vue的Vnode 和 patch机制

真实DOM

浏览器的DOM设计的非常复杂，所以它的创建和修改性能开销也会很大。

下面我们创建一个div，来感受下：

"aligntitlelangtranslatedirdatasethiddentabIndexaccessKeydraggablespellcheckautocapitalizecontentEditableisContentEditableinputModeoffsetParentoffsetTopoffsetLeftoffsetWidthoffsetHeightstyleinnerTextouterTextoncopyoncutonpasteonabortonbluroncanceloncanplayoncanplaythroughonchangeonclickoncloseoncontextmenuoncuechangeondblclickondragondragendondragenterondragleaveondragoverondragstartondropondurationchangeonemptiedonendedonerroronfocusoninputoninvalidonkeydownonkeypressonkeyuponloadonloadeddataonloadedmetadataonloadstartonmousedownonmouseenteronmouseleaveonmousemoveonmouseoutonmouseoveronmouseuponmousewheelonpauseonplayonplayingonprogressonratechangeonresetonresizeonscrollonseekedonseekingonselectonstalledonsubmitonsuspendontimeupdateontoggleonvolumechangeonwaitingonwheelonauxclickongotpointercaptureonlostpointercaptureonpointerdownonpointermoveonpointeruponpointercancelonpointeroveronpointeroutonpointerenteronpointerleaveonselectstartonselectionchangenonceclickfocusblurnamespaceURIprefixlocalNametagNameidclassNameclassListslotattributesshadowRootassignedSlotinnerHTMLouterHTMLscrollTopscrollLeftscrollWidthscrollHeightclientTopclientLeftclientWidthclientHeightattributeStyleMaponbeforecopyonbeforecutonbeforepasteonsearchpreviousElementSiblingnextElementSiblingchildrenfirstElementChildlastElementChildchildElementCountonwebkitfullscreenchangeonwebkitfullscreenerrorsetPointerCapturereleasePointerCapturehasPointerCapturehasAttributesgetAttributeNamesgetAttributegetAttributeNSsetAttributesetAttributeNSremoveAttributeremoveAttributeNShasAttributehasAttributeNStoggleAttributegetAttributeNodegetAttributeNodeNSsetAttributeNodesetAttributeNodeNSremoveAttributeNodeclosestmatcheswebkitMatchesSelectorattachShadowgetElementsByTagNamegetElementsByTagNameNSgetElementsByClassNameinsertAdjacentElementinsertAdjacentTextinsertAdjacentHTMLrequestPointerLockgetClientRectsgetBoundingClientRectscrollIntoViewscrollIntoViewIfNeededanimatecomputedStyleMapbeforeafterreplaceWithremoveprependappendquerySelectorquerySelectorAllwebkitRequestFullScreenwebkitRequestFullscreenonfullscreenchangeonfullscreenerrorscrollscrollToscrollBycreateShadowRootgetDestinationInsertionPointsrequestFullscreenELEMENT_NODEATTRIBUTE_NODETEXT_NODECDATA_SECTION_NODEENTITY_REFERENCE_NODEENTITY_NODEPROCESSING_INSTRUCTION_NODECOMMENT_NODEDOCUMENT_NODEDOCUMENT_TYPE_NODEDOCUMENT_FRAGMENT_NODENOTATION_NODEDOCUMENT_POSITION_DISCONNECTEDDOCUMENT_POSITION_PRECEDINGDOCUMENT_POSITION_FOLLOWINGDOCUMENT_POSITION_CONTAINSDOCUMENT_POSITION_CONTAINED_BYDOCUMENT_POSITION_IMPLEMENTATION_SPECIFICnodeTypenodeNamebaseURIisConnectedownerDocumentparentNodeparentElementchildNodesfirstChildlastChildpreviousSiblingnextSiblingnodeValuetextContenthasChildNodesgetRootNodenormalizecloneNodeisEqualNodeisSameNodecompareDocumentPositioncontainslookupPrefixlookupNamespaceURIisDefaultNamespaceinsertBeforeappendChildreplaceChildremoveChildaddEventListenerremoveEventListenerdispatchEvent"

Vue虚拟DOM

Virtual DOM的概念相信大部人都不陌生，虚拟DOM就是为了解决浏览器性能问题而被设计出来的。
虚拟dom几乎是现代前端框架的标配。本文主要以Vue为例。
vue的Virtual DOM 算法基于 Snabbdom(核心内容实现实现不到三百行非常简短)。

Vue虚拟dom到底是个什么东西？

Vue中虚拟dom的类

class VNode {
  tag: string | void;
  data: VNodeData | void;
  children: ?Array<VNode>;
  text: string | void;
  elm: Node | void;
  ns: string | void;
  context: Component | void; // rendered in this component's scope
  key: string | number | void;
  componentOptions: VNodeComponentOptions | void;
  componentInstance: Component | void; // component instance
  parent: VNode | void; // component placeholder node

  // strictly internal
  raw: boolean; // contains raw HTML? (server only)
  isStatic: boolean; // hoisted static node
  isRootInsert: boolean; // necessary for enter transition check
  isComment: boolean; // empty comment placeholder?
  isCloned: boolean; // is a cloned node?
  isOnce: boolean; // is a v-once node?
  asyncFactory: Function | void; // async component factory function
  asyncMeta: Object | void;
  isAsyncPlaceholder: boolean;
  ssrContext: Object | void;
  fnContext: Component | void; // real context vm for functional nodes
  fnOptions: ?ComponentOptions; // for SSR caching
  devtoolsMeta: ?Object; // used to store functional render context for devtools
  fnScopeId: ?string; // functional scope id support

  constructor (
    tag?: string,
    data?: VNodeData,
    children?: ?Array<VNode>,
    text?: string,
    elm?: Node,
    context?: Component,
    componentOptions?: VNodeComponentOptions,
    asyncFactory?: Function
  ) {
    this.tag = tag
    this.data = data
    this.children = children
    this.text = text
    this.elm = elm
    this.ns = undefined
    this.context = context
    this.fnContext = undefined
    this.fnOptions = undefined
    this.fnScopeId = undefined
    this.key = data && data.key
    this.componentOptions = componentOptions
    this.componentInstance = undefined
    this.parent = undefined
    this.raw = false
    this.isStatic = false
    this.isRootInsert = true
    this.isComment = false
    this.isCloned = false
    this.isOnce = false
    this.asyncFactory = asyncFactory
    this.asyncMeta = undefined
    this.isAsyncPlaceholder = false
  }

  // DEPRECATED: alias for componentInstance for backwards compat.
  /* istanbul ignore next */
  get child (): Component | void {
    return this.componentInstance
  }
}

可以看到Vnode就是简单地用javascript来表示DOM结构。

Vue的patch机制

Vue的patch实现在https://github.com/vuejs/vue/...，近800行代码。
主要分2步：

  1.patchVnode
  2.updateChildren

一些重要api

在分析patch机制之前，先了解下使用到的重要api

createElm 用来新建一个节点

function createElm (
    vnode,
    insertedVnodeQueue,
    parentElm,
    refElm,
    nested,
    ownerArray,
    index
  ) {
    if (isDef(vnode.elm) && isDef(ownerArray)) {
      // This vnode was used in a previous render!
      // now it's used as a new node, overwriting its elm would cause
      // potential patch errors down the road when it's used as an insertion
      // reference node. Instead, we clone the node on-demand before creating
      // associated DOM element for it.
      vnode = ownerArray[index] = cloneVNode(vnode)
    }

    vnode.isRootInsert = !nested // for transition enter check
    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) {
          creatingElmInVPre++
        }
        if (isUnknownElement(vnode, creatingElmInVPre)) {
          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) {
        creatingElmInVPre--
      }
    } 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)
    }
  }

addVnodes 批量新建节点

 function addVnodes (parentElm, refElm, vnodes, startIdx, endIdx, insertedVnodeQueue) {
  for (; startIdx <= endIdx; ++startIdx) {
    createElm(vnodes[startIdx], insertedVnodeQueue, parentElm, refElm, false, vnodes, startIdx)
  }
}

removeNode移除节点

function removeNode (el) {
    const parent = nodeOps.parentNode(el)
    // element may have already been removed due to v-html / v-text
    if (isDef(parent)) {
      nodeOps.removeChild(parent, el)
    }
  }

removeVnodes批量移除节点

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)
        }
      }
    }
  }

patchVnode

 function patchVnode (
    oldVnode,
    vnode,
    insertedVnodeQueue,
    ownerArray,
    index,
    removeOnly
  ) {
    if (oldVnode === vnode) {
      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 (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)) {
        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)
    }
  }

绝大部分新老节点的是相同的即oldVnode === vnode 执行的结果都是 return， 不需要更新。

if (oldVnode === vnode) {
    return;
}

当新老 VNode 节点都是 isStatic（静态的），并且 key 相同时，只要将 componentInstance 与 elm 从老 VNode 节点取过来就可以。

if (vnode.isStatic && oldVnode.isStatic && vnode.key === oldVnode.key) {
    vnode.elm = oldVnode.elm;
    vnode.componentInstance = oldVnode.componentInstance;
    return;
}

当oldCh 与 ch 都存在且不相同时，使用 updateChildren 函数来更新子节点

if (oldCh && ch && (oldCh !== ch)) {
    updateChildren(elm, oldCh, ch);
}

updateChildren

updateChildren方法

 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)) {
        oldStartVnode = oldCh[++oldStartIdx] // Vnode has been moved left
      } else if (isUndef(oldEndVnode)) {
        oldEndVnode = oldCh[--oldEndIdx]
      } else if (sameVnode(oldStartVnode, newStartVnode)) {
        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 {
        if (isUndef(oldKeyToIdx)) oldKeyToIdx = createKeyToOldIdx(oldCh, oldStartIdx, oldEndIdx)
        idxInOld = isDef(newStartVnode.key)
          ? oldKeyToIdx[newStartVnode.key]
          : findIdxInOld(newStartVnode, oldCh, oldStartIdx, oldEndIdx)
        if (isUndef(idxInOld)) { // New element
          createElm(newStartVnode, insertedVnodeQueue, parentElm, oldStartVnode.elm, false, newCh, newStartIdx)
        } else {
          vnodeToMove = oldCh[idxInOld]
          if (sameVnode(vnodeToMove, newStartVnode)) {
            patchVnode(vnodeToMove, newStartVnode, insertedVnodeQueue, newCh, newStartIdx)
            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, false, newCh, newStartIdx)
          }
        }
        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)
    }
  }

代码有点长，我们通过一个🌰来简单的了解下

<div v-if="true">
    <div>1</div>
    <div>2</div>
    <h1>3</h1>
    <div>4</div>
    <div>5</div>
</div>
<div v-else>    
    <div>1</div>
    <div>2</div>
    <div>4</div>
    <div>5</div>
</div>

1.虚拟DOM 与 真实DOM 一一对应，比较从两端开始，向中间靠拢，直到oldStartIdx > oldEndIdx 或者 newStartIdx > newEndIdx 停止。最后，相同的节点不变，不同的节点进行新增，删除，修改。

2.对指针分别指向，新旧Vnode的2端。第一次比较，第一个节点相同，这时oldStartIdx++，newStartIdx++ 指针向右移动一位。

3.然后进行第二次比较，第二个节点也相同，oldStartIdx++，newStartIdx++ 指针继续向右移动一位。

4.发现第三个节点不同，这时看看最后一个节点是否相同

5.发现最后一个节点相同，oldEndIdx--，newEndIdx-- 指针向左移动一位。

6.接着比较，发现最后一个节点相同，oldEndIdx--，newEndIdx-- 指针向左移动一位。

这时，newEndIdx > newStartIdx 比较停止。

  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)
    }

针对这个🌰的情况，通过removeVnodes执行批量移除操作。

有兴趣的同👟可以自己去github上👀Vue的源码。仔细地梳理一下patch的过程，相信一定能够理解得更加透彻的。