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.jsexport 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
以上就是本文的全部内容，希望对大家的学习有所帮助，也希望大家多多支持。
