【校招VIP】vue核心面试题：diff算法- 校招VIP

转载声明：文章来源：https://blog.csdn.net/qq_42072086/article/details/107965196

一、diff算法的时间复杂度

两个树的完全的 diff 算法是一个时间复杂度为 O(n3) , Vue 进行了优化·O(n3) 复杂度的问题转换成 O(n) 复杂度的问题(只比较同级不考虑跨级问题)，因为在前端操作dom的时候了，不会把当前元素作为上一级元素或下一级元素，很少会跨越层级地移动Dom元素，常见的都是同级的比较。所以 Virtual Dom只会对同一个层级的元素进行对比。

二、vue中diff算法的原理

在数据发生变化，vue是先根据真实DOM生成一颗 virtual DOM ，当 virtual DOM 某个节点的数据改变后会生成一个新的 Vnode ，然后 Vnode 和 oldVnode 作对比，发现有不一样的地方就直接修改在真实的DOM上，然后使 oldVnode 的值为 Vnode ，来实现更新节点。

1.原理简述：

（1）先去同级比较，然后再去比较子节点

（2）先去判断一方有子节点一方没有子节点的情况

（3）比较都有子节点的情况

（4）递归比较子节点

2.源码：

源码文件地址：src/core/vdom/patch.js
源码总结：

在进行节点比较时，是通过patch这个方法实现，其中如果不是真实元素并且用sameVnode看两个是否是同一个元素，如果是然后会调用patchVnode进行比较，比较的是虚拟节点不是真实节点，如果不值得去比较则用 Vnode 替换 oldVnode。patchVnode会比较这两个节点，判断 Vnode 和 oldVnode 是否指向同一个对象，如果是，那么直接 return，复用老的真是元素。如果不是会用新的子节点和旧的字节做比较。如果他们都有文本节点并且不相等，那么将el的文本节点设置为Vnode的文本节点；如果两个都有子节点的情况下会调用updateChildren方法比较他们的子节点；只有新的节点有子节点而旧节点没有子节点，就会把新增的子节点插入到旧的dom中；如果当前新的节点中没有子节点，旧的中有子节点，然后就会把旧的节点删除掉；

updateChildren方法中，先定义了四对指针，先比较新的和旧的第一个子节点是否一样，如果一样会移动前面两个指针，以此类推，如果比对最后新的节点多了一个子节点就把它插入旧的中。如果开始第一个子节点不一样就从后面开始进行比较（从尾子节点开始比较），比到最后把新增的节点插入到旧的dom中。还有这种情况就是头和尾节点都不相等的情况下，用当前的头和旧的尾节点进行比较，如果一样就把旧的尾节点移到前面去，然后将旧的尾指针向前移动一位，当前头指针移动到下一个子节点上。另一种就是用旧的结尾和新的开头节点进行比较，四种比较策略。如果存在key，就会拿的当前子节点的key去旧的子节点中找，如果找到就将它移动到旧节点的前面，然后就将指针移向当前节点的第二个子节点上，以此类推，如果当前子节点没有就将它直接插入到旧的节点中，最后把旧的节点中多余出的子节点删除掉。

（1）patch

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return function patch (oldVnode, vnode, hydrating, removeOnly) {
    if (isUndef(vnode)) {
      if (isDef(oldVnode)) invokeDestroyHook(oldVnode)
      return
    }
  
    let isInitialPatch = false
    const insertedVnodeQueue = []
  
    if (isUndef(oldVnode)) {
      // empty mount (likely as component), create new root element
      isInitialPatch = true
      createElm(vnode, insertedVnodeQueue)
    } else {
      const isRealElement = isDef(oldVnode.nodeType)
      // 如果不是真实元素并且两个是同一个元素的话，然后会调用patchVnode进行比较
      // 比较是虚拟节点不是真实节点
      if (!isRealElement && sameVnode(oldVnode, vnode)) {
        // patch existing root node
        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)) {
            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)
  
        // 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)) {
          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
  }

（2）patchVnode

function patchVnode (
    oldVnode,
    vnode,
    insertedVnodeQueue,
    ownerArray,
    index,
    removeOnly
  ) {
    if (oldVnode === vnode) { // 如果相同直接return
      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.
    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)
    }
    // 进行子节点的比较
    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)) {// 如果两个都有子节点的情况
        // 比较它们的子节点
        if (oldCh !== ch) updateChildren(elm, oldCh, ch, insertedVnodeQueue, removeOnly)
      } else if (isDef(ch)) { // 如果只有新的有子节点
        if (process.env.NODE_ENV !== 'production') {
          checkDuplicateKeys(ch) // 就把新增的子节点插入到旧的节点中
        }
        if (isDef(oldVnode.text)) nodeOps.setTextContent(elm, '')
        addVnodes(elm, null, ch, 0, ch.length - 1, insertedVnodeQueue)
      } else if (isDef(oldCh)) { // 如果只有老的有子节点
        removeVnodes(oldCh, 0, oldCh.length - 1) // 就会把旧的节点直接删除
      } else if (isDef(oldVnode.text)) {  // 如果旧节点有子文本节点
        nodeOps.setTextContent(elm, '') // 就把elm置空
      }
    } 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)
    }
  }

（3）updateChildren（双指针）

function updateChildren (parentElm, oldCh, newCh, insertedVnodeQueue, removeOnly) {
    // 做了四对指针
    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 {
        // 在旧的节点中找索引
        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
          // 如果找不到直接进行插入
          createElm(newStartVnode, insertedVnodeQueue, parentElm, oldStartVnode.elm, false, newCh, newStartIdx)
        } else {
          vnodeToMove = oldCh[idxInOld]
          // 如果找的到就看他是否相等，如果相等就直接进行移动
          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
            // 如果索引相同，但元素不同，会重新创建一个插入到旧的节点中
            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(oldCh, oldStartIdx, oldEndIdx)
    }
  }

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