function updateChildren (parentElm, oldCh, newCh, insertedVnodeQueue, removeOnly) {
    // 為oldCh和newCh分別建立索引，為之後遍歷的依據
    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, elmToMove, refElm
    
    // 直到oldCh或者newCh被遍歷完後跳出迴圈
    while (oldStartIdx <= oldEndIdx && newStartIdx <= newEndIdx) {
      if (isUndef(oldStartVnode)) {
        oldStartVnode = oldCh[++
 
oldStartIdx] // Vnode has been moved left
      } else if (isUndef(oldEndVnode)) {
        oldEndVnode = oldCh[--oldEndIdx]
      } else if (sameVnode(oldStartVnode, newStartVnode)) {
        patchVnode(oldStartVnode, newStartVnode, insertedVnodeQueue)
        oldStartVnode = oldCh[++oldStartIdx]
        newStartVnode =
 
 newCh[++newStartIdx]
      } else if (sameVnode(oldEndVnode, newEndVnode)) {
        patchVnode(oldEndVnode, newEndVnode, insertedVnodeQueue)
        oldEndVnode = oldCh[--oldEndIdx]
        newEndVnode = newCh[--newEndIdx]
      } else if (sameVnode(oldStartVnode, newEndVnode)) { // Vnode moved right
        patchVnode(oldStartVnode, newEndVnode, insertedVnodeQueue)
        canMove && nodeOps.insertBefore(parentElm, oldStartVnode.elm, nodeOps.nextSibling(oldEndVnode.elm))
        oldStartVnode = oldCh[++oldStartIdx]
        newEndVnode = newCh[--newEndIdx]
      } else if (sameVnode(oldEndVnode, newStartVnode)) { // Vnode moved left
        patchVnode(oldEndVnode, newStartVnode, insertedVnodeQueue)
        // 插入到老的開始節點的前面
        canMove && nodeOps.insertBefore(parentElm, oldEndVnode.elm, oldStartVnode.elm)
        oldEndVnode = oldCh[--oldEndIdx]
        newStartVnode = newCh[++newStartIdx]
      } else {
        // 如果以上條件都不滿足，那麼這個時候開始比較key值，首先建立key和index索引的對應關係
        if (isUndef(oldKeyToIdx)) oldKeyToIdx = createKeyToOldIdx(oldCh, oldStartIdx, oldEndIdx)
        idxInOld = isDef(newStartVnode.key) ? oldKeyToIdx[newStartVnode.key] : null
        // 如果idxInOld不存在
        // 1. newStartVnode上存在這個key,但是oldKeyToIdx中不存在
        // 2. newStartVnode上並沒有設定key屬性
        if (isUndef(idxInOld)) { // New element
          // 建立新的dom節點
          // 插入到oldStartVnode.elm前面
          // 參見createElm方法
          createElm(newStartVnode, insertedVnodeQueue, parentElm, oldStartVnode.elm)
          newStartVnode = newCh[++newStartIdx]
        } else {
          elmToMove = oldCh[idxInOld]
          /* istanbul ignore if */
          if (process.env.NODE_ENV !== 'production' && !elmToMove) {
            warn(
              'It seems there are duplicate keys that is causing an update error. ' +
              'Make sure each v-for item has a unique key.'
            )
          
          // 將找到的key一致的oldVnode再和newStartVnode進行diff
          if (sameVnode(elmToMove, newStartVnode)) {
            patchVnode(elmToMove, newStartVnode, insertedVnodeQueue)
            oldCh[idxInOld] = undefined
            // 移動node節點
            canMove && nodeOps.insertBefore(parentElm, newStartVnode.elm, oldStartVnode.elm)
            newStartVnode = newCh[++newStartIdx]
          } else {
            // same key but different element. treat as new element
            // 建立新的dom節點
            createElm(newStartVnode, insertedVnodeQueue, parentElm, oldStartVnode.elm)
            newStartVnode = newCh[++newStartIdx]
          }
        }
      }
    }
    // 如果最後遍歷的oldStartIdx大於oldEndIdx的話
    if (oldStartIdx > oldEndIdx) {        // 如果是老的vdom先被遍歷完
      refElm = isUndef(newCh[newEndIdx + 1]) ? null : newCh[newEndIdx + 1].elm
      // 新增newVnode中剩餘的節點到parentElm中
      addVnodes(parentElm, refElm, newCh, newStartIdx, newEndIdx, insertedVnodeQueue)
    } else if (newStartIdx > newEndIdx) { // 如果是新的vdom先被遍歷完，則刪除oldVnode裡面所有的節點
      // 刪除剩餘的節點
      removeVnodes(parentElm, oldCh, oldStartIdx, oldEndIdx)
    }
}