虚拟DOM#

Vue的虚拟DOM实质上就是生成一个AST（抽象语法树Abstract Syntax Tree）

我们可以用这一串代码来看看原生DOM有多复杂：

let div = document.createElement('div')
let str = ''
for (const key in div) {
  str += key + ''
}
console.log(str)

解决方案就是 我们可以用 JS的计算性能来换取操作 DOM所消耗的性能，既然我们逃不掉操作 DOM这道坎,但是我们可以尽可能少的操作 DOM，这时候就是Diff算法发挥的时候了

Diff算法#

Diff 算法，在 Vue 里面就是叫做 patch ，它的核心就是参考 Snabbdom，通过新旧虚拟 DOM 对比(即 patch 过程)，找出最小变化的地方转为进行 DOM 操作

在 Vue1 里是没有 patch 的，每个依赖都有单独的 Watcher 负责更新，当项目规模变大的时候性能就跟不上了，所以在 Vue2 里为了提升性能，改为每个组件只有一个Watcher，那我们需要更新的时候，怎么才能精确找到组件里发生变化的位置呢？所以 patch 它来了

比如有上图这样的 DOM 结构，是怎么计算出变化？简单说就是

• 遍历老的虚拟 DOM
• 遍历新的虚拟 DOM
• 然后根据变化，比如上面的改变和新增，再重新排序

可是这样会有很大问题，假如有1000个节点，就需要计算 1000³ 次，也就是10亿次，这样是无法让人接受的，所以 Vue 或者 React 里使用 Diff 算法的时候都遵循深度优先，同层比较的策略做了一些优化，来计算出最小变化

Diff算法怎么优化？#

1. 只比较同一层级，不跨级比较

如图，Diff 过程只会把同颜色框起来的同一层级的 DOM 进行比较，这样来简化比较次数，这是第一个方面

2. 比较标签名

如果同一层级的比较标签名不同，就直接移除老的虚拟 DOM 对应的节点，不继续按这个树状结构做深度比较，这是简化比较次数的第二个方面

3. 比较 key

如果标签名相同，key 也相同，就会认为是相同节点，也不继续按这个树状结构做深度比较，比如我们写 v-for 的时候会比较 key，不写 key 就会报错，这也就是因为 Diff 算法需要比较 key

面试中有一道特别常见的题，就是让你说一下 key 的作用，实际上考查的就是大家对虚拟 DOM 和 patch 细节的掌握程度，能够反应出我们面试者的理解层次，所以这里扩展一下 key

Key的作用#

在Vue源码的renderer.ts的1600行左右：

const patchUnkeyedChildren = (
    c1: VNode[],
    c2: VNodeArrayChildren,
    container: RendererElement,
    anchor: RendererNode | null,
    parentComponent: ComponentInternalInstance | null,
    parentSuspense: SuspenseBoundary | null,
    namespace: ElementNamespace,
    slotScopeIds: string[] | null,
    optimized: boolean,
  ) => {
    c1 = c1 || EMPTY_ARR
    c2 = c2 || EMPTY_ARR
    const oldLength = c1.length
    const newLength = c2.length
    const commonLength = Math.min(oldLength, newLength)
    let i
    for (i = 0; i < commonLength; i++) {
      const nextChild = (c2[i] = optimized
        ? cloneIfMounted(c2[i] as VNode)
        : normalizeVNode(c2[i]))
      patch(
        c1[i],
        nextChild,
        container,
        null,
        parentComponent,
        parentSuspense,
        namespace,
        slotScopeIds,
        optimized,
      )
    }
    if (oldLength > newLength) {
      // remove old
      unmountChildren(
        c1,
        parentComponent,
        parentSuspense,
        true,
        false,
        commonLength,
      )
    } else {
      // mount new
      mountChildren(
        c2,
        container,
        anchor,
        parentComponent,
        parentSuspense,
        namespace,
        slotScopeIds,
        optimized,
        commonLength,
      )
    }
  }

对于没有key的patch更新是这样的，也就是简单的去头去尾之后来更新

而对于有key的更新：

const patchKeyedChildren = (
    c1: VNode[],
    c2: VNodeArrayChildren,
    container: RendererElement,
    parentAnchor: RendererNode | null,
    parentComponent: ComponentInternalInstance | null,
    parentSuspense: SuspenseBoundary | null,
    namespace: ElementNamespace,
    slotScopeIds: string[] | null,
    optimized: boolean,
  ) => {
    let i = 0
    const l2 = c2.length
    let e1 = c1.length - 1 // prev ending index
    let e2 = l2 - 1 // next ending index

    // 1. sync from start
    // (a b) c
    // (a b) d e
    while (i <= e1 && i <= e2) {
      const n1 = c1[i]
      const n2 = (c2[i] = optimized
        ? cloneIfMounted(c2[i] as VNode)
        : normalizeVNode(c2[i]))
      if (isSameVNodeType(n1, n2)) {
        patch(
          n1,
          n2,
          container,
          null,
          parentComponent,
          parentSuspense,
          namespace,
          slotScopeIds,
          optimized,
        )
      } else {
        break
      }
      i++
    }

    // 2. sync from end
    // a (b c)
    // d e (b c)
    while (i <= e1 && i <= e2) {
      const n1 = c1[e1]
      const n2 = (c2[e2] = optimized
        ? cloneIfMounted(c2[e2] as VNode)
        : normalizeVNode(c2[e2]))
      if (isSameVNodeType(n1, n2)) {
        patch(
          n1,
          n2,
          container,
          null,
          parentComponent,
          parentSuspense,
          namespace,
          slotScopeIds,
          optimized,
        )
      } else {
        break
      }
      e1--
      e2--
    }

    // 3. common sequence + mount
    // (a b)
    // (a b) c
    // i = 2, e1 = 1, e2 = 2
    // (a b)
    // c (a b)
    // i = 0, e1 = -1, e2 = 0
    if (i > e1) {
      if (i <= e2) {
        const nextPos = e2 + 1
        const anchor = nextPos < l2 ? (c2[nextPos] as VNode).el : parentAnchor
        while (i <= e2) {
          patch(
            null,
            (c2[i] = optimized
              ? cloneIfMounted(c2[i] as VNode)
              : normalizeVNode(c2[i])),
            container,
            anchor,
            parentComponent,
            parentSuspense,
            namespace,
            slotScopeIds,
            optimized,
          )
          i++
        }
      }
    }

    // 4. common sequence + unmount
    // (a b) c
    // (a b)
    // i = 2, e1 = 2, e2 = 1
    // a (b c)
    // (b c)
    // i = 0, e1 = 0, e2 = -1
    else if (i > e2) {
      while (i <= e1) {
        unmount(c1[i], parentComponent, parentSuspense, true)
        i++
      }
    }

    // 5. unknown sequence
    // [i ... e1 + 1]: a b [c d e] f g
    // [i ... e2 + 1]: a b [e d c h] f g
    // i = 2, e1 = 4, e2 = 5
    else {
      const s1 = i // prev starting index
      const s2 = i // next starting index

      // 5.1 build key:index map for newChildren
      const keyToNewIndexMap: Map<PropertyKey, number> = new Map()
      for (i = s2; i <= e2; i++) {
        const nextChild = (c2[i] = optimized
          ? cloneIfMounted(c2[i] as VNode)
          : normalizeVNode(c2[i]))
        if (nextChild.key != null) {
          if (__DEV__ && keyToNewIndexMap.has(nextChild.key)) {
            warn(
              `Duplicate keys found during update:`,
              JSON.stringify(nextChild.key),
              `Make sure keys are unique.`,
            )
          }
          keyToNewIndexMap.set(nextChild.key, i)
        }
      }

      // 5.2 loop through old children left to be patched and try to patch
      // matching nodes & remove nodes that are no longer present
      let j
      let patched = 0
      const toBePatched = e2 - s2 + 1
      let moved = false
      // used to track whether any node has moved
      let maxNewIndexSoFar = 0
      // works as Map<newIndex, oldIndex>
      // Note that oldIndex is offset by +1
      // and oldIndex = 0 is a special value indicating the new node has
      // no corresponding old node.
      // used for determining longest stable subsequence
      const newIndexToOldIndexMap = new Array(toBePatched)
      for (i = 0; i < toBePatched; i++) newIndexToOldIndexMap[i] = 0

      for (i = s1; i <= e1; i++) {
        const prevChild = c1[i]
        if (patched >= toBePatched) {
          // all new children have been patched so this can only be a removal
          unmount(prevChild, parentComponent, parentSuspense, true)
          continue
        }
        let newIndex
        if (prevChild.key != null) {
          newIndex = keyToNewIndexMap.get(prevChild.key)
        } else {
          // key-less node, try to locate a key-less node of the same type
          for (j = s2; j <= e2; j++) {
            if (
              newIndexToOldIndexMap[j - s2] === 0 &&
              isSameVNodeType(prevChild, c2[j] as VNode)
            ) {
              newIndex = j
              break
            }
          }
        }
        if (newIndex === undefined) {
          unmount(prevChild, parentComponent, parentSuspense, true)
        } else {
          newIndexToOldIndexMap[newIndex - s2] = i + 1
          if (newIndex >= maxNewIndexSoFar) {
            maxNewIndexSoFar = newIndex
          } else {
            moved = true
          }
          patch(
            prevChild,
            c2[newIndex] as VNode,
            container,
            null,
            parentComponent,
            parentSuspense,
            namespace,
            slotScopeIds,
            optimized,
          )
          patched++
        }
      }

      // 5.3 move and mount
      // generate longest stable subsequence only when nodes have moved
      const increasingNewIndexSequence = moved
        ? getSequence(newIndexToOldIndexMap)
        : EMPTY_ARR
      j = increasingNewIndexSequence.length - 1
      // looping backwards so that we can use last patched node as anchor
      for (i = toBePatched - 1; i >= 0; i--) {
        const nextIndex = s2 + i
        const nextChild = c2[nextIndex] as VNode
        const anchor =
          nextIndex + 1 < l2 ? (c2[nextIndex + 1] as VNode).el : parentAnchor
        if (newIndexToOldIndexMap[i] === 0) {
          // mount new
          patch(
            null,
            nextChild,
            container,
            anchor,
            parentComponent,
            parentSuspense,
            namespace,
            slotScopeIds,
            optimized,
          )
        } else if (moved) {
          // move if:
          // There is no stable subsequence (e.g. a reverse)
          // OR current node is not among the stable sequence
          if (j < 0 || i !== increasingNewIndexSequence[j]) {
            move(nextChild, container, anchor, MoveType.REORDER)
          } else {
            j--
          }
        }
      }
    }
  }

首先经过去头去尾留下中间部分，然后找出最长递增子序列，在此序列基础上进行增删改来更新，节省了大量性能

鱿鱼须在这里也留下了他的源码：

// https://en.wikipedia.org/wiki/Longest_increasing_subsequence
function getSequence(arr: number[]): number[] {
  const p = arr.slice()
  const result = [0]
  let i, j, u, v, c
  const len = arr.length
  for (i = 0; i < len; i++) {
    const arrI = arr[i]
    if (arrI !== 0) {
      j = result[result.length - 1]
      if (arr[j] < arrI) {
        p[i] = j
        result.push(i)
        continue
      }
      u = 0
      v = result.length - 1
      while (u < v) {
        c = (u + v) >> 1
        if (arr[result[c]] < arrI) {
          u = c + 1
        } else {
          v = c
        }
      }
      if (arrI < arr[result[u]]) {
        if (u > 0) {
          p[i] = result[u - 1]
        }
        result[u] = i
      }
    }
  }
  u = result.length
  v = result[u - 1]
  while (u-- > 0) {
    result[u] = v
    v = p[v]
  }
  return result
}

看到这里不禁惊叹，大佬就是大佬，我们平时刷的累死的leetcode题目，大佬信手拈来搞优化，真是隔行如隔山。