這次帶給大家怎麼使用vue中diff演算法，使用vue中diff演算法的注意事項有哪些，以下就是實戰案例，一起來看一下。
虛擬dom
diff演算法首先要明確一個概念就是diff的物件是虛擬dom，更新真實dom則是diff演算法的結果
Vnode基底類別
constructor (
。。。
) {
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
}這個部分的程式碼主要是為了更好地知道在diff演算法中具體diff的屬性的意義，當然也可以更好地了解vnode實例
整體過程
核心函數是patch函數
#isUndef判斷（是不是undefined或null）
#// empty mount (likely as component), create new root elementcreateElm(vnode, insertedVnodeQueue) 這裡可以發現創建節點不是一個一個插入，而是放入一個隊列中統一批次
#核心函數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)
)
)
)
}這裡是一個外層的比較函數，直接去比較了兩個節點的key，tag（標籤）,data的比較（注意這裡的data指的是VNodeData），input的話直接比較type。
export interface VNodeData {
key?: string | number;
slot?: string;
scopedSlots?: { [key: string]: ScopedSlot };
ref?: string;
tag?: string;
staticClass?: string;
class?: any;
staticStyle?: { [key: string]: any };
style?: object[] | object;
props?: { [key: string]: any };
attrs?: { [key: string]: any };
domProps?: { [key: string]: any };
hook?: { [key: string]: Function };
on?: { [key: string]: Function | Function[] };
nativeOn?: { [key: string]: Function | Function[] };
transition?: object;
show?: boolean;
inlineTemplate?: {
render: Function;
staticRenderFns: Function[];
};
directives?: VNodeDirective[];
keepAlive?: boolean;
}這會確認兩個節點是否有進一步比較的價值，不然直接替換
替換的過程主要是一個createElm函數另外則是銷毀oldVNode
// destroy old node
if (isDef(parentElm)) {
removeVnodes(parentElm, [oldVnode], 0, 0)
} else if (isDef(oldVnode.tag)) {
invokeDestroyHook(oldVnode)
}插入過程簡化來說就是判斷node的type分別呼叫
createComponent（會判斷是否有children然後遞迴呼叫）
createComment
createTextNode
#建立後使用insert函數
之後需要用hydrate函數將虛擬dom和真是dom進行映射
function insert (parent, elm, ref) {
if (isDef(parent)) {
if (isDef(ref)) {
if (ref.parentNode === parent) {
nodeOps.insertBefore(parent, elm, ref)
}
} else {
nodeOps.appendChild(parent, elm)
}
}
}核心函數
function patchVnode (oldVnode, vnode, insertedVnodeQueue, removeOnly) {
if (oldVnode === vnode) {
return
}
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
}
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)
}
if (isUndef(vnode.text)) {
if (isDef(oldCh) && isDef(ch)) {
if (oldCh !== ch) updateChildren(elm, oldCh, ch, insertedVnodeQueue, removeOnly)
} else if (isDef(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)
}
}##const el = vnode.el = oldVnode.el 這是很重要的一步，讓vnode.el引用到現在的真實dom，當el修改時，vnode.el會同步變化。
這部分重點還是關注整個演算法
首先四個指針，oldStart，oldEnd，newStart，newEnd，兩個數組，oldVnode，Vnode。
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
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)
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]
} 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)
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)
}
}一個循環比較的幾個情況和處理（以下的--均指index的--）比較則是比較的node節點，簡略寫法不嚴謹比較用的是sameVnode函數也不是真的全等
整體循環不結束的條件oldStartIdx <= oldEndIdx && newStartIdx <= newEndIdx
循环结束后并没有完成
还有一段判断才算完
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)
}
简单的说就是循环结束后，看四个指针中间的内容，old数组中和new数组中，多退少补而已
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