Do you understand the vue diff algorithm? Principle analysis-Vue.js-php.cn

The diff algorithm is an efficient algorithm that compares tree nodes at the same level, avoiding the need to search and traverse the tree layer by layer. So how much do you know about the diff algorithm? The following article will give you an in-depth analysis of the diff algorithm ofvue. I hope it will be helpful to you!

Do you understand the vue diff algorithm? Principle analysis

1. What is

diffThe algorithm is a comparison of tree nodes at the same level efficient algorithm. (Learning video sharing:vue video tutorial)

It has two characteristics:

Comparisons will only be performed at the same level, and will not be compared across levels
During the diff comparison process, the loop compares from both sides to the middle

diffThe algorithm is applied in many scenarios, invue, acts on the virtualdomand renders the old and newVNodenodes into the realdomNode comparison

2. Comparison method

diffThe overall strategy is: depth first, comparison at the same level

Comparisons will only be performed at the same level, and will not be compared across levels

Do you understand the vue diff algorithm? Principle analysis

## During the comparison process, the loop shrinks from both sides to the middle

Do you understand the vue diff algorithm? Principle analysis

The following is an example of

vueupdated through thediffalgorithm:

New and old

VNodenodes are as shown below:

Do you understand the vue diff algorithm? Principle analysis

After the first cycle, it is found that the old node D is the same as the new node D, and the old node D is directly reused as the first one after

diffThe real node, while the old nodeendIndexis moved to C, and the new node’sstartIndexis moved to C

Do you understand the vue diff algorithm? Principle analysis

After the second cycle, The end of the old node is the same as the beginning of the new node (both C). Similarly, the real node C created after

diffis inserted behind the D node created for the first time. At the same time, theendIndexof the old node moved to B, and thestartIndexof the new node moved to E

Do you understand the vue diff algorithm? Principle analysis

In the third cycle, E was found If it is not found, then you can only directly create a new real node E and insert it after the second created C node. At the same time, the

startIndexof the new node is moved to A. ThestartIndexandendIndexof the old node remain unchanged

Do you understand the vue diff algorithm? Principle analysis

In the fourth cycle, the beginning of the new and old nodes was found ( They are all the same as A), so

diffcreates the real node of A and inserts it behind the previously created E node. At the same time, thestartIndexof the old node is moved to B, and thestartIndexof the new node is moved to B

Do you understand the vue diff algorithm? Principle analysis

In the fifth cycle, the situation is the same The fourth loop is the same, so

diffcreates the B real node and inserts it behind the previously created A node. At the same time, thestartIndexof the old node has been moved to C, and the startIndex of the new node has been moved to F

Do you understand the vue diff algorithm? Principle analysis

startIndexof the new node is already greater thanendIndex, you need to create all the nodes betweennewStartIdxandnewEndIdx, that is, node F, directly create the real node corresponding to the F node and put it behind the B node

Do you understand the vue diff algorithm? Principle analysis

3. Principle Analysis

When the data changes, the

setmethod will callDep. notifyNotifies all subscribersWatcher, and the subscribers will callpatchto patch the realDOMand update the corresponding view

Source code location: src/core/vdom/patch.js

function patch(oldVnode, vnode, hydrating, removeOnly) { if (isUndef(vnode)) { // 没有新节点，直接执行destory钩子函数 if (isDef(oldVnode)) invokeDestroyHook(oldVnode) return } let isInitialPatch = false const insertedVnodeQueue = [] if (isUndef(oldVnode)) { isInitialPatch = true createElm(vnode, insertedVnodeQueue) // 没有旧节点，直接用新节点生成dom元素 } else { const isRealElement = isDef(oldVnode.nodeType) if (!isRealElement && sameVnode(oldVnode, vnode)) { // 判断旧节点和新节点自身一样，一致执行patchVnode patchVnode(oldVnode, vnode, insertedVnodeQueue, null, null, removeOnly) } else { // 否则直接销毁及旧节点，根据新节点生成dom元素 if (isRealElement) { 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 } } oldVnode = emptyNodeAt(oldVnode) } return vnode.elm } } }

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patchThe first two parameters of the function areoldVnodeandVnode, Representing the new node and the previous old node respectively, four main judgments are made:

没有新节点，直接触发旧节点的destory钩子
没有旧节点，说明是页面刚开始初始化的时候，此时，根本不需要比较了，直接全是新建，所以只调用createElm
旧节点和新节点自身一样，通过sameVnode判断节点是否一样，一样时，直接调用patchVnode去处理这两个节点
旧节点和新节点自身不一样，当两个节点不一样的时候，直接创建新节点，删除旧节点

下面主要讲的是patchVnode部分

function patchVnode (oldVnode, vnode, insertedVnodeQueue, removeOnly) { // 如果新旧节点一致，什么都不做 if (oldVnode === vnode) { return } // 让vnode.el引用到现在的真实dom，当el修改时，vnode.el会同步变化 const elm = vnode.elm = oldVnode.elm // 异步占位符 if (isTrue(oldVnode.isAsyncPlaceholder)) { if (isDef(vnode.asyncFactory.resolved)) { hydrate(oldVnode.elm, vnode, insertedVnodeQueue) } else { vnode.isAsyncPlaceholder = true } return } // 如果新旧都是静态节点，并且具有相同的key // 当vnode是克隆节点或是v-once指令控制的节点时，只需要把oldVnode.elm和oldVnode.child都复制到vnode上 // 也不用再有其他操作 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) } 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) } // 如果vnode不是文本节点或者注释节点 if (isUndef(vnode.text)) { // 并且都有子节点 if (isDef(oldCh) && isDef(ch)) { // 并且子节点不完全一致，则调用updateChildren if (oldCh !== ch) updateChildren(elm, oldCh, ch, insertedVnodeQueue, removeOnly) // 如果只有新的vnode有子节点 } else if (isDef(ch)) { if (isDef(oldVnode.text)) nodeOps.setTextContent(elm, '') // elm已经引用了老的dom节点，在老的dom节点上添加子节点 addVnodes(elm, null, ch, 0, ch.length - 1, insertedVnodeQueue) // 如果新vnode没有子节点，而vnode有子节点，直接删除老的oldCh } else if (isDef(oldCh)) { removeVnodes(elm, oldCh, 0, oldCh.length - 1) // 如果老节点是文本节点 } else if (isDef(oldVnode.text)) { nodeOps.setTextContent(elm, '') } // 如果新vnode和老vnode是文本节点或注释节点 // 但是vnode.text != oldVnode.text时，只需要更新vnode.elm的文本内容就可以 } 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) } }

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patchVnode主要做了几个判断：

新节点是否是文本节点，如果是，则直接更新dom的文本内容为新节点的文本内容
新节点和旧节点如果都有子节点，则处理比较更新子节点
只有新节点有子节点，旧节点没有，那么不用比较了，所有节点都是全新的，所以直接全部新建就好了，新建是指创建出所有新DOM，并且添加进父节点
只有旧节点有子节点而新节点没有，说明更新后的页面，旧节点全部都不见了，那么要做的，就是把所有的旧节点删除，也就是直接把DOM删除

子节点不完全一致，则调用updateChildren

function updateChildren (parentElm, oldCh, newCh, insertedVnodeQueue, removeOnly) { let oldStartIdx = 0 // 旧头索引 let newStartIdx = 0 // 新头索引 let oldEndIdx = oldCh.length - 1 // 旧尾索引 let newEndIdx = newCh.length - 1 // 新尾索引 let oldStartVnode = oldCh[0] // oldVnode的第一个child let oldEndVnode = oldCh[oldEndIdx] // oldVnode的最后一个child let newStartVnode = newCh[0] // newVnode的第一个child let newEndVnode = newCh[newEndIdx] // newVnode的最后一个child let oldKeyToIdx, idxInOld, vnodeToMove, refElm // removeOnly is a special flag used only by  // to ensure removed elements stay in correct relative positions // during leaving transitions const canMove = !removeOnly // 如果oldStartVnode和oldEndVnode重合，并且新的也都重合了，证明diff完了，循环结束 while (oldStartIdx <= oldEndIdx && newStartIdx <= newEndIdx) { // 如果oldVnode的第一个child不存在 if (isUndef(oldStartVnode)) { // oldStart索引右移 oldStartVnode = oldCh[++oldStartIdx] // Vnode has been moved left // 如果oldVnode的最后一个child不存在 } else if (isUndef(oldEndVnode)) { // oldEnd索引左移 oldEndVnode = oldCh[--oldEndIdx] // oldStartVnode和newStartVnode是同一个节点 } else if (sameVnode(oldStartVnode, newStartVnode)) { // patch oldStartVnode和newStartVnode， 索引左移，继续循环 patchVnode(oldStartVnode, newStartVnode, insertedVnodeQueue) oldStartVnode = oldCh[++oldStartIdx] newStartVnode = newCh[++newStartIdx] // oldEndVnode和newEndVnode是同一个节点 } else if (sameVnode(oldEndVnode, newEndVnode)) { // patch oldEndVnode和newEndVnode，索引右移，继续循环 patchVnode(oldEndVnode, newEndVnode, insertedVnodeQueue) oldEndVnode = oldCh[--oldEndIdx] newEndVnode = newCh[--newEndIdx] // oldStartVnode和newEndVnode是同一个节点 } else if (sameVnode(oldStartVnode, newEndVnode)) { // Vnode moved right // patch oldStartVnode和newEndVnode patchVnode(oldStartVnode, newEndVnode, insertedVnodeQueue) // 如果removeOnly是false，则将oldStartVnode.eml移动到oldEndVnode.elm之后 canMove && nodeOps.insertBefore(parentElm, oldStartVnode.elm, nodeOps.nextSibling(oldEndVnode.elm)) // oldStart索引右移，newEnd索引左移 oldStartVnode = oldCh[++oldStartIdx] newEndVnode = newCh[--newEndIdx] // 如果oldEndVnode和newStartVnode是同一个节点 } else if (sameVnode(oldEndVnode, newStartVnode)) { // Vnode moved left // patch oldEndVnode和newStartVnode patchVnode(oldEndVnode, newStartVnode, insertedVnodeQueue) // 如果removeOnly是false，则将oldEndVnode.elm移动到oldStartVnode.elm之前 canMove && nodeOps.insertBefore(parentElm, oldEndVnode.elm, oldStartVnode.elm) // oldEnd索引左移，newStart索引右移 oldEndVnode = oldCh[--oldEndIdx] newStartVnode = newCh[++newStartIdx] // 如果都不匹配 } else { if (isUndef(oldKeyToIdx)) oldKeyToIdx = createKeyToOldIdx(oldCh, oldStartIdx, oldEndIdx) // 尝试在oldChildren中寻找和newStartVnode的具有相同的key的Vnode idxInOld = isDef(newStartVnode.key) ? oldKeyToIdx[newStartVnode.key] : findIdxInOld(newStartVnode, oldCh, oldStartIdx, oldEndIdx) // 如果未找到，说明newStartVnode是一个新的节点 if (isUndef(idxInOld)) { // New element // 创建一个新Vnode createElm(newStartVnode, insertedVnodeQueue, parentElm, oldStartVnode.elm) // 如果找到了和newStartVnodej具有相同的key的Vnode，叫vnodeToMove } else { vnodeToMove = oldCh[idxInOld] /* istanbul ignore if */ if (process.env.NODE_ENV !== 'production' && !vnodeToMove) { warn( 'It seems there are duplicate keys that is causing an update error. ' + 'Make sure each v-for item has a unique key.' ) } // 比较两个具有相同的key的新节点是否是同一个节点 //不设key，newCh和oldCh只会进行头尾两端的相互比较，设key后，除了头尾两端的比较外，还会从用key生成的对象oldKeyToIdx中查找匹配的节点，所以为节点设置key可以更高效的利用dom。 if (sameVnode(vnodeToMove, newStartVnode)) { // patch vnodeToMove和newStartVnode patchVnode(vnodeToMove, newStartVnode, insertedVnodeQueue) // 清除 oldCh[idxInOld] = undefined // 如果removeOnly是false，则将找到的和newStartVnodej具有相同的key的Vnode，叫vnodeToMove.elm // 移动到oldStartVnode.elm之前 canMove && nodeOps.insertBefore(parentElm, vnodeToMove.elm, oldStartVnode.elm) // 如果key相同，但是节点不相同，则创建一个新的节点 } else { // same key but different element. treat as new element createElm(newStartVnode, insertedVnodeQueue, parentElm, oldStartVnode.elm) } } // 右移 newStartVnode = newCh[++newStartIdx] } }

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while循环主要处理了以下五种情景：

当新老VNode节点的start相同时，直接patchVnode，同时新老VNode节点的开始索引都加 1
当新老VNode节点的end相同时，同样直接patchVnode，同时新老VNode节点的结束索引都减 1
当老VNode节点的start和新VNode节点的end相同时，这时候在patchVnode后，还需要将当前真实dom节点移动到oldEndVnode的后面，同时老VNode节点开始索引加 1，新VNode节点的结束索引减 1
当老VNode节点的end和新VNode节点的start相同时，这时候在patchVnode后，还需要将当前真实dom节点移动到oldStartVnode的前面，同时老VNode节点结束索引减 1，新VNode节点的开始索引加 1
如果都不满足以上四种情形，那说明没有相同的节点可以复用，则会分为以下两种情况：
- 从旧的VNode为key值，对应index序列为value值的哈希表中找到与newStartVnode一致key的旧的VNode节点，再进行patchVnode，同时将这个真实dom移动到oldStartVnode对应的真实dom的前面
- 调用createElm创建一个新的dom节点放到当前newStartIdx的位置

小结

当数据发生改变时，订阅者watcher就会调用patch给真实的DOM打补丁
通过isSameVnode进行判断，相同则调用patchVnode方法
patchVnode做了以下操作：
- 找到对应的真实dom，称为el
- 如果都有都有文本节点且不相等，将el文本节点设置为Vnode的文本节点
- 如果oldVnode有子节点而VNode没有，则删除el子节点
- 如果oldVnode没有子节点而VNode有，则将VNode的子节点真实化后添加到el
- 如果两者都有子节点，则执行updateChildren函数比较子节点
updateChildren主要做了以下操作：
- 设置新旧VNode的头尾指针
- 新旧头尾指针进行比较，循环向中间靠拢，根据情况调用patchVnode进行patch重复流程、调用createElem创建一个新节点，从哈希表寻找key一致的VNode节点再分情况操作

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