vue的diff演算法知識點總結

NO IMAGE

原始碼:https://github.com/vuejs/vue/blob/dev/src/core/vdom/patch.js

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基類


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的屬性的含義,當然也可以更好地瞭解vnode例項

整體過程

核心函式是patch函式

isUndef判斷(是不是undefined或者null)
// empty mount (likely as component), create new root elementcreateElm(vnode, insertedVnodeQueue) 這裡可以發現建立節點不是一個一個插入,而是放入一個佇列中統一批處理

核心函式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式,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函式 另外則是銷燬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函式

之後需要用hydrate函式將虛擬dom和真是dom進行對映


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)
}
}
}

核心函式


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。

比較二者引用是否一致
之後asyncFactory不知道是做什麼的,所以這個比較看不懂
靜態節點比較key,相同後也不做重新渲染,直接拷貝componentInstance(once命令在此生效)
如果vnode是文字節點或註釋節點,但是vnode.text != oldVnode.text時,只需要更新vnode.elm的文字內容就可以
children的比較

如果只有oldVnode有子節點,那就把這些節點都刪除
如果只有vnode有子節點,那就建立這些子節點,這裡如果oldVnode是個文字節點就把vnode.elm的文字設定為空字串
都有則updateChildren,這個之後詳述
如果oldVnode和vnode都沒有子節點,但是oldVnode是文字節點或註釋節點,就把vnode.elm的文字設定為空字串

updateChildren

這部分重點還是關注整個演算法

首先四個指標,oldStart,oldEnd,newStart,newEnd,兩個陣列,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)
}
}

一個迴圈比較的幾種情況和處理(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函式也不是真的全等

整體迴圈不結束的條件oldStartIdx <= oldEndIdx && newStartIdx <= newEndIdx

oldStart === newStart,oldStart newStart
oldEnd === newEnd,oldEnd– newEnd–
oldStart === newEnd, oldStart插到隊伍末尾 oldStart newEnd–
oldEnd === newStart, oldEnd插到隊伍開頭 oldEnd– newStart
剩下的所有情況都走這個處理簡單的說也就兩種處理,處理後newStart

newStart在old中發現一樣的那麼將這個移動到oldStart前
沒有發現一樣的那麼建立一個放到oldStart之前

迴圈結束後並沒有完成

還有一段判斷才算完


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)
}

簡單的說就是迴圈結束後,看四個指標中間的內容,old陣列中和new陣列中,多退少補而已

總結

整體認識還很粗糙,不過以目前的水平和對vue的瞭解也就只能到這了

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