摘要：前言經常看到講解的虛擬原理的，但很多都是在原代碼的基礎上添加些注釋等等，這里從行代碼開始實現一個的虛擬實現標簽名孩子文本節點對應的真實對象為什么這里默認把置為，不直接根據用把賦值而要等后面時候再賦值呢定義一個類，創建節點分為兩類，一類為節點

經常看到講解Vue2的虛擬Dom diff原理的，但很多都是在原代碼的基礎上添加些注釋等等，這里從0行代碼開始實現一個
Vue2的虛擬DOM

src/core/vdom/Vnode.js

export class VNode{
    constructor (
        tag, //標簽名
        children,//孩子[VNode,VNode],
        text, //文本節點
        elm //對應的真實dom對象
    ){
        this.tag = tag;
        this.children = children
        this.text = text;
        this.elm = elm;
    }
}
export function createTextNode(val){
    //為什么這里默認把elm置為undefined，不直接根據tag 用document.createElement(tagName)把elm賦值？而要等后面createElm時候再賦值呢？
    return new VNode(undefined,undefined,String(val),undefined)
}
export function createCommentNode(tag,children){
    if(children){
        for(var i=0;i
定義一個Vnode類， 創建節點分為兩類，一類為text節點，一類非text節點
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Vnode 生成對應的Vnode對象，上述代碼對應的dom表示

item1
item2
item3

先實現不用diff把Vnode渲染到頁面中來
為什么先來實現不用diff渲染Vnode的部分，這里也是為了統計渲染的時間，來表明一個道理
并不是diff就比非diff要開，虛擬DOM并不是任何時候性能都比非虛擬DOM 要快
先來實現一個工具函數，不熟悉的人可以手工敲下代碼 熟悉下
// 真實的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");
這里我們要實現一個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Vnode節點的根一致  開始diff
        patchVnode(oldVnode,vnode)
    }else{
        //這里就是不借助diff的實現
        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)){
        //非文本節點
        if(isDef(oldCh) && isDef(ch)){
            //都有字節點
            if(oldCh !== ch){
                //更新children
                updateChildren(elm,oldCh,ch,removeOnly);
            }
        }else if(isDef(ch)){
            //新的有子節點，老的沒有
            if(isDef(oldVnode.text)){
                nodeOps.setTextContent(elm,"");
            }
            //添加子節點
            addVnodes(elm,null,ch,0,ch.length-1)
        }else if(isDef(oldCh)){
            //老的有子節點，新的沒有
            removeVnodes(elm,oldCh,0,oldCh.length-1)
        }else if(isDef(oldVnode.text)){
            //否則老的有文本內容 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，添加新節點中沒有比較的節點
        refElm = isUndef(newCh[newEndIdx + 1]) ? null : newCh[newEndIdx+1].elm
        addVnodes(parentElm,refElm,newCh,newStartIdx,newEndIdx)
    }else{
        //新的提前相遇  刪除多余的節點
        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
這就是完整實現了