、夕月的博客

Vue-diff算法分析

2021/2/18

# Heading

源码版本vue：2.6.10

  • 广义上，diff 算法的本质是找出两个对象之间的差异，如linux的diff命令，git的diff命令，Beyond Compare比较等
  • patch即为打补丁，将虚拟dom节点Vnode的变化体现在真实dom中
  • vue中一边diff一边patch，比diff结束再patch少一次树的遍历
  • vue-diff算法的核心是子节点数组对比（updateChildren）,思路是通过 首尾两端交叉对比
  • key主要用于节点复用，在patch过程中可以替代遍历，提升性能（updateChildren - createKeyToOldIdx）

# patch算法

patch.js 暴露一个createPatchFunction方法，返回patch函数

export function createPatchFunction (backend) {
  //省略若干……
  return function patch (oldVnode, vnode, hydrating, removeOnly) {
    //如果新节点不存在，旧节点存在，销毁旧节点
    if (isUndef(vnode)) {
      if (isDef(oldVnode)) invokeDestroyHook(oldVnode)
      return
    }

    let isInitialPatch = false
    const insertedVnodeQueue = []

    //新节点存在
    if (isUndef(oldVnode)) {
      // 如果旧节点不存在，创建节点
      // empty mount (likely as component), create new root element
      isInitialPatch = true
      createElm(vnode, insertedVnodeQueue)
    } else {//旧节点存在
      const isRealElement = isDef(oldVnode.nodeType)
      //节点类型存在，并且一致，进行patchVnode
      if (!isRealElement && sameVnode(oldVnode, vnode)) {
        // patch existing root node
        patchVnode(oldVnode, vnode, insertedVnodeQueue, null, null, removeOnly)
      } else {
        //节点类型是Element，且服务端渲染，移除服务端标志
        //进行hydrate（根据vNode初始化组件、创建节点、孩子节点等）
        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)) {
            oldVnode.removeAttribute(SSR_ATTR)
            hydrating = true
          }
          if (isTrue(hydrating)) {
            if (hydrate(oldVnode, vnode, insertedVnodeQueue)) {
              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
          oldVnode = emptyNodeAt(oldVnode)
        }

        // replacing existing element
        const oldElm = oldVnode.elm
        const parentElm = nodeOps.parentNode(oldElm)

        //创建组件、dom节点，设置scope，创建孩子节点
        // 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)
        )

        // 递归更新父节点占位元素（回调、ref等）
        // update parent placeholder node element, recursively
        if (isDef(vnode.parent)) {
          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) {
              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)) {
          removeVnodes([oldVnode], 0, 0)
        } else if (isDef(oldVnode.tag)) {
          invokeDestroyHook(oldVnode)
        }
      }
    }

    invokeInsertHook(vnode, insertedVnodeQueue, isInitialPatch)
    return vnode.elm
  }
}
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# 对当前节点打补丁

  function patchVnode (
    oldVnode,
    vnode,
    insertedVnodeQueue,
    ownerArray,
    index,
    removeOnly
  ) {
    //引用一致，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)) {//异步占位符
      if (isDef(vnode.asyncFactory.resolved)) {//异步组件工厂函数
        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.
    if (isTrue(vnode.isStatic) &&
      isTrue(oldVnode.isStatic) &&
      vnode.key === oldVnode.key &&
      (isTrue(vnode.isCloned) || isTrue(vnode.isOnce))
    ) {
      // 只对克隆过的节点进行元素复用，否则要重新触发render
      vnode.componentInstance = oldVnode.componentInstance
      return
    }

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

    const oldCh = oldVnode.children
    const ch = vnode.children
    //触发update钩子
    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)) {
        //新旧节点都有孩子节点，进行 **子节点的patch更新**
        //核心updateChildren
        if (oldCh !== ch) updateChildren(elm, oldCh, ch, insertedVnodeQueue, removeOnly)
      } else if (isDef(ch)) {
        //旧节点没孩子，新节点有孩子，循环createElm
        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)) {
        //旧节点有孩子，新节点没有孩子，删除孩子节点
        removeVnodes(oldCh, 0, oldCh.length - 1)
      } else if (isDef(oldVnode.text)) {
        //新节点无text，旧节点有text，置为空
        nodeOps.setTextContent(elm, '')
      }
    } else if (oldVnode.text !== vnode.text) {
      //新节点的text与旧节点不一致，更新
      nodeOps.setTextContent(elm, vnode.text)
    }
    //触发postpatch钩子
    if (isDef(data)) {
      if (isDef(i = data.hook) && isDef(i = i.postpatch)) i(oldVnode, vnode)
    }
  }
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# patch更新孩子节点

  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)
    }
    //新旧节点数组首位各一个指针，一共4个
    while (oldStartIdx <= oldEndIdx && newStartIdx <= newEndIdx) {
      if (isUndef(oldStartVnode)) {
        //旧首节点不存在，右移
        oldStartVnode = oldCh[++oldStartIdx] // Vnode has been moved left
      } else if (isUndef(oldEndVnode)) {
        //旧尾节点不存在，左移
        oldEndVnode = oldCh[--oldEndIdx]
      // sameVnode key相同，tag相同或者组件工厂函数相同。input类型必须type相同
      } else if (sameVnode(oldStartVnode, newStartVnode)) {
        //比较新旧首节点，然后patchVnode
        patchVnode(oldStartVnode, newStartVnode, insertedVnodeQueue, newCh, newStartIdx)
        oldStartVnode = oldCh[++oldStartIdx]
        newStartVnode = newCh[++newStartIdx]
      } else if (sameVnode(oldEndVnode, newEndVnode)) {
        //比较新旧尾节点，然后patchVnode
        patchVnode(oldEndVnode, newEndVnode, insertedVnodeQueue, newCh, newEndIdx)
        oldEndVnode = oldCh[--oldEndIdx]
        newEndVnode = newCh[--newEndIdx]
      } else if (sameVnode(oldStartVnode, newEndVnode)) { // Vnode moved right
        //比较旧首节点和新尾结点，然后patchVnode
        patchVnode(oldStartVnode, newEndVnode, insertedVnodeQueue, newCh, newEndIdx)
        //将旧首节点插入旧尾结点的下一个节点的前面（也就是旧尾之后）
        canMove && nodeOps.insertBefore(parentElm, oldStartVnode.elm, nodeOps.nextSibling(oldEndVnode.elm))
        oldStartVnode = oldCh[++oldStartIdx]
        newEndVnode = newCh[--newEndIdx]
      } else if (sameVnode(oldEndVnode, newStartVnode)) { // Vnode moved left
        //比较旧尾节点和新首结点，然后patchVnode
        patchVnode(oldEndVnode, newStartVnode, insertedVnodeQueue, newCh, newStartIdx)
        //将旧尾节点插入旧首结点的前面
        canMove && nodeOps.insertBefore(parentElm, oldEndVnode.elm, oldStartVnode.elm)
        oldEndVnode = oldCh[--oldEndIdx]
        newStartVnode = newCh[++newStartIdx]
      } else {
        //最后再处理一个新首节点

        //oldKeyToIdx 对象类型，key=旧节点的key，value=旧节点VNode
        if (isUndef(oldKeyToIdx)) oldKeyToIdx = createKeyToOldIdx(oldCh, oldStartIdx, oldEndIdx)
        //用新首节点的key在旧节点中寻找 或者 找寻sameVnode的节点
        idxInOld = isDef(newStartVnode.key)
          ? oldKeyToIdx[newStartVnode.key]
          : findIdxInOld(newStartVnode, oldCh, oldStartIdx, oldEndIdx)
        //找不到用新首节点创建新节点
        if (isUndef(idxInOld)) { // New element
          createElm(newStartVnode, insertedVnodeQueue, parentElm, oldStartVnode.elm, false, newCh, newStartIdx)
        } else {
          vnodeToMove = oldCh[idxInOld]
          //找到节点后，通过sameVnode比较是否同类型节点，是则patchVnode
          if (sameVnode(vnodeToMove, newStartVnode)) {
            patchVnode(vnodeToMove, newStartVnode, insertedVnodeQueue, newCh, newStartIdx)
            //将该节点置空，上面的第一二个条件可以跳过此节点
            oldCh[idxInOld] = undefined
            //插入旧首节点的前面
            canMove && nodeOps.insertBefore(parentElm, vnodeToMove.elm, oldStartVnode.elm)
          } else {
            //相同的key，类型不同的节点视为新节点，直接创建
            // same key but different element. treat as new element
            createElm(newStartVnode, insertedVnodeQueue, parentElm, oldStartVnode.elm, false, newCh, newStartIdx)
          }
        }
        //新首节点处理结束，指针后移
        newStartVnode = newCh[++newStartIdx]
      }
    }
    //旧首大于旧尾，说明新节点首尾未相遇，创建剩余的节点（newStartIdx, newEndIdx）
    if (oldStartIdx > oldEndIdx) {
      refElm = isUndef(newCh[newEndIdx + 1]) ? null : newCh[newEndIdx + 1].elm
      addVnodes(parentElm, refElm, newCh, newStartIdx, newEndIdx, insertedVnodeQueue)
    } else if (newStartIdx > newEndIdx) {
    //新首大于新尾，说明旧节点未相遇，删除剩余的节点（oldStartIdx, oldEndIdx）
      removeVnodes(oldCh, oldStartIdx, oldEndIdx)
    }
  }
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# 附录

NodeType

sameVnode

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)
      )
    )
  )
}
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Vue-响应式原理