引言
vue数据的渲染会引入视图的重新渲染。
从数据到视图的渲染流程可以移步https://www.jb51.net/article/261839.htm，那么从数据的变化到视图的变化是怎样的?
vue在数据的初始化阶段会进行响应式的处理defineReactive：
export function defineReactive (
obj: Object,
key: string,
val: any,
customSetter?: ?Function,
shallow?: boolean
) {
const dep = new Dep()
const property = Object.getOwnPropertyDescriptor(obj, key)
if (property && property.configurable === false) {
return
}
// cater for pre-defined getter/setters
const getter = property && property.get
const setter = property && property.set
if ((!getter || setter) && arguments.length === 2) {
val = obj[key]
}
let childOb = !shallow && observe(val)
Object.defineProperty(obj, key, {
enumerable: true,
configurable: true,
get: function reactiveGetter () {
const value = getter ? getter.call(obj) : val
if (Dep.target) {
dep.depend()
if (childOb) {
childOb.dep.depend()
if (Array.isArray(value)) {
dependArray(value)
}
}
}
return value
},
set: function reactiveSetter (newVal) {
const value = getter ? getter.call(obj) : val
if (newVal === value || (newVal !== newVal && value !== value)) {
return
}
if (process.env.NODE_ENV !== 'production' && customSetter) {
customSetter()
}
// #7981: for accessor properties without setter
if (getter && !setter) return
if (setter) {
setter.call(obj, newVal)
} else {
val = newVal
}
childOb = !shallow && observe(newVal)
dep.notify()
}
})
}
数据的变化会触发set方法，会让发布者dep执行 dep.notify，当vue所有的同步执行完后，在异步队列中按次序执行到vm的渲染流程，订阅者接收到发布者的通知后会执行到this.get()，指的是
updateComponent = () => {
vm._update(vm._render(), hydrating)
}
vm._render()获取到vNode后，会执行vm._update视图的渲染:
Vue.prototype._update = function (vnode: VNode, hydrating?: boolean) {
// ...
const prevVnode = vm._vnode
// ...
if (!prevVnode) {
// initial render
vm.$el = vm.__patch__(vm.$el, vnode, hydrating, false )
} else {
// updates
vm.$el = vm.__patch__(prevVnode, vnode)
}
// ...
}
主要区别在于数据变化引起的视图变化有prevVnode，vm.__patch__(prevVnode, vnode)之后会执行到patch方法：
function patch (oldVnode, vnode, hydrating, removeOnly) {
// ...
if (isUndef(oldVnode)) {
// empty mount (likely as component), create new root element
isInitialPatch = true
createElm(vnode, insertedVnodeQueue)
} else {
const isRealElement = isDef(oldVnode.nodeType)
if (!isRealElement && sameVnode(oldVnode, vnode)) {
// patch existing root node
patchVnode(oldVnode, vnode, insertedVnodeQueue, null, null, removeOnly)
} else {
// ...
// 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)
)
// ...
// destroy old node
if (isDef(parentElm)) {
removeVnodes([oldVnode], 0, 0)
} else if (isDef(oldVnode.tag)) {
invokeDestroyHook(oldVnode)
}
}
}
invokeInsertHook(vnode, insertedVnodeQueue, isInitialPatch)
return vnode.elm
}
在数据变化引起的patch过程中isRealElement显然为false，新旧节点是否相同的另一个判断条件是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)
)
)
)
}
如果sameVnode(oldVnode, vnode)为false，则执行createElm以及后续流程，该流程可以参考模板渲染的流程(请移步https://www.jb51.net/article/261850.htm )。
sameVnode(oldVnode, vnode)为true的时候，执行到patchVnode(oldVnode, vnode, insertedVnodeQueue, null, null, removeOnly)：
function patchVnode (
oldVnode,
vnode,
insertedVnodeQueue,
ownerArray,
index,
removeOnly
) {
// ...
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 (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)) {
removeVnodes(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)
}
}
ch = vnode.children和oldCh = oldVnode.children分别获取到新旧vnode的子元素，ch和oldCh都存在时会执行到updateChildren(elm, oldCh, ch, insertedVnodeQueue, removeOnly)：
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)
}
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 {
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, newCh, newStartIdx)
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(oldCh, oldStartIdx, oldEndIdx)
}
}
这里定义了四个索引oldStartIdx、newStartIdx、oldEndIdx和newEndIdx，也可以称之为指针，通过while循环，进行四个指针的移动：
1、isUndef(oldStartVnode)
如果oldStartVnode不存在，执行oldStartVnode = oldCh[++oldStartIdx]，将oldStartIdx指针向右移动一位，进行下次循环。
2、isUndef(oldEndVnode)
如果oldEndVnode不存在，执行oldEndVnode = oldCh[--oldEndIdx]，将oldEndIdx指针向左移动一位，进行下次循环。
3、sameVnode(oldStartVnode, newStartVnode)
如果满足sameVnode(oldStartVnode, newStartVnode)，执行patchVnode(oldStartVnode, newStartVnode, insertedVnodeQueue, newCh, newStartIdx)开始递归执行，结束后oldStartIdx和newStartIdx分别向右移动一位。
4、sameVnode(oldEndVnode, newEndVnode)
如果满足sameVnode(oldEndVnode, newEndVnode)，执行patchVnode(oldEndVnode, newEndVnode, insertedVnodeQueue, newCh, newStartIdx)开始递归执行，结束后oldEndIdx和newEndIdx分别向左移动一位。
5、sameVnode(oldStartVnode, newEndVnode)
如果满足sameVnode(oldStartVnode, newEndVnode)，执行patchVnode(oldStartVnode, newEndVnode, insertedVnodeQueue, newCh, newStartIdx)开始递归执行，结束后oldStartVnode向右移动一位，newEndIdx向左移动一位。
并且通过nodeOps.insertBefore(parentElm, oldStartVnode.elm, nodeOps.nextSibling(oldEndVnode.elm))的方式将oldStartVnode.elm插入到oldEndVnode.elm节点之后。
6、sameVnode(oldEndVnode, newStartVnode)
如果满足sameVnode(oldEndVnode, newStartVnode)，执行patchVnode(oldEndVnode, newStartVnode, insertedVnodeQueue, newCh, newStartIdx)开始递归执行，结束后newStartIdx向右移动一位，oldEndIdx向左移动一位。
并且通过nodeOps.insertBefore(parentElm, oldEndVnode.elm, oldStartVnode.elm)的方式将 oldEndVnode.elm插入到oldStartVnode.elm节点之前。
7、如果以上都不满足
如果新旧vNode首首、首尾、尾首和尾尾对比都没找到相同的，则在旧vNode的oldStartIdx和oldEndIdx之间去找。 oldKeyToIdx = createKeyToOldIdx(oldCh, oldStartIdx, oldEndIdx)创建以旧vNode的key为key值，位置索引为value的map映射:
function createKeyToOldIdx (children, beginIdx, endIdx) {
let i, key
const map = {}
for (i = beginIdx; i <= endIdx; ++i) {
key = children[i].key
if (isDef(key)) map[key] = i
}
return map
}
如果通过createKeyToOldIdx找不到，则通过findIdxInOld(newStartVnode, oldCh, oldStartIdx, oldEndIdx)和旧vNode的方式去进行比对，并返回位置索引：
function findIdxInOld (node, oldCh, start, end) {
for (let i = start; i < end; i++) {
const c = oldCh[i]
if (isDef(c) && sameVnode(node, c)) return i
}
}
通过oldKeyToIdx[newStartVnode.key]和findIdxInOld (node, oldCh, start, end)的查询会有两种结果：
1、没找到如果没有找到，则以newStartVnode为渲染vNode通过createElm去进行节点的创建。
2、找到了如果找到了，通过vnodeToMove = oldCh[idxInOld]获取到介于oldStartIdx和oldEndIdx之间的可以比对的vnode, 执行完patchVnode(vnodeToMove, newStartVnode, insertedVnodeQueue, newCh, newStartIdx)后将当前位置的oldCh[idxInOld] = undefined。
通过nodeOps.insertBefore(parentElm, vnodeToMove.elm, oldStartVnode.elm)将vnodeToMove.elm移动到oldStartVnode.elm之前。
小结
diff算法从两端进行比对，找不到再从中间寻找，是一种 “滑动窗口” 算法的使用，以达到通过节点移动来实现原地复用的目的。
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