function updateChildren (parentElm, oldCh, newCh, insertedVnodeQueue, removeOnly) {
  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 oldKeyToIdx, idxInOld, vnodeToMove, refElm

    const canMove = !removeOnly

  while (oldStartIdx <= oldEndIdx && newStartIdx <= newEndIdx) { // 開(kāi)始索引大于結(jié)束索引，進(jìn)不了
   if (isUndef(oldStartVnode)) {
    oldStartVnode = oldCh[++oldStartIdx] // Vnode已經(jīng)被移走了。
   } else if (isUndef(oldEndVnode)) {
    oldEndVnode = oldCh[--oldEndIdx]
   } else if (sameVnode(oldStartVnode, newStartVnode)) {
    patchVnode(oldStartVnode, newStartVnode, insertedVnodeQueue)
    oldStartVnode = oldCh[++oldStartIdx] // 索引加1。是去對(duì)比下一個(gè)節(jié)點(diǎn)。比如之前start=a[0]，那現(xiàn)在start=a[1],改變start的值后再去對(duì)比start這個(gè)vnode
    newStartVnode = newCh[++newStartIdx]
     
   } else if (sameVnode(oldEndVnode, newEndVnode)) { 
    patchVnode(oldEndVnode, newEndVnode, insertedVnodeQueue)
    oldEndVnode = oldCh[--oldEndIdx]
    newEndVnode = newCh[--newEndIdx]
   } else if (sameVnode(oldStartVnode, newEndVnode)) { 
    patchVnode(oldStartVnode, newEndVnode, insertedVnodeQueue)
    canMove && nodeOps.insertBefore(parentElm, oldStartVnode.elm, nodeOps.nextSibling(oldEndVnode.elm))// 把節(jié)點(diǎn)b移到樹(shù)的最右邊
    oldStartVnode = oldCh[++oldStartIdx]
    newEndVnode = newCh[--newEndIdx]
     
   } else if (sameVnode(oldEndVnode, newStartVnode)) {  old.end.d=new.start.d
    patchVnode(oldEndVnode, newStartVnode, insertedVnodeQueue)
    canMove && nodeOps.insertBefore(parentElm, oldEndVnode.elm, oldStartVnode.elm)// Vnode moved left,把d移到c的左邊。=old.start->old.end
    oldEndVnode = oldCh[--oldEndIdx] 
    newStartVnode = newCh[++newStartIdx] 

   } else {
    if (isUndef(oldKeyToIdx)) oldKeyToIdx = createKeyToOldIdx(oldCh, oldStartIdx, oldEndIdx)
    idxInOld = isDef(newStartVnode.key)
     ? oldKeyToIdx[newStartVnode.key]
     : findIdxInOld(newStartVnode, oldCh, oldStartIdx, oldEndIdx)
    if (isUndef(idxInOld)) { 
     createElm(newStartVnode, insertedVnodeQueue, parentElm, oldStartVnode.elm) // 創(chuàng)建新節(jié)點(diǎn)，后面執(zhí)行了nodeOps.insertBefore(parent, elm, ref)
    } else {
     vnodeToMove = oldCh[idxInOld]
     /* istanbul ignore if */
     if (process.env.NODE_ENV !== 'production' && !vnodeToMove) {
      warn(
       'It seems there are duplicate keys that is causing an update error. ' +
       'Make sure each v-for item has a unique key.'
      )
     }
     if (sameVnode(vnodeToMove, newStartVnode)) {
      patchVnode(vnodeToMove, newStartVnode, insertedVnodeQueue)
      oldCh[idxInOld] = undefined
      canMove && nodeOps.insertBefore(parentElm, vnodeToMove.elm, oldStartVnode.elm)
     } else {
      // same key but different element. treat as new element
      createElm(newStartVnode, insertedVnodeQueue, parentElm, oldStartVnode.elm)
     }
    }
    newStartVnode = newCh[++newStartIdx] 
   
   }
  }
  if (oldStartIdx > oldEndIdx) {
   refElm = isUndef(newCh[newEndIdx + 1]) ? null : newCh[newEndIdx + 1].elm
   addVnodes(parentElm, refElm, newCh, newStartIdx, newEndIdx, insertedVnodeQueue)
  } else if (newStartIdx > newEndIdx) {
   removeVnodes(parentElm, oldCh, oldStartIdx, oldEndIdx) // 刪除舊的c,removeNode(ch.elm)

  }
 }

function sameVnode (a, b) {
 return (
  a.key === b.key && (
   (
    a.tag === b.tag &&
    a.isComment === b.isComment &&
    isDef(a.data) === isDef(b.data) &&
    sameInputType(a, b)
   ) || (
    isTrue(a.isAsyncPlaceholder) &&
    a.asyncFactory === b.asyncFactory &&
    isUndef(b.asyncFactory.error)
   )
  )
 )
}

/**
   * 比較新舊vnode節(jié)點(diǎn)，根據(jù)不同的狀態(tài)對(duì)dom做合理的更新操作（添加，移動(dòng)，刪除）整個(gè)過(guò)程還會(huì)依次調(diào)用prepatch,update,postpatch等鉤子函數(shù)，在編譯階段生成的一些靜態(tài)子樹(shù)，在這個(gè)過(guò)程
   * @param oldVnode 中由于不會(huì)改變而直接跳過(guò)比對(duì)，動(dòng)態(tài)子樹(shù)在比較過(guò)程中比較核心的部分就是當(dāng)新舊vnode同時(shí)存在children，通過(guò)updateChildren方法對(duì)子節(jié)點(diǎn)做更新，
   * @param vnode
   * @param insertedVnodeQueue
   * @param removeOnly
   */
 function patchVnode (oldVnode, vnode, insertedVnodeQueue, removeOnly) {
  if (oldVnode === vnode) {
   return
  }

  const elm = vnode.elm = oldVnode.elm

  if (isTrue(oldVnode.isAsyncPlaceholder)) {
   if (isDef(vnode.asyncFactory.resolved)) {
    hydrate(oldVnode.elm, vnode, insertedVnodeQueue)
   } else {
    vnode.isAsyncPlaceholder = true
   }
   return
  }

   // 用于靜態(tài)樹(shù)的重用元素。
    // 注意，如果vnode是克隆的，我們只做這個(gè)。
    // 如果新節(jié)點(diǎn)不是克隆的，則表示呈現(xiàn)函數(shù)。
    // 由熱重加載api重新設(shè)置，我們需要進(jìn)行適當(dāng)?shù)闹匦落秩尽?
  if (isTrue(vnode.isStatic) &&
   isTrue(oldVnode.isStatic) &&
   vnode.key === oldVnode.key &&
   (isTrue(vnode.isCloned) || isTrue(vnode.isOnce))
  ) {
   vnode.componentInstance = oldVnode.componentInstance
   return
  }

  let i
  const data = vnode.data
  if (isDef(data) && isDef(i = data.hook) && isDef(i = i.prepatch)) {
   i(oldVnode, vnode)
  }

  const oldCh = oldVnode.children
  const ch = vnode.children
  if (isDef(data) && isPatchable(vnode)) {
   for (i = 0; i < cbs.update.length; ++i) cbs.update[i](oldVnode, vnode)
   if (isDef(i = data.hook) && isDef(i = i.update)) i(oldVnode, vnode)
  }
  if (isUndef(vnode.text)) {
   if (isDef(oldCh) && isDef(ch)) {
    if (oldCh !== ch) updateChildren(elm, oldCh, ch, insertedVnodeQueue, removeOnly)
   } else if (isDef(ch)) {
    if (isDef(oldVnode.text)) nodeOps.setTextContent(elm, '')
    addVnodes(elm, null, ch, 0, ch.length - 1, insertedVnodeQueue)
   } else if (isDef(oldCh)) {
    removeVnodes(elm, oldCh, 0, oldCh.length - 1)
   } else if (isDef(oldVnode.text)) {
    nodeOps.setTextContent(elm, '')
   }
  } else if (oldVnode.text !== vnode.text) {
   nodeOps.setTextContent(elm, vnode.text)
  }
  if (isDef(data)) {
   if (isDef(i = data.hook) && isDef(i = i.postpatch)) i(oldVnode, vnode)
  }
 }

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

/**
   *
   * @param vnode根據(jù)vnode的數(shù)據(jù)結(jié)構(gòu)創(chuàng)建真實(shí)的dom節(jié)點(diǎn)，如果vnode有children則會(huì)遍歷這些子節(jié)點(diǎn)，遞歸調(diào)用createElm方法，
   * @param insertedVnodeQueue記錄子節(jié)點(diǎn)創(chuàng)建順序的隊(duì)列，每創(chuàng)建一個(gè)dom元素就會(huì)往隊(duì)列中插入當(dāng)前的vnode，當(dāng)整個(gè)vnode對(duì)象全部轉(zhuǎn)換成為真實(shí)的dom 樹(shù)時(shí)，會(huì)依次調(diào)用這個(gè)隊(duì)列中vnode hook的insert方法
   * @param parentElm
   * @param refElm
   * @param nested
   */

   let inPre = 0
 function createElm (vnode, insertedVnodeQueue, parentElm, refElm, nested) {
  vnode.isRootInsert = !nested // 過(guò)渡進(jìn)入檢查
  if (createComponent(vnode, insertedVnodeQueue, parentElm, refElm)) {
   return
  }

  const data = vnode.data
  const children = vnode.children
  const tag = vnode.tag
  if (isDef(tag)) {
   if (process.env.NODE_ENV !== 'production') {
    if (data && data.pre) {
     inPre++
    }
    if (
     !inPre &&
     !vnode.ns &&
     !(
      config.ignoredElements.length &&
      config.ignoredElements.some(ignore => {
       return isRegExp(ignore)
        ? ignore.test(tag)
        : ignore === tag
      })
     ) &&
     config.isUnknownElement(tag)
    ) {
     warn(
      'Unknown custom element: <' + tag + '> - did you ' +
      'register the component correctly? For recursive components, ' +
      'make sure to provide the "name" option.',
      vnode.context
     )
    }
   }
   vnode.elm = vnode.ns
    ? nodeOps.createElementNS(vnode.ns, tag)
    : nodeOps.createElement(tag, vnode)
   setScope(vnode)

   /* istanbul ignore if */
   if (__WEEX__) {
    // in Weex, the default insertion order is parent-first.
    // List items can be optimized to use children-first insertion
    // with append="tree".
    const appendAsTree = isDef(data) && isTrue(data.appendAsTree)
    if (!appendAsTree) {
     if (isDef(data)) {
      invokeCreateHooks(vnode, insertedVnodeQueue)
     }
     insert(parentElm, vnode.elm, refElm)
    }
    createChildren(vnode, children, insertedVnodeQueue)
    if (appendAsTree) {
     if (isDef(data)) {
      invokeCreateHooks(vnode, insertedVnodeQueue)
     }
     insert(parentElm, vnode.elm, refElm)
    }
   } else {
    createChildren(vnode, children, insertedVnodeQueue)
    if (isDef(data)) {
     invokeCreateHooks(vnode, insertedVnodeQueue)
    }
    insert(parentElm, vnode.elm, refElm)
   }

   if (process.env.NODE_ENV !== 'production' && data && data.pre) {
    inPre--
   }
  } else if (isTrue(vnode.isComment)) {
   vnode.elm = nodeOps.createComment(vnode.text)
   insert(parentElm, vnode.elm, refElm)
  } else {
   vnode.elm = nodeOps.createTextNode(vnode.text)
   insert(parentElm, vnode.elm, refElm)
  }
 }
function insert (parent, elm, ref) {
  if (isDef(parent)) {
   if (isDef(ref)) {
    if (ref.parentNode === parent) {
     nodeOps.insertBefore(parent, elm, ref)
    }
   } else {
    nodeOps.appendChild(parent, elm)
   }
  }
 }

function removeVnodes (parentElm, vnodes, startIdx, endIdx) {
  for (; startIdx <= endIdx; ++startIdx) {
   const ch = vnodes[startIdx]
   if (isDef(ch)) {
    if (isDef(ch.tag)) {
     removeAndInvokeRemoveHook(ch)
     invokeDestroyHook(ch)
    } else { // Text node
     removeNode(ch.elm)
    }
   }
  }
 }