Vue.prototype._update = function (vnode: VNode, hydrating?: boolean) {
  const vm: Component = this
  const prevEl = vm.$el
  const prevVnode = vm._vnode
  const restoreActiveInstance = setActiveInstance(vm)
  vm._vnode = vnode
  // Vue.prototype.__patch__ is injected in entry points
  // based on the rendering backend used.
  /*基于后端渲染Vue.prototype.__patch__被用來作為一個入口*/
  if (!prevVnode) {
    // initial render
    vm.$el = vm.__patch__(vm.$el, vnode, hydrating, false /* removeOnly */)
  } else {
    // updates
    vm.$el = vm.__patch__(prevVnode, vnode)
  }
  restoreActiveInstance()
  // update __vue__ reference
  /*更新新的實例對象的__vue__*/
  if (prevEl) {
    prevEl.__vue__ = null
  }
  if (vm.$el) {
    vm.$el.__vue__ = vm
  }
  // if parent is an HOC, update its $el as well
  if (vm.$vnode && vm.$parent && vm.$vnode === vm.$parent._vnode) {
    vm.$parent.$el = vm.$el
  }
  // updated hook is called by the scheduler to ensure that children are
  // updated in a parent's updated hook.
}

return function patch (oldVnode, vnode, hydrating, removeOnly) {
  /*vnode不存在調(diào)用銷毀鉤子刪除節(jié)點*/
  if (isUndef(vnode)) {
    if (isDef(oldVnode)) invokeDestroyHook(oldVnode)
    return
  }

  let isInitialPatch = false
  const insertedVnodeQueue = []

  /*oldVnode不存在，直接創(chuàng)建新節(jié)點*/
  if (isUndef(oldVnode)) {
    // empty mount (likely as component), create new root element
    isInitialPatch = true
    createElm(vnode, insertedVnodeQueue)
  } else {
    /*標(biāo)記舊的VNode是否有nodeType*/
    const isRealElement = isDef(oldVnode.nodeType)
    if (!isRealElement && sameVnode(oldVnode, vnode)) {
      // patch existing root node
      /*是同一個節(jié)點的時候直接修改現(xiàn)有的節(jié)點*/
      patchVnode(oldVnode, vnode, insertedVnodeQueue, null, null, removeOnly)
    } else {
      if (isRealElement) {
        // mounting to a real element
        // check if this is server-rendered content and if we can perform
        // a successful hydration.
        if (oldVnode.nodeType === 1 && oldVnode.hasAttribute(SSR_ATTR)) {
          /*當(dāng)舊的VNode是服務(wù)端渲染的元素，hydrating記為true*/
          oldVnode.removeAttribute(SSR_ATTR)
          hydrating = true
        }
        if (isTrue(hydrating)) {
          /*需要合并到真實Dom上*/
          if (hydrate(oldVnode, vnode, insertedVnodeQueue)) {
            /*調(diào)用insert鉤子*/
            invokeInsertHook(vnode, insertedVnodeQueue, true)
            return oldVnode
          } else if (process.env.NODE_ENV !== 'production') {
            warn(
              'The client-side rendered virtual DOM tree is not matching ' +
              'server-rendered content. This is likely caused by incorrect ' +
              'HTML markup, for example nesting block-level elements inside ' +
              '<p>, or missing <tbody>. Bailing hydration and performing ' +
              'full client-side render.'
            )
          }
        }
        // either not server-rendered, or hydration failed.
        // create an empty node and replace it
        /*如果不是服務(wù)端渲染或者合并到真實Dom失敗，則創(chuàng)建一個空的VNode節(jié)點替換它*/
        oldVnode = emptyNodeAt(oldVnode)
      }

      // replacing existing element
      /*取代現(xiàn)有元素*/
      const oldElm = oldVnode.elm
      const parentElm = nodeOps.parentNode(oldElm)

      // create new node
      createElm(
        vnode,
        insertedVnodeQueue,
        // extremely rare edge case: do not insert if old element is in a
        // leaving transition. Only happens when combining transition +
        // keep-alive + HOCs. (#4590)
        oldElm._leaveCb ? null : parentElm,
        nodeOps.nextSibling(oldElm)
      )

      // update parent placeholder node element, recursively
      if (isDef(vnode.parent)) {
        /*組件根節(jié)點被替換，遍歷更新父節(jié)點element*/
        let ancestor = vnode.parent
        const patchable = isPatchable(vnode)
        while (ancestor) {
          for (let i = 0; i < cbs.destroy.length; ++i) {
            cbs.destroy[i](ancestor)
          }
          ancestor.elm = vnode.elm
          if (patchable) {
            /*調(diào)用create回調(diào)*/
            for (let i = 0; i < cbs.create.length; ++i) {
              cbs.create[i](emptyNode, ancestor)
            }
            // #6513
            // invoke insert hooks that may have been merged by create hooks.
            // e.g. for directives that uses the "inserted" hook.
            const insert = ancestor.data.hook.insert
            if (insert.merged) {
              // start at index 1 to avoid re-invoking component mounted hook
              for (let i = 1; i < insert.fns.length; i++) {
                insert.fns[i]()
              }
            }
          } else {
            registerRef(ancestor)
          }
          ancestor = ancestor.parent
        }
      }

      // destroy old node
      if (isDef(parentElm)) {
        /*移除老節(jié)點*/
        removeVnodes([oldVnode], 0, 0)
      } else if (isDef(oldVnode.tag)) {
        /*調(diào)用destroy鉤子*/
        invokeDestroyHook(oldVnode)
      }
    }
  }

  /*調(diào)用insert鉤子*/
  invokeInsertHook(vnode, insertedVnodeQueue, isInitialPatch)
  return vnode.elm
}

/*
  判斷兩個VNode節(jié)點是否是同一個節(jié)點，需要滿足以下條件
  key相同
  tag（當(dāng)前節(jié)點的標(biāo)簽名）相同
  isComment（是否為注釋節(jié)點）相同
  是否data（當(dāng)前節(jié)點對應(yīng)的對象，包含了具體的一些數(shù)據(jù)信息，是一個VNodeData類型，可以參考VNodeData類型中的數(shù)據(jù)信息）都有定義
  當(dāng)標(biāo)簽是<input>的時候，type必須相同
*/
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)
      )
    )
  )
}

// Some browsers do not support dynamically changing type for <input>
// so they need to be treated as different nodes
/*
  判斷當(dāng)標(biāo)簽是<input>的時候，type是否相同
  某些瀏覽器不支持動態(tài)修改<input>類型，所以他們被視為不同類型
*/
function sameInputType (a, b) {
  if (a.tag !== 'input') return true
  let i
  const typeA = isDef(i = a.data) && isDef(i = i.attrs) && i.type
  const typeB = isDef(i = b.data) && isDef(i = i.attrs) && i.type
  return typeA === typeB || isTextInputType(typeA) && isTextInputType(typeB)
}

 // diff算法 比較節(jié)點
function patchVnode (
  oldVnode,
  vnode,
  insertedVnodeQueue,
  ownerArray,
  index,
  removeOnly
) {
  /*兩個VNode節(jié)點相同則直接返回*/
  if (oldVnode === vnode) {
    return
  }

  if (isDef(vnode.elm) && isDef(ownerArray)) {
    // clone reused vnode
    vnode = ownerArray[index] = cloneVNode(vnode)
  }

  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
  }

  // reuse element for static trees.
  // note we only do this if the vnode is cloned -
  // if the new node is not cloned it means the render functions have been
  // reset by the hot-reload-api and we need to do a proper re-render.
  /*
    如果新舊VNode都是靜態(tài)的，同時它們的key相同（代表同一節(jié)點），
    并且新的VNode是clone或者是標(biāo)記了once（標(biāo)記v-once屬性，只渲染一次），
    那么只需要替換elm以及componentInstance即可。
  */
  if (isTrue(vnode.isStatic) &&
    isTrue(oldVnode.isStatic) &&
    vnode.key === oldVnode.key &&
    (isTrue(vnode.isCloned) || isTrue(vnode.isOnce))
  ) {
    vnode.componentInstance = oldVnode.componentInstance
    return
  }

  // 執(zhí)行一些組件鉤子
  /*如果存在data.hook.prepatch則要先執(zhí)⾏*/
  let i
  const data = vnode.data
  if (isDef(data) && isDef(i = data.hook) && isDef(i = i.prepatch)) {
    /*i = data.hook.prepatch，如果存在的話，見"./create-component componentVNodeHooks"。*/
    i(oldVnode, vnode)
  }

  // 查找新舊節(jié)點是否存在孩子
  const oldCh = oldVnode.children
  const ch = vnode.children

  // 屬性更新
  if (isDef(data) && isPatchable(vnode)) {
    // cbs中關(guān)于屬性更新的數(shù)組拿出來[attrFn, classFn, ...]
    /*調(diào)用update回調(diào)以及update鉤子*/
    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)
  }

  // 判斷是否元素
  if (isUndef(vnode.text)) { /*如果這個VNode節(jié)點沒有text文本時*/
    // 雙方都有孩子
    if (isDef(oldCh) && isDef(ch)) {
      /*新老節(jié)點均有children子節(jié)點，則對子節(jié)點進行diff操作，調(diào)用updateChildren*/
      if (oldCh !== ch) updateChildren(elm, oldCh, ch, insertedVnodeQueue, removeOnly)
    } else if (isDef(ch)) {
      /*如果老節(jié)點沒有子節(jié)點而新節(jié)點存在子節(jié)點，先清空elm的文本內(nèi)容，然后為當(dāng)前節(jié)點加入子節(jié)點*/
      if (process.env.NODE_ENV !== 'production') {
        checkDuplicateKeys(ch)
      }
      if (isDef(oldVnode.text)) nodeOps.setTextContent(elm, '')
      addVnodes(elm, null, ch, 0, ch.length - 1, insertedVnodeQueue)
    } else if (isDef(oldCh)) {
      /*當(dāng)新節(jié)點沒有子節(jié)點而老節(jié)點有子節(jié)點的時候，則移除所有ele的子節(jié)點*/
      removeVnodes(oldCh, 0, oldCh.length - 1)
    } else if (isDef(oldVnode.text)) {
      /*當(dāng)新老節(jié)點都無子節(jié)點的時候，只是文本的替換，因為這個邏輯中新節(jié)點text不存在，所以直接去除ele的文本*/
      nodeOps.setTextContent(elm, '')
    }
  } else if (oldVnode.text !== vnode.text) {
    /*當(dāng)新老節(jié)點text不一樣時，直接替換這段文本*/
    nodeOps.setTextContent(elm, vnode.text)
  }
  /*調(diào)用postpatch鉤子*/
  if (isDef(data)) {
    if (isDef(i = data.hook) && isDef(i = i.postpatch)) i(oldVnode, vnode)
  }
}

/*
  diff核心方法，比較優(yōu)化
*/
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

  // removeOnly is a special flag used only by <transition-group>
  // to ensure removed elements stay in correct relative positions
  // during leaving transitions
  const canMove = !removeOnly

  if (process.env.NODE_ENV !== 'production') {
    checkDuplicateKeys(newCh)
  }

  while (oldStartIdx <= oldEndIdx && newStartIdx <= newEndIdx) {
    if (isUndef(oldStartVnode)) {
      /*向右靠攏*/
      oldStartVnode = oldCh[++oldStartIdx] // Vnode has been moved left
    } else if (isUndef(oldEndVnode)) {
      /*向左靠攏*/
      oldEndVnode = oldCh[--oldEndIdx]
    } else if (sameVnode(oldStartVnode, newStartVnode)) {
      /*前四種情況其實是指定key的時候，判定為同一個VNode，則直接patchVnode即可，分別比較oldCh以及newCh的兩頭節(jié)點2*2=4種情況*/
      patchVnode(oldStartVnode, newStartVnode, insertedVnodeQueue, newCh, newStartIdx)
      oldStartVnode = oldCh[++oldStartIdx]
      newStartVnode = newCh[++newStartIdx]
    } else if (sameVnode(oldEndVnode, newEndVnode)) {
      patchVnode(oldEndVnode, newEndVnode, insertedVnodeQueue, newCh, newEndIdx)
      oldEndVnode = oldCh[--oldEndIdx]
      newEndVnode = newCh[--newEndIdx]
    } else if (sameVnode(oldStartVnode, newEndVnode)) { // Vnode moved right
      patchVnode(oldStartVnode, newEndVnode, insertedVnodeQueue, newCh, newEndIdx)
      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, newCh, newStartIdx)
      canMove && nodeOps.insertBefore(parentElm, oldEndVnode.elm, oldStartVnode.elm)
      oldEndVnode = oldCh[--oldEndIdx]
      newStartVnode = newCh[++newStartIdx]
    } else {
      /*
        生成一個key與舊VNode的key對應(yīng)的哈希表（只有第一次進來undefined的時候會生成，也為后面檢測重復(fù)的key值做鋪墊）
        比如childre是這樣的 [{xx: xx, key: 'key0'}, {xx: xx, key: 'key1'}, {xx: xx, key: 'key2'}]  beginIdx = 0   endIdx = 2  
        結(jié)果生成{key0: 0, key1: 1, key2: 2}
      */
      if (isUndef(oldKeyToIdx)) oldKeyToIdx = createKeyToOldIdx(oldCh, oldStartIdx, oldEndIdx)

      /*如果newStartVnode新的VNode節(jié)點存在key并且這個key在oldVnode中能找到則返回這個節(jié)點的idxInOld（即第幾個節(jié)點，下標(biāo)）*/
      idxInOld = isDef(newStartVnode.key)
        ? oldKeyToIdx[newStartVnode.key]
        : findIdxInOld(newStartVnode, oldCh, oldStartIdx, oldEndIdx)
      if (isUndef(idxInOld)) { // New element
        /*newStartVnode沒有key或者是該key沒有在老節(jié)點中找到則創(chuàng)建一個新的節(jié)點*/
        createElm(newStartVnode, insertedVnodeQueue, parentElm, oldStartVnode.elm, false, newCh, newStartIdx)
      } else {
        /*獲取同key的老節(jié)點*/
        vnodeToMove = oldCh[idxInOld]
        if (sameVnode(vnodeToMove, newStartVnode)) {
          /*如果新VNode與得到的有相同key的節(jié)點是同一個VNode則進行patchVnode*/
          patchVnode(vnodeToMove, newStartVnode, insertedVnodeQueue, newCh, newStartIdx)
          /*因為已經(jīng)patchVnode進去了，所以將這個老節(jié)點賦值undefined，之后如果還有新節(jié)點與該節(jié)點key相同可以檢測出來提示已有重復(fù)的key*/
          oldCh[idxInOld] = undefined
          /*當(dāng)有標(biāo)識位canMove實可以直接插入oldStartVnode對應(yīng)的真實Dom節(jié)點前面*/
          canMove && nodeOps.insertBefore(parentElm, vnodeToMove.elm, oldStartVnode.elm)
        } else {
          // same key but different element. treat as new element
          /*當(dāng)新的VNode與找到的同樣key的VNode不是sameVNode的時候（比如說tag不一樣或者是有不一樣type的input標(biāo)簽），創(chuàng)建一個新的節(jié)點*/
          createElm(newStartVnode, insertedVnodeQueue, parentElm, oldStartVnode.elm, false, newCh, newStartIdx)
        }
      }
      newStartVnode = newCh[++newStartIdx]
    }
  }
  if (oldStartIdx > oldEndIdx) {
    /*全部比較完成以后，發(fā)現(xiàn)oldStartIdx > oldEndIdx的話，說明老節(jié)點已經(jīng)遍歷完了，新節(jié)點比老節(jié)點多，所以這時候多出來的新節(jié)點需要一個一個創(chuàng)建出來加入到真實Dom中*/
    refElm = isUndef(newCh[newEndIdx + 1]) ? null : newCh[newEndIdx + 1].elm
    addVnodes(parentElm, refElm, newCh, newStartIdx, newEndIdx, insertedVnodeQueue)
  } else if (newStartIdx > newEndIdx) {
    /*如果全部比較完成以后發(fā)現(xiàn)newStartIdx > newEndIdx，則說明新節(jié)點已經(jīng)遍歷完了，老節(jié)點多余新節(jié)點，這個時候需要將多余的老節(jié)點從真實Dom中移除*/
    removeVnodes(oldCh, oldStartIdx, oldEndIdx)
  }
}

  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)
    canMove && nodeOps.insertBefore(parentElm, oldEndVnode.elm, oldStartVnode.elm)
    oldEndVnode = oldCh[--oldEndIdx]
    newStartVnode = newCh[++newStartIdx]
  }

if (isUndef(oldKeyToIdx)) oldKeyToIdx = createKeyToOldIdx(oldCh, oldStartIdx, oldEndIdx)
function createKeyToOldIdx (children, beginIdx, endIdx) {
  let i, key
  const map = {}
  for (i = beginIdx; i <= endIdx; ++i) {
    key = children[i].key
    if (isDef(key)) map[key] = i
  }
  return map
}

  idxInOld = isDef(newStartVnode.key) ? oldKeyToIdx[newStartVnode.key] : null

  if (isUndef(idxInOld)) { // New element
    createElm(newStartVnode, insertedVnodeQueue, parentElm, oldStartVnode.elm)
    newStartVnode = newCh[++newStartIdx]
  } 

  elmToMove = oldCh[idxInOld]
  if (sameVnode(elmToMove, newStartVnode)) {
    patchVnode(elmToMove, newStartVnode, insertedVnodeQueue)
    oldCh[idxInOld] = undefined
    canMove && nodeOps.insertBefore(parentElm, newStartVnode.elm, oldStartVnode.elm)
    newStartVnode = newCh[++newStartIdx]
  } else {
    // same key but different element. treat as new element
    createElm(newStartVnode, insertedVnodeQueue, parentElm, oldStartVnode.elm)
    newStartVnode = newCh[++newStartIdx]
  }