深入理解Vue2.x的虛擬DOM diff原理

更新時(shí)間：2017年09月27日 10:35:10 作者：小溪流

本篇文章主要介紹了Vue2.x的虛擬DOM diff原理，小編覺得挺不錯(cuò)的，現(xiàn)在分享給大家，也給大家做個(gè)參考。一起跟隨小編過來看看吧

前言

經(jīng)?？吹街v解Vue2的虛擬Dom diff原理的，但很多都是在原代碼的基礎(chǔ)上添加些注釋等等，這里從0行代碼開始實(shí)現(xiàn)一個(gè)Vue2的虛擬DOM

實(shí)現(xiàn)VNode

src/core/vdom/Vnode.js

export class VNode{
 constructor (
  tag, //標(biāo)簽名
  children,//孩子[VNode,VNode],
  text, //文本節(jié)點(diǎn)
  elm //對(duì)應(yīng)的真實(shí)dom對(duì)象
 ){
  this.tag = tag;
  this.children = children
  this.text = text;
  this.elm = elm;
 }
}
export function createTextNode(val){
 //為什么這里默認(rèn)把elm置為undefined，不直接根據(jù)tag 用document.createElement(tagName)把elm賦值？而要等后面createElm時(shí)候再賦值呢？
 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)
}

定義一個(gè)Vnode類，創(chuàng)建節(jié)點(diǎn)分為兩類，一類為text節(jié)點(diǎn)，一類非text節(jié)點(diǎn)

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就可以根據(jù)Vnode 生成對(duì)應(yīng)的Vnode對(duì)象，上述代碼對(duì)應(yīng)的dom表示

<ul>

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

先實(shí)現(xiàn)不用diff把Vnode渲染到頁面中來

為什么先來實(shí)現(xiàn)不用diff渲染Vnode的部分，這里也是為了統(tǒng)計(jì)渲染的時(shí)間，來表明一個(gè)道理。并不是diff就比非diff要開，虛擬DOM并不是任何時(shí)候性能都比非虛擬DOM 要快

先來實(shí)現(xiàn)一個(gè)工具函數(shù)，不熟悉的人可以手工敲下代碼熟悉下

// 真實(shí)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); //渲染頁面
console.timeEnd('start');

這里我們要實(shí)現(xiàn)一個(gè)patch方法，把Vnode渲染到頁面中

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)){
  //如果兩個(gè)Vnode節(jié)點(diǎn)的根一致 開始diff
  patchVnode(oldVnode,vnode)
 }else{
  //這里就是不借助diff的實(shí)現(xiàn)
  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)){
  //非文本節(jié)點(diǎn)
  if(isDef(oldCh) && isDef(ch)){
   //都有字節(jié)點(diǎn)
   if(oldCh !== ch){
    //更新children
    updateChildren(elm,oldCh,ch,removeOnly);
   }
  }else if(isDef(ch)){
   //新的有子節(jié)點(diǎn)，老的沒有
   if(isDef(oldVnode.text)){
    nodeOps.setTextContent(elm,'');
   }
   //添加子節(jié)點(diǎn)
   addVnodes(elm,null,ch,0,ch.length-1)
  }else if(isDef(oldCh)){
   //老的有子節(jié)點(diǎn)，新的沒有
   removeVnodes(elm,oldCh,0,oldCh.length-1)
  }else if(isDef(oldVnode.text)){
   //否則老的有文本內(nèi)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){
  //這里認(rèn)真讀下，沒什么難度的，不行的話 也可以搜索下圖文描述這段過程的

 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);
   //更換順序
   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，添加新節(jié)點(diǎn)中沒有比較的節(jié)點(diǎn)
  refElm = isUndef(newCh[newEndIdx + 1]) ? null : newCh[newEndIdx+1].elm
  addVnodes(parentElm,refElm,newCh,newStartIdx,newEndIdx)
 }else{
  //新的提前相遇 刪除多余的節(jié)點(diǎn)
  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)){
  // 非文本節(jié)點(diǎn)
  vnode.elm = nodeOps.createElement(tag); // 其實(shí)可以初始化的時(shí)候就賦予
  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)
  }
 }
}

這就是完整實(shí)現(xiàn)了

以上就是本文的全部?jī)?nèi)容，希望對(duì)大家的學(xué)習(xí)有所幫助，也希望大家多多支持腳本之家。

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