInitialPatch = false;
 if(sameVnode(oldVnode,vnode)){
 //如果两个Vnode节点的根一致 开始diff
 patchVnode(oldVnode,vnode)
 }else{
 //这里就是不借助diff的实现
 const oldElm = oldVnode.elm;
 const parentElm = nodeOps.parentNode(oldElm);
 createElm(
 vnode,
 parentElm,
 nodeOps.nextSibling(oldElm)
 )
 if(parentElm != null){
 removeVnodes(parentElm,[oldVnode],0,0)
 }
 }
 return vnode.elm;
}
function patchVnode(oldVnode,vnode,removeOnly){
 if(oldVnode === vnode){
 return
 }
 const elm = vnode.elm = oldVnode.elm
 const oldCh = oldVnode.children;
 const ch = vnode.children

 if(isUndef(vnode.text)){
 //非文本节点
 if(isDef(oldCh) && isDef(ch)){
 //都有字节点
 if(oldCh !== ch){
 //更新children
 updateChildren(elm,oldCh,ch,removeOnly);
 }
 }else if(isDef(ch)){
 //新的有子节点，老的没有
 if(isDef(oldVnode.text)){
 nodeOps.setTextContent(elm,'');
 }
 //添加子节点
 addVnodes(elm,null,ch,0,ch.length-1)
 }else if(isDef(oldCh)){
 //老的有子节点，新的没有
 removeVnodes(elm,oldCh,0,oldCh.length-1)
 }else if(isDef(oldVnode.text)){
 //否则老的有文本内容 直接置空就行
 nodeOps.setTextContent(elm,'');
 }
 }else if(oldVnode.text !== vnode.text){
 //直接修改文本
 nodeOps.setTextContent(elm,vnode.text);
 }
}

function updateChildren(parentElm,oldCh,newCh,removeOnly){
 //这里认真读下，没什么难度的，不行的话 也可以搜索下图文描述这段过程的

 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 refElm;
 const canMove = !removeOnly
 while(oldStartIdx <= oldEndIdx && newStartIdx <= newEndIdx){
 if(isUndef(oldStartVnode)){
 oldStartVnode = oldCh[++oldStartIdx]
 }else if(isUndef(oldEndVnode)){
 oldEndVnode = oldCh[--oldEndIdx]
 }else if(sameVnode(oldStartVnode,newStartVnode)){
 patchVnode(oldStartVnode,newStartVnode)
 oldStartVnode = oldCh[++oldStartIdx]
 newStartVnode = newCh[++newStartIdx]
 }else if(sameVnode(oldEndVnode,newEndVnode)){
 patchVnode(oldEndVnode,newEndVnode)
 oldEndVnode = oldCh[--oldEndIdx];
 newEndVnode = newCh[--newEndIdx];
 }else if(sameVnode(oldStartVnode,newEndVnode)){
 patchVnode(oldStartVnode,newEndVnode);
 //更换顺序
 canMove && nodeOps.insertBefore(parentElm,oldStartVnode.elm,nodeOps.nextSibling(oldEndVnode.elm))
 oldStartVnode = oldCh[++oldStartIdx]
 newEndVnode = newCh[--newEndIdx]
 }else if(sameVnode(oldEndVnode,newStartVnode)){
 patchVnode(oldEndVnode,newStartVnode)
 canMove && nodeOps.insertBefore(parentElm,oldEndVnode.elm,oldStartVnode.elm)
 oldEndVnode = oldCh[--oldEndIdx]
 newStartVnode = newCh[++newStartIdx]
 }else{
 createElm(newStartVnode,parentElm,oldStartVnode.elm)
 newStartVnode = newCh[++newStartIdx];
 }
 }

 if(oldStartIdx > oldEndIdx){
 //老的提前相遇，添加新节点中没有比较的节点
 refElm = isUndef(newCh[newEndIdx + 1]) ? null : newCh[newEndIdx+1].elm
 addVnodes(parentElm,refElm,newCh,newStartIdx,newEndIdx)
 }else{
 //新的提前相遇 删除多余的节点
 removeVnodes(parentElm,oldCh,oldStartIdx,oldEndIdx)
 }
}
function removeVnodes(parentElm,vnodes,startIdx,endIdx){
 for(;startIdx<=endIdx;++startIdx){
 const ch = vnodes[startIdx];
 if(isDef(ch)){
 removeNode(ch.elm)
 }
 }
}

function addVnodes(parentElm,refElm,vnodes,startIdx,endIdx){
 for(;startIdx <=endIdx;++startIdx ){
 createElm(vnodes[startIdx],parentElm,refElm)
 }
}

function sameVnode(vnode1,vnode2){
 return vnode1.tag === vnode2.tag
}
function removeNode(el){
 const parent = nodeOps.parentNode(el)
 if(parent){
 nodeOps.removeChild(parent,el)
 }
}
function removeVnodes(parentElm,vnodes,startIdx,endIdx){
 for(;startIdx<=endIdx;++startIdx){
 const ch = vnodes[startIdx]
 if(isDef(ch)){
 removeNode(ch.elm)
 }
 }
}
function isDef (s){
 return s != null
}
function isUndef(s){
 return s == null
}
function createChildren(vnode,children){
 if(Array.isArray(children)){
 for(let i=0;i<children.length;i++){
 createElm(children[i],vnode.elm,null)
 }
 }
}
function createElm(vnode,parentElm,refElm){
 const children = vnode.children
 const tag = vnode.tag
 if(isDef(tag)){
 // 非文本节点
 vnode.elm = nodeOps.createElement(tag); // 其实可以初始化的时候就赋予
 createChildren(vnode,children);
 insert(parentElm,vnode.elm,refElm)
 }else{
 vnode.elm = nodeOps.createTextNode(vnode.text)
 insert(parentElm,vnode.elm,refElm)
 }
}
function insert(parent,elm,ref){
 if(parent){
 if(ref){
 nodeOps.insertBefore(parent,elm,ref)
 }else{
 nodeOps.appendChild(parent,elm)
 }
 }
}

这就是完整实现了