1.diff算法是什么？
diff算法是一种通过**同层的树节点**进行比较的高效算法
Diff 算法探讨的就是虚拟 DOM 树发生变化后，生成 DOM 树更新补丁的方式。对比新旧两株虚拟 DOM 树的变更差异，将更新补丁作用于真实 DOM，以最小成本完成视图更新。
1.1特点
策略：深度优先，同层比较
1.2原理分析
updateChildren
updateChildren使用同级比对，同级比对图示
使用首尾指针法：
-
依次对比，比较成功后退出当前比较
-
渲染结构以newvnode为准
-
每次比较成功后，start和end向中间靠拢
-
当新旧节点中有一个start跑到end的右侧时，终止比较
-
如果都匹配不到，则旧的虚拟dom key只去对比新的虚拟dom key值，如果key相同，则复用
好，了解了这些规则，让我们利用一个案例看看 ，源码会如何执行吧。
简单贴一下源码吧，以下是更新子节点 diff核心算法，有一些我自己写的注释和问题
function updateChildren(
parentElm,
oldCh,
newCh,
insertedVnodeQueue,
removeOnly
) {
//节点的序号
let oldStartIdx = 0
let oldEndIdx = oldCh.length - 1
let newStartIdx = 0
let newEndIdx = newCh.length - 1
//节点的value
let oldStartVnode = oldCh[0]
let oldEndVnode = oldCh[oldEndIdx]
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 (__DEV__) {
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 {
//todo oldKeyToIdx这是啥？
if (isUndef(oldKeyToIdx))
//这是取得什么？{key:index}
oldKeyToIdx = createKeyToOldIdx(oldCh, oldStartIdx, oldEndIdx)
//isDef：不是undefind or null
/***
* 如果有key 获取key对应的index,
* 如果没有key，则
*/
idxInOld = isDef(newStartVnode.key)
? oldKeyToIdx[newStartVnode.key]
: findIdxInOld(newStartVnode, oldCh, oldStartIdx, oldEndIdx)
if (isUndef(idxInOld)) {//如果 oldCh里面没有此节点
// New element
createElm(
newStartVnode,
insertedVnodeQueue,
parentElm,
oldStartVnode.elm,
false,
newCh,
newStartIdx
)
} else {//如果 oldCh有此节点
//那就拿到值
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)
}
}
function checkDuplicateKeys(children) {
const seenKeys = {}
for (let i = 0; i < children.length; i++) {
const vnode = children[i]
const key = vnode.key
if (isDef(key)) {
if (seenKeys[key]) {
warn(
`Duplicate keys detected: '${key}'. This may cause an update error.`,
vnode.context
)
} else {
seenKeys[key] = true
}
}
}
}
//从oldCh找到与node相同的值，返回old对应的i
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
}
}
function patchVnode(
oldVnode,
vnode,
insertedVnodeQueue,
ownerArray,
index,
removeOnly?: any
) {
if (oldVnode === vnode) {
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)
}
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 (__DEV__) {
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)
}
}