虚拟DOM详解

虚拟DOM简介

　　Virtual Dom可以看做一棵模拟了DOM树的JavaScript对象树，其主要是通过vnode,实现一个无状态的组件，当组件状态发生更新时，然后触发Virtual Dom数据的变化，然后通过Virtual Dom和真实DOM的比对，再对真实DOM更新。虚拟DOM其实就是一种模拟DOM的JavaScript数据结构。

　　像SnabbDOM这种库的虚拟DOM是如下的数据结构：

• sel 元素选择器 
• data 元素属性 ●
• children 元素子节点 ●
• text 元素文本 ●
• elm 对应dom元素 ●
• key

SnabbDOM源码概述

　　说到SnabbDOM可能大家不太知道，但是大名鼎鼎的VUE就是使用SnabbDOM来提供虚拟DOM。SnabbDOM中的VNode结构如下：

export interface VNodeData {
  props?: Props;
  attrs?: Attrs;
  class?: Classes;
  style?: VNodeStyle;
  dataset?: Dataset;
  on?: On;
  hero?: Hero;
  attachData?: AttachData;
  hook?: Hooks;
  key?: Key;
  ns?: string; // for SVGs
  fn?: () => VNode; // for thunks
  args?: Array<any>; // for thunks
  [key: string]: any; // for any other 3rd party module
}

export function vnode(sel: string | undefined,
                      data: any | undefined,
                      children: Array<VNode | string> | undefined,
                      text: string | undefined,
                      elm: Element | Text | undefined): VNode {
  let key = data === undefined ? undefined : data.key;
  return {sel: sel, data: data, children: children,
          text: text, elm: elm, key: key};
}

export default vnode;

　　但是这里并没有直接提供对外接口，而是提供了h方法来创建VNode：

export function h(sel: string): VNode;
export function h(sel: string, data: VNodeData): VNode;
export function h(sel: string, text: string): VNode;
export function h(sel: string, children: Array<VNode | undefined | null>): VNode;
export function h(sel: string, data: VNodeData, text: string): VNode;
export function h(sel: string, data: VNodeData, children: Array<VNode | undefined | null>): VNode;
export function h(sel: any, b?: any, c?: any): VNode {
  var data: VNodeData = {}, children: any, text: any, i: number;
  if (c !== undefined) {
    data = b;
    if (is.array(c)) { children = c; }
    else if (is.primitive(c)) { text = c; }
    else if (c && c.sel) { children = [c]; }
  } else if (b !== undefined) {
    if (is.array(b)) { children = b; }
    else if (is.primitive(b)) { text = b; }
    else if (b && b.sel) { children = [b]; }
    else { data = b; }
  }
  if (is.array(children)) {
    for (i = 0; i < children.length; ++i) {
      if (is.primitive(children[i])) children[i] = vnode(undefined, undefined, undefined, children[i], undefined);
    }
  }
  if (
    sel[0] === 's' && sel[1] === 'v' && sel[2] === 'g' &&
    (sel.length === 3 || sel[3] === '.' || sel[3] === '#')
  ) {
    addNS(data, children, sel);
  }
  return vnode(sel, data, children, text, undefined);
};

　　那么在具体使用场景如下：

const overviewView = (movies) =>
  h('div.page', {style: fadeInOutStyle}, [
    h('div.header', [
      h('div.header-content.overview', {
        style: fadeInOutStyle,
      }, [
        h('div.header-title', {
          style: {transform: 'translateY(-2em)',
                  delayed: {transform: 'translate(0)'},
                  destroy: {transform: 'translateY(-2em)'}}
        }, 'Top 10 movies'),
        h('div.spacer'),
      ]),
    ]),
    h('div.page-content', [
      h('div.list', {
        style: {opacity: '0', delayed: {opacity: '1'},
                remove: {opacity: '0', position: 'absolute', top: '0', left: '0'}}
      }, movies.map((movie) =>
        h('div.row', {
          on: {click: [select, movie]},
        }, [
          h('div.hero.rank', [
            h('span.hero', {hero: {id: 'rank'+movie.rank}}, movie.rank)
          ]),
          h('div.hero', {hero: {id: movie.title}}, movie.title)
        ])
      )),
    ]),
  ]);

　　其中跟虚拟DOM各个周期中需要使用的hook方法都放在Hook.ts这个文件下：

import {VNode} from './vnode';

export type PreHook = () => any;
export type InitHook = (vNode: VNode) => any;
export type CreateHook = (emptyVNode: VNode, vNode: VNode) => any;
export type InsertHook = (vNode: VNode) => any;
export type PrePatchHook = (oldVNode: VNode, vNode: VNode) => any;
export type UpdateHook = (oldVNode: VNode, vNode: VNode) => any;
export type PostPatchHook = (oldVNode: VNode, vNode: VNode) => any;
export type DestroyHook = (vNode: VNode) => any;
export type RemoveHook = (vNode: VNode, removeCallback: () => void) => any;
export type PostHook = () => any;

export interface Hooks {
  pre?: PreHook;
  init?: InitHook;
  create?: CreateHook;
  insert?: InsertHook;
  prepatch?: PrePatchHook;
  update?: UpdateHook;
  postpatch?: PostPatchHook;
  destroy?: DestroyHook;
  remove?: RemoveHook;
  post?: PostHook;
}

　　具体的钩子方法则是放在src/modules目录下的各个文件中。

　　其中核心代码，包括整个Diff算法都放在snabbdom.ts文件中：

/* global module, document, Node */
import {Module} from './modules/module';
import {Hooks} from './hooks';
import vnode, {VNode, VNodeData, Key} from './vnode';
import * as is from './is';
import htmlDomApi, {DOMAPI} from './htmldomapi';

function isUndef(s: any): boolean { return s === undefined; }
function isDef(s: any): boolean { return s !== undefined; }

type VNodeQueue = Array<VNode>;

const emptyNode = vnode('', {}, [], undefined, undefined);

function sameVnode(vnode1: VNode, vnode2: VNode): boolean {
  return vnode1.key === vnode2.key && vnode1.sel === vnode2.sel;
}

function isVnode(vnode: any): vnode is VNode {
  return vnode.sel !== undefined;
}

type KeyToIndexMap = {[key: string]: number};

type ArraysOf<T> = {
  [K in keyof T]: (T[K])[];
}

type ModuleHooks = ArraysOf<Module>;

function createKeyToOldIdx(children: Array<VNode>, beginIdx: number, endIdx: number): KeyToIndexMap {
  let i: number, map: KeyToIndexMap = {}, key: Key | undefined, ch;
  for (i = beginIdx; i <= endIdx; ++i) {
    ch = children[i];
    if (ch != null) {
      key = ch.key;
      if (key !== undefined) map[key] = i;
    }
  }
  return map;
}

const hooks: (keyof Module)[] = ['create', 'update', 'remove', 'destroy', 'pre', 'post'];

export {h} from './h';
export {thunk} from './thunk';

export function init(modules: Array<Partial<Module>>, domApi?: DOMAPI) {
  let i: number, j: number, cbs = ({} as ModuleHooks);

  const api: DOMAPI = domApi !== undefined ? domApi : htmlDomApi;
  // 将各个模块的增删改查钩子方法挂在模块钩子集合中
  for (i = 0; i < hooks.length; ++i) {
    cbs[hooks[i]] = [];
    for (j = 0; j < modules.length; ++j) {
      const hook = modules[j][hooks[i]];
      if (hook !== undefined) {
        (cbs[hooks[i]] as Array<any>).push(hook);
      }
    }
  }

  function emptyNodeAt(elm: Element) {
    const id = elm.id ? '#' + elm.id : '';
    const c = elm.className ? '.' + elm.className.split(' ').join('.') : '';
    return vnode(api.tagName(elm).toLowerCase() + id + c, {}, [], undefined, elm);
  }

  function createRmCb(childElm: Node, listeners: number) {
    return function rmCb() {
      if (--listeners === 0) {
        const parent = api.parentNode(childElm);
        api.removeChild(parent, childElm);
      }
    };
  }

  function createElm(vnode: VNode, insertedVnodeQueue: VNodeQueue): Node {
    let i: any, data = vnode.data;
    if (data !== undefined) {
      if (isDef(i = data.hook) && isDef(i = i.init)) {
        i(vnode);
        data = vnode.data;
      }
    }
    let children = vnode.children, sel = vnode.sel;
    if (sel === '!') {
      if (isUndef(vnode.text)) {
        vnode.text = '';
      }
      vnode.elm = api.createComment(vnode.text as string);
    } else if (sel !== undefined) {
      // Parse selector
      const hashIdx = sel.indexOf('#');
      const dotIdx = sel.indexOf('.', hashIdx);
      const hash = hashIdx > 0 ? hashIdx : sel.length;
      const dot = dotIdx > 0 ? dotIdx : sel.length;
      const tag = hashIdx !== -1 || dotIdx !== -1 ? sel.slice(0, Math.min(hash, dot)) : sel;
      const elm = vnode.elm = isDef(data) && isDef(i = (data as VNodeData).ns) ? api.createElementNS(i, tag)
                                                                               : api.createElement(tag);
      if (hash < dot) elm.setAttribute('id', sel.slice(hash + 1, dot));
      if (dotIdx > 0) elm.setAttribute('class', sel.slice(dot + 1).replace(/./g, ' '));
      for (i = 0; i < cbs.create.length; ++i) cbs.create[i](emptyNode, vnode);
      if (is.array(children)) {
        for (i = 0; i < children.length; ++i) {
          const ch = children[i];
          if (ch != null) {
            api.appendChild(elm, createElm(ch as VNode, insertedVnodeQueue));
          }
        }
      } else if (is.primitive(vnode.text)) {
        api.appendChild(elm, api.createTextNode(vnode.text));
      }
      i = (vnode.data as VNodeData).hook; // Reuse variable
      if (isDef(i)) {
        if (i.create) i.create(emptyNode, vnode);
        if (i.insert) insertedVnodeQueue.push(vnode);
      }
    } else {
      vnode.elm = api.createTextNode(vnode.text as string);
    }
    return vnode.elm;
  }

  function addVnodes(parentElm: Node,
                     before: Node | null,
                     vnodes: Array<VNode>,
                     startIdx: number,
                     endIdx: number,
                     insertedVnodeQueue: VNodeQueue) {
    for (; startIdx <= endIdx; ++startIdx) {
      const ch = vnodes[startIdx];
      if (ch != null) {
        api.insertBefore(parentElm, createElm(ch, insertedVnodeQueue), before);
      }
    }
  }

  function invokeDestroyHook(vnode: VNode) {
    let i: any, j: number, data = vnode.data;
    if (data !== undefined) {
      if (isDef(i = data.hook) && isDef(i = i.destroy)) i(vnode);
      for (i = 0; i < cbs.destroy.length; ++i) cbs.destroy[i](vnode);
      if (vnode.children !== undefined) {
        for (j = 0; j < vnode.children.length; ++j) {
          i = vnode.children[j];
          if (i != null && typeof i !== "string") {
            invokeDestroyHook(i);
          }
        }
      }
    }
  }

  function removeVnodes(parentElm: Node,
                        vnodes: Array<VNode>,
                        startIdx: number,
                        endIdx: number): void {
    for (; startIdx <= endIdx; ++startIdx) {
      let i: any, listeners: number, rm: () => void, ch = vnodes[startIdx];
      if (ch != null) {
        if (isDef(ch.sel)) {
          invokeDestroyHook(ch);
          listeners = cbs.remove.length + 1;
          rm = createRmCb(ch.elm as Node, listeners);
          for (i = 0; i < cbs.remove.length; ++i) cbs.remove[i](ch, rm);
          if (isDef(i = ch.data) && isDef(i = i.hook) && isDef(i = i.remove)) {
            i(ch, rm);
          } else {
            rm();
          }
        } else { // Text node
          api.removeChild(parentElm, ch.elm as Node);
        }
      }
    }
  }

  function updateChildren(parentElm: Node,
                          oldCh: Array<VNode>,
                          newCh: Array<VNode>,
                          insertedVnodeQueue: VNodeQueue) {
    // 声明指针
    let oldStartIdx = 0, 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 oldKeyToIdx: any;
    let idxInOld: number;
    let elmToMove: VNode;
    let before: any;

    while (oldStartIdx <= oldEndIdx && newStartIdx <= newEndIdx) {
      // 如果节点已经被标记处理过，直接跳过
      if (oldStartVnode == null) {
        oldStartVnode = oldCh[++oldStartIdx]; // Vnode might have been moved left
      } else if (oldEndVnode == null) {
        oldEndVnode = oldCh[--oldEndIdx];
      } else if (newStartVnode == null) {
        newStartVnode = newCh[++newStartIdx];
      } else if (newEndVnode == null) {
        newEndVnode = newCh[--newEndIdx];

      } else if (sameVnode(oldStartVnode, newStartVnode)) {// 处理头头相同情况
        patchVnode(oldStartVnode, newStartVnode, insertedVnodeQueue);
        oldStartVnode = oldCh[++oldStartIdx];
        newStartVnode = newCh[++newStartIdx];
      } else if (sameVnode(oldEndVnode, newEndVnode)) { // 尾尾相同
        patchVnode(oldEndVnode, newEndVnode, insertedVnodeQueue);
        oldEndVnode = oldCh[--oldEndIdx];
        newEndVnode = newCh[--newEndIdx];
      } else if (sameVnode(oldStartVnode, newEndVnode)) {// 头尾相同 // Vnode moved right
        patchVnode(oldStartVnode, newEndVnode, insertedVnodeQueue);
        // 将oldStart指向的节点插入到oldEnd指向节点前面
        api.insertBefore(parentElm, oldStartVnode.elm as Node, api.nextSibling(oldEndVnode.elm as Node));
        oldStartVnode = oldCh[++oldStartIdx];
        newEndVnode = newCh[--newEndIdx];
      } else if (sameVnode(oldEndVnode, newStartVnode)) { // Vnode moved left 尾头相同
        patchVnode(oldEndVnode, newStartVnode, insertedVnodeQueue);
        // 将oldEnd指向节点插入到oldStart之前
        api.insertBefore(parentElm, oldEndVnode.elm as Node, oldStartVnode.elm as Node);
        oldEndVnode = oldCh[--oldEndIdx];
        newStartVnode = newCh[++newStartIdx];
      } else {
        if (oldKeyToIdx === undefined) { // 制作key-index的哈西集合
          oldKeyToIdx = createKeyToOldIdx(oldCh, oldStartIdx, oldEndIdx);
        }
        idxInOld = oldKeyToIdx[newStartVnode.key as string];
        if (isUndef(idxInOld)) { // New element
          // 新节点则插入到oldStart指向的dom节点之前
          api.insertBefore(parentElm, createElm(newStartVnode, insertedVnodeQueue), oldStartVnode.elm as Node);
          newStartVnode = newCh[++newStartIdx];
        } else {
          elmToMove = oldCh[idxInOld];
          if (elmToMove.sel !== newStartVnode.sel) {
            api.insertBefore(parentElm, createElm(newStartVnode, insertedVnodeQueue), oldStartVnode.elm as Node);
          } else {
            // 打补丁
            // 对旧的位置坐标记设为undefined
            // 将更新后的节点移动到oldStart之前
            patchVnode(elmToMove, newStartVnode, insertedVnodeQueue);
            oldCh[idxInOld] = undefined as any;
            api.insertBefore(parentElm, (elmToMove.elm as Node), oldStartVnode.elm as Node);
          }
          newStartVnode = newCh[++newStartIdx];
        }
      }
    }
    if (oldStartIdx > oldEndIdx) {// 新的有剩余则需要将这些节点插入到dom树中
      before = newCh[newEndIdx+1] == null ? null : newCh[newEndIdx+1].elm;
      addVnodes(parentElm, before, newCh, newStartIdx, newEndIdx, insertedVnodeQueue);
    } else if (newStartIdx > newEndIdx) {// 老的有节点则应该移除
      removeVnodes(parentElm, oldCh, oldStartIdx, oldEndIdx);
    }
  }

  function patchVnode(oldVnode: VNode, vnode: VNode, insertedVnodeQueue: VNodeQueue) {
    let i: any, hook: any;
    // 调用prepatch钩子
    if (isDef(i = vnode.data) && isDef(hook = i.hook) && isDef(i = hook.prepatch)) {
      i(oldVnode, vnode);
    }
    const elm = vnode.elm = (oldVnode.elm as Node);
    let oldCh = oldVnode.children;
    let ch = vnode.children;
    if (oldVnode === vnode) return;// 没有变化直接返回
    if (vnode.data !== undefined) { // 对两个虚拟dom树的根节点进行更新
      for (i = 0; i < cbs.update.length; ++i) cbs.update[i](oldVnode, vnode);
      i = vnode.data.hook;
      if (isDef(i) && isDef(i = i.update)) i(oldVnode, vnode);
    }
    if (isUndef(vnode.text)) { // 是否有文本节点
      if (isDef(oldCh) && isDef(ch)) { // 都有子节点对子节点进行diff算法
        if (oldCh !== ch) updateChildren(elm, oldCh as Array<VNode>, ch as Array<VNode>, insertedVnodeQueue);
      } else if (isDef(ch)) { // 新虚拟dom有子节点旧的没有，则把新的子节点挂在dom上
        if (isDef(oldVnode.text)) api.setTextContent(elm, '');
        addVnodes(elm, null, ch as Array<VNode>, 0, (ch as Array<VNode>).length - 1, insertedVnodeQueue);
      } else if (isDef(oldCh)) {// 老的里面有子节点，新的没有直接干掉老的
        removeVnodes(elm, oldCh as Array<VNode>, 0, (oldCh as Array<VNode>).length - 1);
      } else if (isDef(oldVnode.text)) { // 老的有文本信息直接设置为空字符串
        api.setTextContent(elm, '');
      }
    } else if (oldVnode.text !== vnode.text) {// 使用新的文本信息替换旧的
      api.setTextContent(elm, vnode.text as string);
    }
    if (isDef(hook) && isDef(i = hook.postpatch)) { // 调用postpatch方法
      i(oldVnode, vnode);
    }
  }

  return function patch(oldVnode: VNode | Element, vnode: VNode): VNode {
    let i: number, elm: Node, parent: Node;
    const insertedVnodeQueue: VNodeQueue = [];
    for (i = 0; i < cbs.pre.length; ++i) cbs.pre[i]();// 调用所有模块的pre钩子方法

    if (!isVnode(oldVnode)) { // 非虚拟dom直接清空
      oldVnode = emptyNodeAt(oldVnode);
    }

    if (sameVnode(oldVnode, vnode)) { // 两个虚拟dom树的根节点完全一样才会进行打补丁
      patchVnode(oldVnode, vnode, insertedVnodeQueue);
    } else { // 如果跟节点不同直接干掉旧的根节点，重新创建dom元素插入到dom树中
      elm = oldVnode.elm as Node;
      parent = api.parentNode(elm);

      createElm(vnode, insertedVnodeQueue);

      if (parent !== null) {
        api.insertBefore(parent, vnode.elm as Node, api.nextSibling(elm));
        removeVnodes(parent, [oldVnode], 0, 0);
      }
    }

    // 对每个插入的元素使用相应模块中的insert钩子方法进行更新
    for (i = 0; i < insertedVnodeQueue.length; ++i) {
      (((insertedVnodeQueue[i].data as VNodeData).hook as Hooks).insert as any)(insertedVnodeQueue[i]);
    }
    for (i = 0; i < cbs.post.length; ++i) cbs.post[i](); // 调用所有钩子模块的post方法
    return vnode;
  };
}

Diff算法

　　SnabbDOM的diff算法主要有两个特点：

• 同级比较
• 就近复用

　　

　　同级比较的意思是，对于两颗DOM树，只会比较同一层级的节点，如果节点类型不同直接干掉旧的节点，而不是继续比较。那么就近复用意味着如果节点类型相同就会对这个节点进行改造，而不是严格的比较各个属性书否相同。那么这里涉及snabbdom中的两个主要函数：

function patch(oldVnode: VNode | Element, vnode: VNode): VNode {
    let i: number, elm: Node, parent: Node;
    const insertedVnodeQueue: VNodeQueue = [];
    for (i = 0; i < cbs.pre.length; ++i) cbs.pre[i]();// 调用所有模块的pre钩子方法

    if (!isVnode(oldVnode)) { // 非虚拟dom直接清空
      oldVnode = emptyNodeAt(oldVnode);
    }

    if (sameVnode(oldVnode, vnode)) { // 两个虚拟dom树的根节点完全一样才会进行打补丁
      patchVnode(oldVnode, vnode, insertedVnodeQueue);
    } else { // 如果跟节点不同直接干掉旧的根节点，重新创建dom元素插入到dom树中
      elm = oldVnode.elm as Node;
      parent = api.parentNode(elm);

      createElm(vnode, insertedVnodeQueue);

      if (parent !== null) {
        api.insertBefore(parent, vnode.elm as Node, api.nextSibling(elm));
        removeVnodes(parent, [oldVnode], 0, 0);
      }
    }

    // 对每个插入的元素使用相应模块中的insert钩子方法进行更新
    for (i = 0; i < insertedVnodeQueue.length; ++i) {
      (((insertedVnodeQueue[i].data as VNodeData).hook as Hooks).insert as any)(insertedVnodeQueue[i]);
    }
    for (i = 0; i < cbs.post.length; ++i) cbs.post[i](); // 调用所有钩子模块的post方法
    return vnode;
  };

function patchVnode(oldVnode: VNode, vnode: VNode, insertedVnodeQueue: VNodeQueue) {
    let i: any, hook: any;
    // 调用prepatch钩子
    if (isDef(i = vnode.data) && isDef(hook = i.hook) && isDef(i = hook.prepatch)) {
      i(oldVnode, vnode);
    }
    const elm = vnode.elm = (oldVnode.elm as Node);
    let oldCh = oldVnode.children;
    let ch = vnode.children;
    if (oldVnode === vnode) return;// 没有变化直接返回
    if (vnode.data !== undefined) { // 对两个虚拟dom树的根节点进行更新
      for (i = 0; i < cbs.update.length; ++i) cbs.update[i](oldVnode, vnode);
      i = vnode.data.hook;
      if (isDef(i) && isDef(i = i.update)) i(oldVnode, vnode);
    }
    if (isUndef(vnode.text)) { // 是否有文本节点
      if (isDef(oldCh) && isDef(ch)) { // 都有子节点对子节点进行diff算法
        if (oldCh !== ch) updateChildren(elm, oldCh as Array<VNode>, ch as Array<VNode>, insertedVnodeQueue);
      } else if (isDef(ch)) { // 新虚拟dom有子节点旧的没有，则把新的子节点挂在dom上
        if (isDef(oldVnode.text)) api.setTextContent(elm, '');
        addVnodes(elm, null, ch as Array<VNode>, 0, (ch as Array<VNode>).length - 1, insertedVnodeQueue);
      } else if (isDef(oldCh)) {// 老的里面有子节点，新的没有直接干掉老的
        removeVnodes(elm, oldCh as Array<VNode>, 0, (oldCh as Array<VNode>).length - 1);
      } else if (isDef(oldVnode.text)) { // 老的有文本信息直接设置为空字符串
        api.setTextContent(elm, '');
      }
    } else if (oldVnode.text !== vnode.text) {// 使用新的文本信息替换旧的
      api.setTextContent(elm, vnode.text as string);
    }
    if (isDef(hook) && isDef(i = hook.postpatch)) { // 调用postpatch方法
      i(oldVnode, vnode);
    }
  }

　　其中diff的核心算法就在updateChildren中

function updateChildren(parentElm: Node,
                          oldCh: Array<VNode>,
                          newCh: Array<VNode>,
                          insertedVnodeQueue: VNodeQueue) {
    // 声明指针
    let oldStartIdx = 0, 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 oldKeyToIdx: any;
    let idxInOld: number;
    let elmToMove: VNode;
    let before: any;

    while (oldStartIdx <= oldEndIdx && newStartIdx <= newEndIdx) {
      // 如果节点已经被标记处理过，直接跳过
      if (oldStartVnode == null) {
        oldStartVnode = oldCh[++oldStartIdx]; // Vnode might have been moved left
      } else if (oldEndVnode == null) {
        oldEndVnode = oldCh[--oldEndIdx];
      } else if (newStartVnode == null) {
        newStartVnode = newCh[++newStartIdx];
      } else if (newEndVnode == null) {
        newEndVnode = newCh[--newEndIdx];

      } else if (sameVnode(oldStartVnode, newStartVnode)) {// 处理头头相同情况
        patchVnode(oldStartVnode, newStartVnode, insertedVnodeQueue);
        oldStartVnode = oldCh[++oldStartIdx];
        newStartVnode = newCh[++newStartIdx];
      } else if (sameVnode(oldEndVnode, newEndVnode)) { // 尾尾相同
        patchVnode(oldEndVnode, newEndVnode, insertedVnodeQueue);
        oldEndVnode = oldCh[--oldEndIdx];
        newEndVnode = newCh[--newEndIdx];
      } else if (sameVnode(oldStartVnode, newEndVnode)) {// 头尾相同 // Vnode moved right
        patchVnode(oldStartVnode, newEndVnode, insertedVnodeQueue);
        // 将oldStart指向的节点插入到oldEnd指向节点前面
        api.insertBefore(parentElm, oldStartVnode.elm as Node, api.nextSibling(oldEndVnode.elm as Node));
        oldStartVnode = oldCh[++oldStartIdx];
        newEndVnode = newCh[--newEndIdx];
      } else if (sameVnode(oldEndVnode, newStartVnode)) { // Vnode moved left 尾头相同
        patchVnode(oldEndVnode, newStartVnode, insertedVnodeQueue);
        // 将oldEnd指向节点插入到oldStart之前
        api.insertBefore(parentElm, oldEndVnode.elm as Node, oldStartVnode.elm as Node);
        oldEndVnode = oldCh[--oldEndIdx];
        newStartVnode = newCh[++newStartIdx];
      } else {
        if (oldKeyToIdx === undefined) { // 制作key-index的哈西集合
          oldKeyToIdx = createKeyToOldIdx(oldCh, oldStartIdx, oldEndIdx);
        }
        idxInOld = oldKeyToIdx[newStartVnode.key as string];
        if (isUndef(idxInOld)) { // New element
          // 新节点则插入到oldStart指向的dom节点之前
          api.insertBefore(parentElm, createElm(newStartVnode, insertedVnodeQueue), oldStartVnode.elm as Node);
          newStartVnode = newCh[++newStartIdx];
        } else {
          elmToMove = oldCh[idxInOld];
          if (elmToMove.sel !== newStartVnode.sel) {
            api.insertBefore(parentElm, createElm(newStartVnode, insertedVnodeQueue), oldStartVnode.elm as Node);
          } else {
            // 打补丁
            // 对旧的位置坐标记设为undefined
            // 将更新后的节点移动到oldStart之前
            patchVnode(elmToMove, newStartVnode, insertedVnodeQueue);
            oldCh[idxInOld] = undefined as any;
            api.insertBefore(parentElm, (elmToMove.elm as Node), oldStartVnode.elm as Node);
          }
          newStartVnode = newCh[++newStartIdx];
        }
      }
    }
    if (oldStartIdx > oldEndIdx) {// 新的有剩余则需要将这些节点插入到dom树中
      before = newCh[newEndIdx+1] == null ? null : newCh[newEndIdx+1].elm;
      addVnodes(parentElm, before, newCh, newStartIdx, newEndIdx, insertedVnodeQueue);
    } else if (newStartIdx > newEndIdx) {// 老的有节点则应该移除
      removeVnodes(parentElm, oldCh, oldStartIdx, oldEndIdx);
    }
  }

　　这个方法有些复杂，下面文字描述未必那么清晰，大家可以看我录制的视频——虚拟DOM详解。

　　下图中白色图表代表目前实际的DOM节点，oldStart和oldEnd指向之前的虚拟DOM树，newStart和newEnd分别指向新的虚拟DOM节点。

　　现在二者进行比较，首先处理头头、尾尾相同的节点，如果头尾尾头相同则同时移动新旧的指针

　　接下来处理头尾尾头相同的节点，把newStartIdx和oldEndIdx相同的节点插入到oldStartIdx指向节点之前，newStartIdx向后移动，oldEndIdx向前移动；把newEndIdx和oldStartIdx相同的节点插入到oldEndIdx指向的节点之后，同时oldStartIdx向后移动，newEndIdx向前移动。

　　处理完毕后的指针状态：

　　接下来需要处理newStartIdx指向的节点11，那么这时候先去oldStartIdx和oldEndIdx的区间内寻找有没有这个节点，如果没有那么这个节点属于插入节点，这个节点会被插入到oldStartIdx指向的节点的前面，同时newStartIdx向后移动

　　处理完11后，newStartIdx指向4，这个时候从oldStartIdx和oldEndIdx中能够找到这个节点，这说明它的位置被移动了，那么这时候只需要移动这个节点，把它移动到oldStartIdx所指向的节点之前，同时对就的虚拟DOM节点的位置进行标记，这里是设置为undefined。继续移动newStartIdx。

　　接下来对7、 5、 6 都进行相同操作，这时候newStartIdx指向3的位置

　　那么这个时候又变成了头头相同的情况，只需要同时将newStartIdx和oldStartIdx向后移动。

　　那么这时候newStartIdx越过了newEndIdx，到这里循环结束，这时候oldStartIdx和oldEndIdx中剩下的节点都是需要删除的节点。因为之前都已经打上了标记，所以这里只要节点8是需要删除的。当然也有时候会遇到oldStartIdx和oldEndIdx先相遇，这时候在newStartIdx和newEndIdx中的节点都是需要插入的。

　　至此整个比较算法结束。

参考资料

深入 Vue2.x 的虚拟 DOM diff 原理

vue的Virtual Dom实现snabbdom解密

snabbdom

vue中Virtual DOM源码学习

下面是我录制的视频，有动画的可以更清晰的展示这个过程。