Vue3源码通过render patch 了解diff

const {
  insert: hostInsert,
  remove: hostRemove,
  patchProp: hostPatchProp,
  createElement: hostCreateElement,
  createText: hostCreateText,
  createComment: hostCreateComment,
  setText: hostSetText,
  setElementText: hostSetElementText,
  parentNode: hostParentNode,
  nextSibling: hostNextSibling,
  setScopeId: hostSetScopeId = NOOP,
  cloneNode: hostCloneNode,
  insertStaticContent: hostInsertStaticContent
} = options
// render
const render: RootRenderFunction = (vnode, container, isSVG) => {
  if (vnode == null) {
    if (container._vnode) {
      unmount(container._vnode, null, null, true)
    }
  } else {
    // 新旧节点的对比
    patch(container._vnode || null, vnode, container, null, null, null, isSVG)
  }
  flushPostFlushCbs()
  // 记录旧节点
  container._vnode = vnode
}

// Note: functions inside this closure should use `const xxx = () => {}`
// style in order to prevent being inlined by minifiers.
const patch: PatchFn = (
  n1,
  n2,
  container,
  anchor = null,
  parentComponent = null,
  parentSuspense = null,
  isSVG = false,
  slotScopeIds = null,
  optimized = __DEV__ && isHmrUpdating ? false : !!n2.dynamicChildren
) => {
  // 新旧节点相同，直接返回
  if (n1 === n2) {
    return
  }
  // 旧节点存在，且新旧节点类型不同，卸载旧节点，锚点anchor后移
  // patching & not same type, unmount old tree
  if (n1 && !isSameVNodeType(n1, n2)) {
    anchor = getNextHostNode(n1)
    unmount(n1, parentComponent, parentSuspense, true)
    n1 = null
  }
  // 是否静态节点优化
  if (n2.patchFlag === PatchFlags.BAIL) {
    optimized = false
    n2.dynamicChildren = null
  }
  // 
  const { type, ref, shapeFlag } = n2
  switch (type) {
    case Text:
      processText(n1, n2, container, anchor)
      break
    case Comment:
      processCommentNode(n1, n2, container, anchor)
      break
    case Static:
      if (n1 == null) {
        mountStaticNode(n2, container, anchor, isSVG)
      } else if (__DEV__) {
        patchStaticNode(n1, n2, container, isSVG)
      }
      break
    case Fragment:
      processFragment(
        n1,
        n2,
        container,
        anchor,
        parentComponent,
        parentSuspense,
        isSVG,
        slotScopeIds,
        optimized
      )
      break
    default:
      if (shapeFlag & ShapeFlags.ELEMENT) {
        processElement(
          n1,
          n2,
          container,
          anchor,
          parentComponent,
          parentSuspense,
          isSVG,
          slotScopeIds,
          optimized
        )
      } else if (shapeFlag & ShapeFlags.COMPONENT) {
        processComponent(
          n1,
          n2,
          container,
          anchor,
          parentComponent,
          parentSuspense,
          isSVG,
          slotScopeIds,
          optimized
        )
      } else if (shapeFlag & ShapeFlags.TELEPORT) {
        ;(type as typeof TeleportImpl).process(
          n1 as TeleportVNode,
          n2 as TeleportVNode,
          container,
          anchor,
          parentComponent,
          parentSuspense,
          isSVG,
          slotScopeIds,
          optimized,
          internals
        )
      } else if (__FEATURE_SUSPENSE__ && shapeFlag & ShapeFlags.SUSPENSE) {
        ;(type as typeof SuspenseImpl).process(
          n1,
          n2,
          container,
          anchor,
          parentComponent,
          parentSuspense,
          isSVG,
          slotScopeIds,
          optimized,
          internals
        )
      } else if (__DEV__) {
        warn('Invalid VNode type:', type, `(${typeof type})`)
      }
  }
  // 处理 template 中的 ref 
  // set ref
  if (ref != null && parentComponent) {
    setRef(ref, n1 && n1.ref, parentSuspense, n2 || n1, !n2)
  }
}

const processText: ProcessTextOrCommentFn = (n1, n2, container, anchor) => {
  if (n1 == null) {
    hostInsert(
      (n2.el = hostCreateText(n2.children as string)),
      container,
      anchor
    )
  } else {
    const el = (n2.el = n1.el!)
    if (n2.children !== n1.children) {
      hostSetText(el, n2.children as string)
    }
  }
}

const processCommentNode: ProcessTextOrCommentFn = (
    n1,
    n2,
    container,
    anchor
  ) => {
    if (n1 == null) {
      hostInsert(
        (n2.el = hostCreateComment((n2.children as string) || '')),
        container,
        anchor
      )
    } else {
      // there's no support for dynamic comments
      n2.el = n1.el
    }
  }

const mountStaticNode = (
  n2: VNode,
  container: RendererElement,
  anchor: RendererNode | null,
  isSVG: boolean
) => {
  // static nodes are only present when used with compiler-dom/runtime-dom
  // which guarantees presence of hostInsertStaticContent.
  ;[n2.el, n2.anchor] = hostInsertStaticContent!(
    n2.children as string,
    container,
    anchor,
    isSVG,
    n2.el,
    n2.anchor
  )
}

const patchStaticNode = (
  n1: VNode,
  n2: VNode,
  container: RendererElement,
  isSVG: boolean
) => {
  // static nodes are only patched during dev for HMR
  if (n2.children !== n1.children) {
    const anchor = hostNextSibling(n1.anchor!)
    // 移除已有的静态节点，并插入新的节点
    // remove existing
    removeStaticNode(n1)
    // insert new
    ;[n2.el, n2.anchor] = hostInsertStaticContent!(
      n2.children as string,
      container,
      anchor,
      isSVG
    )
  } else {
    n2.el = n1.el
    n2.anchor = n1.anchor
  }
}
// removeStaticNode：从 n1.el 至 n1.anchor 的内容被遍历移除
const removeStaticNode = ({ el, anchor }: VNode) => {
  let next
  while (el && el !== anchor) {
    next = hostNextSibling(el)
    hostRemove(el)
    el = next
  }
  hostRemove(anchor!)
}

const processFragment = (
  n1: VNode | null,
  n2: VNode,
  container: RendererElement,
  anchor: RendererNode | null,
  parentComponent: ComponentInternalInstance | null,
  parentSuspense: SuspenseBoundary | null,
  isSVG: boolean,
  slotScopeIds: string[] | null,
  optimized: boolean
) => {
  // 锚点
  const fragmentStartAnchor = (n2.el = n1 ? n1.el : hostCreateText(''))!
  const fragmentEndAnchor = (n2.anchor = n1 ? n1.anchor : hostCreateText(''))!
  let { patchFlag, dynamicChildren, slotScopeIds: fragmentSlotScopeIds } = n2
  // 开发环境热更新时，强制全量diff
  if (
    __DEV__ &&
    // #5523 dev root fragment may inherit directives
    (isHmrUpdating || patchFlag & PatchFlags.DEV_ROOT_FRAGMENT)
  ) {
    // HMR updated / Dev root fragment (w/ comments), force full diff
    patchFlag = 0
    optimized = false
    dynamicChildren = null
  }
  // 检查是否是插槽
  // check if this is a slot fragment with :slotted scope ids
  if (fragmentSlotScopeIds) {
    slotScopeIds = slotScopeIds
      ? slotScopeIds.concat(fragmentSlotScopeIds)
      : fragmentSlotScopeIds
  }
  // 当旧的片段为空时，挂载新的片段的子节点
  if (n1 == null) {
    hostInsert(fragmentStartAnchor, container, anchor)
    hostInsert(fragmentEndAnchor, container, anchor)
    // a fragment can only have array children
    // since they are either generated by the compiler, or implicitly created
    // from arrays.
    mountChildren(
      n2.children as VNodeArrayChildren,
      container,
      fragmentEndAnchor,
      parentComponent,
      parentSuspense,
      isSVG,
      slotScopeIds,
      optimized
    )
  } else {
    // 当旧片段不为空时，启用优化则使用patchBlockChildren
    if (
      patchFlag > 0 &&
      patchFlag & PatchFlags.STABLE_FRAGMENT &&
      dynamicChildren &&
      // #2715 the previous fragment could've been a BAILed one as a result
      // of renderSlot() with no valid children
      n1.dynamicChildren
    ) {
      // a stable fragment (template root or <template v-for>) doesn't need to
      // patch children order, but it may contain dynamicChildren.
      patchBlockChildren(
        n1.dynamicChildren,
        dynamicChildren,
        container,
        parentComponent,
        parentSuspense,
        isSVG,
        slotScopeIds
      )
      // 开发环境，热更新 处理静态子节点
      if (__DEV__ && parentComponent && parentComponent.type.__hmrId) {
        traverseStaticChildren(n1, n2)
      } else if (
        // #2080 if the stable fragment has a key, it's a <template v-for> that may
        //  get moved around. Make sure all root level vnodes inherit el.
        // #2134 or if it's a component root, it may also get moved around
        // as the component is being moved.
        n2.key != null ||
        (parentComponent && n2 === parentComponent.subTree)
      ) {
        traverseStaticChildren(n1, n2, true )
      }
    } else {
      // 不可优化时，使用patchChildren处理
      // keyed / unkeyed, or manual fragments.
      // for keyed & unkeyed, since they are compiler generated from v-for,
      // each child is guaranteed to be a block so the fragment will never
      // have dynamicChildren.
      patchChildren(
        n1,
        n2,
        container,
        fragmentEndAnchor,
        parentComponent,
        parentSuspense,
        isSVG,
        slotScopeIds,
        optimized
      )
    }
  }
}

// The fast path for blocks.
const patchBlockChildren: PatchBlockChildrenFn = (
  oldChildren,
  newChildren,
  fallbackContainer,
  parentComponent,
  parentSuspense,
  isSVG,
  slotScopeIds
) => {
  for (let i = 0; i < newChildren.length; i++) {
    const oldVNode = oldChildren[i]
    const newVNode = newChildren[i]
    // Determine the container (parent element) for the patch.
    const container =
      // oldVNode may be an errored async setup() component inside Suspense
      // which will not have a mounted element
      oldVNode.el &&
      // - In the case of a Fragment, we need to provide the actual parent
      // of the Fragment itself so it can move its children.
      (oldVNode.type === Fragment ||
        // - In the case of different nodes, there is going to be a replacement
        // which also requires the correct parent container
        !isSameVNodeType(oldVNode, newVNode) ||
        // - In the case of a component, it could contain anything.
        oldVNode.shapeFlag & (ShapeFlags.COMPONENT | ShapeFlags.TELEPORT))
        ? hostParentNode(oldVNode.el)!
        : // In other cases, the parent container is not actually used so we
          // just pass the block element here to avoid a DOM parentNode call.
          fallbackContainer
    patch(
      oldVNode,
      newVNode,
      container,
      null,
      parentComponent,
      parentSuspense,
      isSVG,
      slotScopeIds,
      true
    )
  }
}

const patchChildren: PatchChildrenFn = (
  n1,
  n2,
  container,
  anchor,
  parentComponent,
  parentSuspense,
  isSVG,
  slotScopeIds,
  optimized = false
) => {
  const c1 = n1 && n1.children
  const prevShapeFlag = n1 ? n1.shapeFlag : 0
  const c2 = n2.children
  const { patchFlag, shapeFlag } = n2
  // 走绿色通道：用patchFlag来保证children是数组
  // fast path
  if (patchFlag > 0) {
    if (patchFlag & PatchFlags.KEYED_FRAGMENT) {
      // 有key属性的时候，根据key来进行diff
      // this could be either fully-keyed or mixed (some keyed some not)
      // presence of patchFlag means children are guaranteed to be arrays
      patchKeyedChildren(
        c1 as VNode[],
        c2 as VNodeArrayChildren,
        container,
        anchor,
        parentComponent,
        parentSuspense,
        isSVG,
        slotScopeIds,
        optimized
      )
      return
    } else if (patchFlag & PatchFlags.UNKEYED_FRAGMENT) {
      // 没有key
      // unkeyed
      patchUnkeyedChildren(
        c1 as VNode[],
        c2 as VNodeArrayChildren,
        container,
        anchor,
        parentComponent,
        parentSuspense,
        isSVG,
        slotScopeIds,
        optimized
      )
      return
    }
  }
  // 没有patchFlag的保证，则children可能为文本、数组或空
  // 根据形状标识来判断
  // children has 3 possibilities: text, array or no children.
  if (shapeFlag & ShapeFlags.TEXT_CHILDREN) {
    // 文本子节点的绿色通道
    // text children fast path
    if (prevShapeFlag & ShapeFlags.ARRAY_CHILDREN) {
      unmountChildren(c1 as VNode[], parentComponent, parentSuspense)
    }
    if (c2 !== c1) {
      hostSetElementText(container, c2 as string)
    }
  } else {
    // 旧的子节点是数组
    if (prevShapeFlag & ShapeFlags.ARRAY_CHILDREN) {
      // prev children was array
      if (shapeFlag & ShapeFlags.ARRAY_CHILDREN) {
        // 新旧子节点都是数组，需要进行全量diff
        // two arrays, cannot assume anything, do full diff
        patchKeyedChildren(
          c1 as VNode[],
          c2 as VNodeArrayChildren,
          container,
          anchor,
          parentComponent,
          parentSuspense,
          isSVG,
          slotScopeIds,
          optimized
        )
      } else {
        // 新的子节点为空，则只需要卸载旧的子节点
        // no new children, just unmount old
        unmountChildren(c1 as VNode[], parentComponent, parentSuspense, true)
      }
    } else {
      // 旧的子节点为文本节点或者空，新的为数组或空
      // prev children was text OR null
      // new children is array OR null
      if (prevShapeFlag & ShapeFlags.TEXT_CHILDREN) {
        // 旧的为文本节点，先将其文本置空
        hostSetElementText(container, '')
      }
      // 新的为数组，则通过mountChildren挂载子节点
      // mount new if array
      if (shapeFlag & ShapeFlags.ARRAY_CHILDREN) {
        mountChildren(
          c2 as VNodeArrayChildren,
          container,
          anchor,
          parentComponent,
          parentSuspense,
          isSVG,
          slotScopeIds,
          optimized
        )
      }
    }
  }
}

// can be all-keyed or mixed
const patchKeyedChildren = (
  c1: VNode[],
  c2: VNodeArrayChildren,
  container: RendererElement,
  parentAnchor: RendererNode | null,
  parentComponent: ComponentInternalInstance | null,
  parentSuspense: SuspenseBoundary | null,
  isSVG: boolean,
  slotScopeIds: string[] | null,
  optimized: boolean
) => {
  let i = 0
  const l2 = c2.length
  // 两组各自的尾节点
  let e1 = c1.length - 1 // prev ending index
  let e2 = l2 - 1 // next ending index
  // (从前往后)
  // 以两组中最短的一组为基准
  // 从头结点开始，依次比较同一位置的节点类型，若头节点类型相同，则对两个节点进行patch进行比较;
  // 若类型不同则退出循环
  // 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,
        isSVG,
        slotScopeIds,
        optimized
      )
    } else {
      break
    }
    // 
    i++
  }
  // (从后往前)
  // 从尾节点开始，尾节点类型相同，则通过patch比较尾节点；
  // 若类型不同则退出循环
  // 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,
        isSVG,
        slotScopeIds,
        optimized
      )
    } else {
      break
    }
    e1--
    e2--
  }
  // 经过前后两轮比较之后，剩下的就是中间那部分类型不同的子节点了
  // 若旧的子节点组已经遍历完，而新的子节点组还有剩余内容
  // 通过patch处理剩下的新的子节点中的内容，由于旧的子节点为空，
  // 因此相当于在patch内部挂载剩余的新的子节点
  // 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,
          isSVG,
          slotScopeIds,
          optimized
        )
        i++
      }
    }
  }
  // 旧的子节点还有剩余内容而新的子节点组已经遍历完，则卸载旧子节点组剩余的那部分
  // 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++
    }
  }
  // 新旧子节点组都没有遍历完，如下注释中[]里的部分
  // 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 {
    // 拿到上次比较完的起点
    const s1 = i // prev starting index
    const s2 = i // next starting index
    // 5.1 build key:index map for newChildren
    const keyToNewIndexMap: Map<string | number | symbol, number> = new Map()
    // 用Map存储新的子节点组的key和对应的index， key=>index 并给出重复的key的警告
    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
    // 已比较的数量
    let patched = 0
    // 未比较的数量
    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
    // 以新的子节点组中未完成比较的节点为基准
    const newIndexToOldIndexMap = new Array(toBePatched)
    // 先用0来填充，标记为没有key的节点。 ps:直接fill(0)不就好了么
    for (i = 0; i < toBePatched; i++) newIndexToOldIndexMap[i] = 0
    // 处理旧的子节点组
    for (i = s1; i <= e1; i++) {
      const prevChild = c1[i]
      // 当已经比较完了(patched >= toBePatched)，卸载旧的子节点
      if (patched >= toBePatched) {
        // all new children have been patched so this can only be a removal
        unmount(prevChild, parentComponent, parentSuspense, true)
        continue
      }
      let newIndex
      // 当旧的子节点的key存在，取出key在新的子节点组中对应的index
      if (prevChild.key != null) {
        newIndex = keyToNewIndexMap.get(prevChild.key)
      } else {
        // 若旧的子节点没有key，找出没有key且类型相同的节点对应在新子节点组中的index
        // 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
          }
        }
      }
      // newIndex不存在，即根据key来找，发现旧的子节点不可复用，则卸载旧的子节点
      if (newIndex === undefined) {
        unmount(prevChild, parentComponent, parentSuspense, true)
      } else {
        // 找到了可复用的节点，在newIndexToOldIndexMap中标记 i+1，
        // 用于最大上升子序列算法
        newIndexToOldIndexMap[newIndex - s2] = i + 1
        // 刷新目前找到的最大的新子节点的index，做节点移动标记
        if (newIndex >= maxNewIndexSoFar) {
          maxNewIndexSoFar = newIndex
        } else {
          moved = true
        }
        // 再递归详细比较两个节点
        patch(
          prevChild,
          c2[newIndex] as VNode,
          container,
          null,
          parentComponent,
          parentSuspense,
          isSVG,
          slotScopeIds,
          optimized
        )
        // 已对比的数量+1
        patched++
      }
    }
    // 当需要移动时，采用最大递增子序列算法，从而最大限度减少节点移动次数
    // 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
    // 倒序遍历，好处是可以使用上一次对比的节点作为锚点
    // 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) {
        // 等于0说明未被旧的子节点匹配到，属于全新的不可复用的子节点，则通过patch进行挂载
        // mount new
        patch(
          null,
          nextChild,
          container,
          anchor,
          parentComponent,
          parentSuspense,
          isSVG,
          slotScopeIds,
          optimized
        )
      } else if (moved) {
        // 当计算出来的最大上升子序列为空数组，
        // 或者当前节点不处于最大上升子序列中
        // 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--
        }
      }
    }
  }
}

const patchUnkeyedChildren = (
  c1: VNode[],
  c2: VNodeArrayChildren,
  container: RendererElement,
  anchor: RendererNode | null,
  parentComponent: ComponentInternalInstance | null,
  parentSuspense: SuspenseBoundary | null,
  isSVG: boolean,
  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,
      isSVG,
      slotScopeIds,
      optimized
    )
  }
  if (oldLength > newLength) {
    // remove old
    unmountChildren(
      c1,
      parentComponent,
      parentSuspense,
      true,
      false,
      commonLength
    )
  } else {
    // mount new
    mountChildren(
      c2,
      container,
      anchor,
      parentComponent,
      parentSuspense,
      isSVG,
      slotScopeIds,
      optimized,
      commonLength
    )
  }
}

const mountChildren: MountChildrenFn = (
  children,
  container,
  anchor,
  parentComponent,
  parentSuspense,
  isSVG,
  slotScopeIds,
  optimized,
  start = 0
) => {
  for (let i = start; i < children.length; i++) {
    const child = (children[i] = optimized
      ? cloneIfMounted(children[i] as VNode)
      : normalizeVNode(children[i]))
    patch(
      null,
      child,
      container,
      anchor,
      parentComponent,
      parentSuspense,
      isSVG,
      slotScopeIds,
      optimized
    )
  }
}

const unmountChildren: UnmountChildrenFn = (
  children,
  parentComponent,
  parentSuspense,
  doRemove = false,
  optimized = false,
  start = 0
) => {
  for (let i = start; i < children.length; i++) {
    unmount(children[i], parentComponent, parentSuspense, doRemove, optimized)
  }
}

const move: MoveFn = (
  vnode,
  container,
  anchor,
  moveType,
  parentSuspense = null
) => {
  const { el, type, transition, children, shapeFlag } = vnode
  // 对于组件节点，递归处理subTree
  if (shapeFlag & ShapeFlags.COMPONENT) {
    move(vnode.component!.subTree, container, anchor, moveType)
    return
  }
  // 处理异步组件<Suspense>
  if (__FEATURE_SUSPENSE__ && shapeFlag & ShapeFlags.SUSPENSE) {
    vnode.suspense!.move(container, anchor, moveType)
    return
  }
  // 处理<Teleport>
  if (shapeFlag & ShapeFlags.TELEPORT) {
    ;(type as typeof TeleportImpl).move(vnode, container, anchor, internals)
    return
  }
  // 文档片段，处理起始锚点和子节点
  if (type === Fragment) {
    hostInsert(el!, container, anchor)
    for (let i = 0; i < (children as VNode[]).length; i++) {
      move((children as VNode[])[i], container, anchor, moveType)
    }
    hostInsert(vnode.anchor!, container, anchor)
    return
  }
  // 静态节点
  if (type === Static) {
    moveStaticNode(vnode, container, anchor)
    return
  }
  // 处理<Transition>的钩子
  // single nodes
  const needTransition =
    moveType !== MoveType.REORDER &&
    shapeFlag & ShapeFlags.ELEMENT &&
    transition
  if (needTransition) {
    if (moveType === MoveType.ENTER) {
      transition!.beforeEnter(el!)
      hostInsert(el!, container, anchor)
      queuePostRenderEffect(() => transition!.enter(el!), parentSuspense)
    } else {
      const { leave, delayLeave, afterLeave } = transition!
      const remove = () => hostInsert(el!, container, anchor)
      const performLeave = () => {
        leave(el!, () => {
          remove()
          afterLeave && afterLeave()
        })
      }
      if (delayLeave) {
        delayLeave(el!, remove, performLeave)
      } else {
        performLeave()
      }
    }
  } else {
    hostInsert(el!, container, anchor)
  }
}

const processElement = (
    n1: VNode | null,
    n2: VNode,
    container: RendererElement,
    anchor: RendererNode | null,
    parentComponent: ComponentInternalInstance | null,
    parentSuspense: SuspenseBoundary | null,
    isSVG: boolean,
    slotScopeIds: string[] | null,
    optimized: boolean
  ) => {
    isSVG = isSVG || (n2.type as string) === 'svg'
    if (n1 == null) {
      mountElement(
        n2,
        container,
        anchor,
        parentComponent,
        parentSuspense,
        isSVG,
        slotScopeIds,
        optimized
      )
    } else {
      patchElement(
        n1,
        n2,
        parentComponent,
        parentSuspense,
        isSVG,
        slotScopeIds,
        optimized
      )
    }
  }

const mountElement = (
    vnode: VNode,
    container: RendererElement,
    anchor: RendererNode | null,
    parentComponent: ComponentInternalInstance | null,
    parentSuspense: SuspenseBoundary | null,
    isSVG: boolean,
    slotScopeIds: string[] | null,
    optimized: boolean
  ) => {
    let el: RendererElement
    let vnodeHook: VNodeHook | undefined | null
    const { type, props, shapeFlag, transition, patchFlag, dirs } = vnode
    if (
      !__DEV__ &&
      vnode.el &&
      hostCloneNode !== undefined &&
      patchFlag === PatchFlags.HOISTED
    ) {
      // vnode的el元素存在，仅在生产环境下对可复用的静态节点进行复制
      // If a vnode has non-null el, it means it's being reused.
      // Only static vnodes can be reused, so its mounted DOM nodes should be
      // exactly the same, and we can simply do a clone here.
      // only do this in production since cloned trees cannot be HMR updated.
      el = vnode.el = hostCloneNode(vnode.el)
    } else {
      // vnode上的元素不存在则新建
      el = vnode.el = hostCreateElement(
        vnode.type as string,
        isSVG,
        props && props.is,
        props
      )
      // 注释：由于某些props依赖于子节点的渲染，先挂载子节点
      // mount children first, since some props may rely on child content
      // being already rendered, e.g. `<select value>`
      if (shapeFlag & ShapeFlags.TEXT_CHILDREN) {
        // 设置元素文本
        hostSetElementText(el, vnode.children as string)
      } else if (shapeFlag & ShapeFlags.ARRAY_CHILDREN) {
        // 挂载子节点
        mountChildren(
          vnode.children as VNodeArrayChildren,
          el,
          null,
          parentComponent,
          parentSuspense,
          isSVG && type !== 'foreignObject',
          slotScopeIds,
          optimized
        )
      }
      // 指令的created阶段
      if (dirs) {
        invokeDirectiveHook(vnode, null, parentComponent, 'created')
      }
      // 处理元素的props
      // props
      if (props) {
        for (const key in props) {
          if (key !== 'value' && !isReservedProp(key)) {
            hostPatchProp(
              el,
              key,
              null,
              props[key],
              isSVG,
              vnode.children as VNode[],
              parentComponent,
              parentSuspense,
              unmountChildren
            )
          }
        }
        
        if ('value' in props) {
          hostPatchProp(el, 'value', null, props.value)
        }
        if ((vnodeHook = props.onVnodeBeforeMount)) {
          invokeVNodeHook(vnodeHook, parentComponent, vnode)
        }
      }
      // scopeId
      setScopeId(el, vnode, vnode.scopeId, slotScopeIds, parentComponent)
    }
    // __DEV__环境下处理 __vnode属性和父组件为不可枚举
    if (__DEV__ || __FEATURE_PROD_DEVTOOLS__) {
      Object.defineProperty(el, '__vnode', {
        value: vnode,
        enumerable: false
      })
      Object.defineProperty(el, '__vueParentComponent', {
        value: parentComponent,
        enumerable: false
      })
    }
    // 执行指令中的 beforeMount 阶段
    if (dirs) {
      invokeDirectiveHook(vnode, null, parentComponent, 'beforeMount')
    }
    // #1583 For inside suspense + suspense not resolved case, enter hook should call when suspense resolved
    // #1689 For inside suspense + suspense resolved case, just call it
    // 是否需要执行动画组件钩子
    const needCallTransitionHooks =
      (!parentSuspense || (parentSuspense && !parentSuspense.pendingBranch)) &&
      transition &&
      !transition.persisted
    if (needCallTransitionHooks) {
      transition!.beforeEnter(el)
    }
    hostInsert(el, container, anchor)
    if (
      (vnodeHook = props && props.onVnodeMounted) ||
      needCallTransitionHooks ||
      dirs
    ) {
      // 加入组件更新后的副作用执行队列，在合适的时机执行入队的函数
      // 这里是一些钩子函数、trasition的钩子、指令在mounted阶段的钩子
      queuePostRenderEffect(() => {
        vnodeHook && invokeVNodeHook(vnodeHook, parentComponent, vnode)
        needCallTransitionHooks && transition!.enter(el)
        dirs && invokeDirectiveHook(vnode, null, parentComponent, 'mounted')
      }, parentSuspense)
    }
  }

const patchElement = (
  n1: VNode,
  n2: VNode,
  parentComponent: ComponentInternalInstance | null,
  parentSuspense: SuspenseBoundary | null,
  isSVG: boolean,
  slotScopeIds: string[] | null,
  optimized: boolean
) => {
  const el = (n2.el = n1.el!)
  let { patchFlag, dynamicChildren, dirs } = n2
  // #1426 take the old vnode's patch flag into account since user may clone a
  // compiler-generated vnode, which de-opts to FULL_PROPS
  patchFlag |= n1.patchFlag & PatchFlags.FULL_PROPS
  const oldProps = n1.props || EMPTY_OBJ
  const newProps = n2.props || EMPTY_OBJ
  let vnodeHook: VNodeHook | undefined | null
  // 关闭recurse，在 beforeUpdated 阶段不允许自己调用
  // disable recurse in beforeUpdate hooks
  parentComponent && toggleRecurse(parentComponent, false)
  // beforeUpdated钩子
  if ((vnodeHook = newProps.onVnodeBeforeUpdate)) {
    invokeVNodeHook(vnodeHook, parentComponent, n2, n1)
  }
  // 指令的 beforeUpdated 钩子
  if (dirs) {
    invokeDirectiveHook(n2, n1, parentComponent, 'beforeUpdate')
  }
  // 允许自己调用
  parentComponent && toggleRecurse(parentComponent, true)
  // 开发环境呢下，关闭优化，全量diff
  if (__DEV__ && isHmrUpdating) {
    // HMR updated, force full diff
    patchFlag = 0
    optimized = false
    dynamicChildren = null
  }
  const areChildrenSVG = isSVG && n2.type !== 'foreignObject'
  // 新节点的动态子节点不为空，则比较新旧节点的动态子节点
  if (dynamicChildren) {
    patchBlockChildren(
      n1.dynamicChildren!,
      dynamicChildren,
      el,
      parentComponent,
      parentSuspense,
      areChildrenSVG,
      slotScopeIds
    )
    // 开发环境  递归遍历静态子节点
    if (__DEV__ && parentComponent && parentComponent.type.__hmrId) {
      traverseStaticChildren(n1, n2)
    }
    // 没有优化，全量 diff
  } else if (!optimized) {
    // full diff
    patchChildren(
      n1,
      n2,
      el,
      null,
      parentComponent,
      parentSuspense,
      areChildrenSVG,
      slotScopeIds,
      false
    )
  }
  // 注释：patchFlag 标识的存在意味着元素的 render 代码是由 compiler 生成的，
  // 且可以在 patch 时走快道，此时能保证新旧节点形状相同，即它们在源模板中正好处于相同的位置
  // 此时的对比是有着各种优化的
  if (patchFlag > 0) {
    // the presence of a patchFlag means this element's render code was
    // generated by the compiler and can take the fast path.
    // in this path old node and new node are guaranteed to have the same shape
    // (i.e. at the exact same position in the source template)
    if (patchFlag & PatchFlags.FULL_PROPS) {
      // 当props中含有动态的key，需要进行全量 diff
      // element props contain dynamic keys, full diff needed
      patchProps(
        el,
        n2,
        oldProps,
        newProps,
        parentComponent,
        parentSuspense,
        isSVG
      )
    } else {
      // 处理动态类名绑定
      // class
      // this flag is matched when the element has dynamic class bindings.
      if (patchFlag & PatchFlags.CLASS) {
        if (oldProps.class !== newProps.class) {
          hostPatchProp(el, 'class', null, newProps.class, isSVG)
        }
      }
      // 处理动态的 style 绑定
      // style
      // this flag is matched when the element has dynamic style bindings
      if (patchFlag & PatchFlags.STYLE) {
        hostPatchProp(el, 'style', oldProps.style, newProps.style, isSVG)
      }
      // 处理动态的 prop/attr 绑定，有迭代缓存，优化比较速度
      // 如果 `prop/attr`的 key 是动态的，那么这种优化则会失效
      // props
      // This flag is matched when the element has dynamic prop/attr bindings
      // other than class and style. The keys of dynamic prop/attrs are saved for
      // faster iteration.
      // Note dynamic keys like :[foo]="bar" will cause this optimization to
      // bail out and go through a full diff because we need to unset the old key
      if (patchFlag & PatchFlags.PROPS) {
        // if the flag is present then dynamicProps must be non-null
        const propsToUpdate = n2.dynamicProps!
        for (let i = 0; i < propsToUpdate.length; i++) {
          const key = propsToUpdate[i]
          const prev = oldProps[key]
          const next = newProps[key]
          // value属性会被强行对比
          // #1471 force patch value
          if (next !== prev || key === 'value') {
            hostPatchProp(
              el,
              key,
              prev,
              next,
              isSVG,
              n1.children as VNode[],
              parentComponent,
              parentSuspense,
              unmountChildren
            )
          }
        }
      }
    }
    // 处理文本：仅在元素只有文本子节点时触发
    // text
    // This flag is matched when the element has only dynamic text children.
    if (patchFlag & PatchFlags.TEXT) {
      if (n1.children !== n2.children) {
        hostSetElementText(el, n2.children as string)
      }
    }
  } else if (!optimized && dynamicChildren == null) {
    // 没有优化，全量 diff
    // unoptimized, full diff
    patchProps(
      el,
      n2,
      oldProps,
      newProps,
      parentComponent,
      parentSuspense,
      isSVG
    )
  }
  // updated 钩子 入队
  if ((vnodeHook = newProps.onVnodeUpdated) || dirs) {
    queuePostRenderEffect(() => {
      vnodeHook && invokeVNodeHook(vnodeHook, parentComponent, n2, n1)
      dirs && invokeDirectiveHook(n2, n1, parentComponent, 'updated')
    }, parentSuspense)
  }
}

const patchProps = (
  el: RendererElement,
  vnode: VNode,
  oldProps: Data,
  newProps: Data,
  parentComponent: ComponentInternalInstance | null,
  parentSuspense: SuspenseBoundary | null,
  isSVG: boolean
) => {
  if (oldProps !== newProps) {
    // 遍历新的props
    for (const key in newProps) {
      // empty string is not valid prop
      if (isReservedProp(key)) continue
      const next = newProps[key]
      const prev = oldProps[key]
      // 先不比较 value
      // defer patching value
      if (next !== prev && key !== 'value') {
        hostPatchProp(
          el,
          key,
          prev,
          next,
          isSVG,
          vnode.children as VNode[],
          parentComponent,
          parentSuspense,
          unmountChildren
        )
      }
    }
    // 遍历旧的props
    if (oldProps !== EMPTY_OBJ) {
      for (const key in oldProps) {
        if (!isReservedProp(key) && !(key in newProps)) {
          hostPatchProp(
            el,
            key,
            oldProps[key],
            null,
            isSVG,
            vnode.children as VNode[],
            parentComponent,
            parentSuspense,
            unmountChildren
          )
        }
      }
    }
    // 最后处理 value
    if ('value' in newProps) {
      hostPatchProp(el, 'value', oldProps.value, newProps.value)
    }
  }
}

const processComponent = (
  n1: VNode | null,
  n2: VNode,
  container: RendererElement,
  anchor: RendererNode | null,
  parentComponent: ComponentInternalInstance | null,
  parentSuspense: SuspenseBoundary | null,
  isSVG: boolean,
  slotScopeIds: string[] | null,
  optimized: boolean
) => {
  n2.slotScopeIds = slotScopeIds
  if (n1 == null) {
    if (n2.shapeFlag & ShapeFlags.COMPONENT_KEPT_ALIVE) {
      ;(parentComponent!.ctx as KeepAliveContext).activate(
        n2,
        container,
        anchor,
        isSVG,
        optimized
      )
    } else {
      mountComponent(
        n2,
        container,
        anchor,
        parentComponent,
        parentSuspense,
        isSVG,
        optimized
      )
    }
  } else {
    updateComponent(n1, n2, optimized)
  }
}

const mountComponent: MountComponentFn = (
  initialVNode,
  container,
  anchor,
  parentComponent,
  parentSuspense,
  isSVG,
  optimized
) => {
  // 2.x compat may pre-create the component instance before actually
  // mounting
  const compatMountInstance =
    __COMPAT__ && initialVNode.isCompatRoot && initialVNode.component
  const instance: ComponentInternalInstance =
    compatMountInstance ||
    (initialVNode.component = createComponentInstance(
      initialVNode,
      parentComponent,
      parentSuspense
    ))
  // 注册热更新
  if (__DEV__ && instance.type.__hmrId) {
    registerHMR(instance)
  }
  // 挂载性能检测
  if (__DEV__) {
    pushWarningContext(initialVNode)
    startMeasure(instance, `mount`)
  }
  // 注入renderer的内部内容
  // inject renderer internals for keepAlive
  if (isKeepAlive(initialVNode)) {
    ;(instance.ctx as KeepAliveContext).renderer = internals
  }
  
  // 处理props和插槽
  // resolve props and slots for setup context
  if (!(__COMPAT__ && compatMountInstance)) {
    // 检测初始化性能
    if (__DEV__) {
      startMeasure(instance, `init`)
    }
    // 处理setup：这个函数里使用其它方法，初始化了props和插槽，且调用了setup
    setupComponent(instance)
    if (__DEV__) {
      endMeasure(instance, `init`)
    }
  }
  // 处理异步的setup
  // setup() is async. This component relies on async logic to be resolved
  // before proceeding
  if (__FEATURE_SUSPENSE__ && instance.asyncDep) {
    parentSuspense && parentSuspense.registerDep(instance, setupRenderEffect)
    // Give it a placeholder if this is not hydration
    // TODO handle self-defined fallback
    if (!initialVNode.el) {
      const placeholder = (instance.subTree = createVNode(Comment))
      processCommentNode(null, placeholder, container!, anchor)
    }
    return
  }
  // 接下来根据setup返回内容进行渲染
  // todo 阅读该函数的内容
  setupRenderEffect(
    instance,
    initialVNode,
    container,
    anchor,
    parentSuspense,
    isSVG,
    optimized
  )
  // mount性能检测结束点
  if (__DEV__) {
    popWarningContext()
    endMeasure(instance, `mount`)
  }
}

const updateComponent = (n1: VNode, n2: VNode, optimized: boolean) => {
  const instance = (n2.component = n1.component)!
  if (shouldUpdateComponent(n1, n2, optimized)) {
    if (
      __FEATURE_SUSPENSE__ &&
      instance.asyncDep &&
      !instance.asyncResolved
    ) {
      // async & still pending - just update props and slots
      // since the component's reactive effect for render isn't set-up yet
      if (__DEV__) {
        pushWarningContext(n2)
      }
      // 更新组件的预渲染：即处理props和插槽
      updateComponentPreRender(instance, n2, optimized)
      if (__DEV__) {
        popWarningContext()
      }
      return
    } else {
      // normal update
      instance.next = n2
      // in case the child component is also queued, remove it to avoid
      // double updating the same child component in the same flush.
      invalidateJob(instance.update)
      // instance.update is the reactive effect.
      instance.update()
    }
  } else {
    // no update needed. just copy over properties
    n2.el = n1.el
    instance.vnode = n2
  }
}

const updateComponentPreRender = (
  instance: ComponentInternalInstance,
  nextVNode: VNode,
  optimized: boolean
) => {
  nextVNode.component = instance
  const prevProps = instance.vnode.props
  instance.vnode = nextVNode
  instance.next = null
  updateProps(instance, nextVNode.props, prevProps, optimized)
  updateSlots(instance, nextVNode.children, optimized)
  pauseTracking()
  // props update may have triggered pre-flush watchers.
  // flush them before the render update.
  flushPreFlushCbs(undefined, instance.update)
  resetTracking()
}

const setupRenderEffect: SetupRenderEffectFn = (
  instance,
  initialVNode,
  container,
  anchor,
  parentSuspense,
  isSVG,
  optimized
) => {
  const componentUpdateFn = () => {
    if (!instance.isMounted) {
      let vnodeHook: VNodeHook | null | undefined
      const { el, props } = initialVNode
      const { bm, m, parent } = instance
      const isAsyncWrapperVNode = isAsyncWrapper(initialVNode)
      // 在beforeMounted期间 不允许effect自己调用
      toggleRecurse(instance, false)
      // beforeMount hook
      if (bm) {
        invokeArrayFns(bm)
      }
      // onVnodeBeforeMount
      if (
        !isAsyncWrapperVNode &&
        (vnodeHook = props && props.onVnodeBeforeMount)
      ) {
        invokeVNodeHook(vnodeHook, parent, initialVNode)
      }
      if (
        __COMPAT__ &&
        isCompatEnabled(DeprecationTypes.INSTANCE_EVENT_HOOKS, instance)
      ) {
        instance.emit('hook:beforeMount')
      }
      toggleRecurse(instance, true)
      if (el && hydrateNode) {
        // vnode has adopted host node - perform hydration instead of mount.
        const hydrateSubTree = () => {
          if (__DEV__) {
            startMeasure(instance, `render`)
          }
          instance.subTree = renderComponentRoot(instance)
          if (__DEV__) {
            endMeasure(instance, `render`)
          }
          if (__DEV__) {
            startMeasure(instance, `hydrate`)
          }
          hydrateNode!(
            el as Node,
            instance.subTree,
            instance,
            parentSuspense,
            null
          )
          if (__DEV__) {
            endMeasure(instance, `hydrate`)
          }
        }
        if (isAsyncWrapperVNode) {
          ;(initialVNode.type as ComponentOptions).__asyncLoader!().then(
            // note: we are moving the render call into an async callback,
            // which means it won't track dependencies - but it's ok because
            // a server-rendered async wrapper is already in resolved state
            // and it will never need to change.
            () => !instance.isUnmounted && hydrateSubTree()
          )
        } else {
          hydrateSubTree()
        }
      } else {
        if (__DEV__) {
          startMeasure(instance, `render`)
        }
        const subTree = (instance.subTree = renderComponentRoot(instance))
        if (__DEV__) {
          endMeasure(instance, `render`)
        }
        if (__DEV__) {
          startMeasure(instance, `patch`)
        }
        patch(
          null,
          subTree,
          container,
          anchor,
          instance,
          parentSuspense,
          isSVG
        )
        if (__DEV__) {
          endMeasure(instance, `patch`)
        }
        initialVNode.el = subTree.el
      }
      // mounted钩子入队
      // mounted hook
      if (m) {
        queuePostRenderEffect(m, parentSuspense)
      }
      // onVnodeMounted
      if (
        !isAsyncWrapperVNode &&
        (vnodeHook = props && props.onVnodeMounted)
      ) {
        const scopedInitialVNode = initialVNode
        queuePostRenderEffect(
          () => invokeVNodeHook(vnodeHook!, parent, scopedInitialVNode),
          parentSuspense
        )
      }
      if (
        __COMPAT__ &&
        isCompatEnabled(DeprecationTypes.INSTANCE_EVENT_HOOKS, instance)
      ) {
        queuePostRenderEffect(
          () => instance.emit('hook:mounted'),
          parentSuspense
        )
      }
      // <KeepAlive>组件的activated钩子，可能包含从子组件注入的钩子
      // activated hook for keep-alive roots.
      // #1742 activated hook must be accessed after first render
      // since the hook may be injected by a child keep-alive
      if (
        initialVNode.shapeFlag & ShapeFlags.COMPONENT_SHOULD_KEEP_ALIVE ||
        (parent &&
          isAsyncWrapper(parent.vnode) &&
          parent.vnode.shapeFlag & ShapeFlags.COMPONENT_SHOULD_KEEP_ALIVE)
      ) {
        instance.a && queuePostRenderEffect(instance.a, parentSuspense)
        // 兼容
        if (
          __COMPAT__ &&
          isCompatEnabled(DeprecationTypes.INSTANCE_EVENT_HOOKS, instance)
        ) {
          queuePostRenderEffect(
            () => instance.emit('hook:activated'),
            parentSuspense
          )
        }
      }
      // 变更组件挂载状态
      instance.isMounted = true
      if (__DEV__ || __FEATURE_PROD_DEVTOOLS__) {
        devtoolsComponentAdded(instance)
      }
      // #2458: deference mount-only object parameters to prevent memleaks
      initialVNode = container = anchor = null as any
    } else {
      // updateComponent
      // This is triggered by mutation of component's own state (next: null)
      // OR parent calling processComponent (next: VNode)
      let { next, bu, u, parent, vnode } = instance
      let originNext = next
      let vnodeHook: VNodeHook | null | undefined
      if (__DEV__) {
        pushWarningContext(next || instance.vnode)
      }
      // beforeUpdated 期间也不允许effect自调用
      // Disallow component effect recursion during pre-lifecycle hooks.
      toggleRecurse(instance, false)
      if (next) {
        next.el = vnode.el
        updateComponentPreRender(instance, next, optimized)
      } else {
        next = vnode
      }
      // beforeUpdate hook
      if (bu) {
        invokeArrayFns(bu)
      }
      // onVnodeBeforeUpdate
      if ((vnodeHook = next.props && next.props.onVnodeBeforeUpdate)) {
        invokeVNodeHook(vnodeHook, parent, next, vnode)
      }
      // 考虑兼容
      if (
        __COMPAT__ &&
        isCompatEnabled(DeprecationTypes.INSTANCE_EVENT_HOOKS, instance)
      ) {
        instance.emit('hook:beforeUpdate')
      }
      toggleRecurse(instance, true)
      // render
      if (__DEV__) {
        startMeasure(instance, `render`)
      }
      const nextTree = renderComponentRoot(instance)
      if (__DEV__) {
        endMeasure(instance, `render`)
      }
      const prevTree = instance.subTree
      instance.subTree = nextTree
      if (__DEV__) {
        startMeasure(instance, `patch`)
      }
      // 更新则比较新旧subTree
      patch(
        prevTree,
        nextTree,
        // parent may have changed if it's in a teleport
        hostParentNode(prevTree.el!)!,
        // anchor may have changed if it's in a fragment
        getNextHostNode(prevTree),
        instance,
        parentSuspense,
        isSVG
      )
      if (__DEV__) {
        endMeasure(instance, `patch`)
      }
      next.el = nextTree.el
      if (originNext === null) {
        // self-triggered update. In case of HOC, update parent component
        // vnode el. HOC is indicated by parent instance's subTree pointing
        // to child component's vnode
        updateHOCHostEl(instance, nextTree.el)
      }
      // 处理updated钩子
      // updated hook
      if (u) {
        queuePostRenderEffect(u, parentSuspense)
      }
      // onVnodeUpdated
      if ((vnodeHook = next.props && next.props.onVnodeUpdated)) {
        queuePostRenderEffect(
          () => invokeVNodeHook(vnodeHook!, parent, next!, vnode),
          parentSuspense
        )
      }
      if (
        __COMPAT__ &&
        isCompatEnabled(DeprecationTypes.INSTANCE_EVENT_HOOKS, instance)
      ) {
        queuePostRenderEffect(
          () => instance.emit('hook:updated'),
          parentSuspense
        )
      }
      if (__DEV__ || __FEATURE_PROD_DEVTOOLS__) {
        devtoolsComponentUpdated(instance)
      }
      if (__DEV__) {
        popWarningContext()
      }
    }
  }
  // 使用componentUpdateFn创建effect
  // create reactive effect for rendering
  const effect = (instance.effect = new ReactiveEffect(
    componentUpdateFn,
    () => queueJob(update),
    instance.scope // track it in component's effect scope
  ))
  const update: SchedulerJob = (instance.update = () => effect.run())
  update.id = instance.uid
  // allowRecurse
  // #1801, #2043 component render effects should allow recursive updates
  toggleRecurse(instance, true)
  // 用于开发调试
  if (__DEV__) {
    effect.onTrack = instance.rtc
      ? e => invokeArrayFns(instance.rtc!, e)
      : void 0
    effect.onTrigger = instance.rtg
      ? e => invokeArrayFns(instance.rtg!, e)
      : void 0
    update.ownerInstance = instance
  }
  // 调用一次更新
  update()
}

const unmount: UnmountFn = (
  vnode,
  parentComponent,
  parentSuspense,
  doRemove = false,
  optimized = false
) => {
  const {
    type,
    props,
    ref,
    children,
    dynamicChildren,
    shapeFlag,
    patchFlag,
    dirs
  } = vnode
  // 置空ref
  // unset ref
  if (ref != null) {
    setRef(ref, null, parentSuspense, vnode, true)
  }
  // 组件被缓存，则调用<KeepAlive>的失活方法 deactivate
  if (shapeFlag & ShapeFlags.COMPONENT_SHOULD_KEEP_ALIVE) {
    ;(parentComponent!.ctx as KeepAliveContext).deactivate(vnode)
    return
  }
  // 是否调用指令和钩子
  const shouldInvokeDirs = shapeFlag & ShapeFlags.ELEMENT && dirs
  const shouldInvokeVnodeHook = !isAsyncWrapper(vnode)
  // beforeUnmounted 钩子
  let vnodeHook: VNodeHook | undefined | null
  if (
    shouldInvokeVnodeHook &&
    (vnodeHook = props && props.onVnodeBeforeUnmount)
  ) {
    invokeVNodeHook(vnodeHook, parentComponent, vnode)
  }
  if (shapeFlag & ShapeFlags.COMPONENT) {
    // 卸载组件
    unmountComponent(vnode.component!, parentSuspense, doRemove)
  } else {
    // 卸载异步组件
    if (__FEATURE_SUSPENSE__ && shapeFlag & ShapeFlags.SUSPENSE) {
      vnode.suspense!.unmount(parentSuspense, doRemove)
      return
    }
    // 处理指令的 beforeUnmounted 阶段
    if (shouldInvokeDirs) {
      invokeDirectiveHook(vnode, null, parentComponent, 'beforeUnmount')
    }
    // 卸载 Teleport
    if (shapeFlag & ShapeFlags.TELEPORT) {
      ;(vnode.type as typeof TeleportImpl).remove(
        vnode,
        parentComponent,
        parentSuspense,
        optimized,
        internals,
        doRemove
      )
    } else if (
      dynamicChildren &&
      // #1153: fast path should not be taken for non-stable (v-for) fragments
      (type !== Fragment ||
        (patchFlag > 0 && patchFlag & PatchFlags.STABLE_FRAGMENT))
    ) {
      // 对于优化过的块状节点，仅需移除动态子节点
      // fast path for block nodes: only need to unmount dynamic children.
      unmountChildren(
        dynamicChildren,
        parentComponent,
        parentSuspense,
        false,
        true
      )
    } else if (
      // 文档片段  移除其子节点
      (type === Fragment &&
        patchFlag &
          (PatchFlags.KEYED_FRAGMENT | PatchFlags.UNKEYED_FRAGMENT)) ||
      (!optimized && shapeFlag & ShapeFlags.ARRAY_CHILDREN)
    ) {
      unmountChildren(children as VNode[], parentComponent, parentSuspense)
    }
    // 处理节点自身
    if (doRemove) {
      remove(vnode)
    }
  }
  // 处理 unmounted 钩子以及指令中的 unmounted 阶段
  if (
    (shouldInvokeVnodeHook &&
      (vnodeHook = props && props.onVnodeUnmounted)) ||
    shouldInvokeDirs
  ) {
    queuePostRenderEffect(() => {
      vnodeHook && invokeVNodeHook(vnodeHook, parentComponent, vnode)
      shouldInvokeDirs &&
        invokeDirectiveHook(vnode, null, parentComponent, 'unmounted')
    }, parentSuspense)
  }
}

const remove: RemoveFn = vnode => {
  const { type, el, anchor, transition } = vnode
  if (type === Fragment) {
    if (
      __DEV__ &&
      vnode.patchFlag > 0 &&
      vnode.patchFlag & PatchFlags.DEV_ROOT_FRAGMENT &&
      transition &&
      !transition.persisted
    ) {
      // __DEV__环境
      // 遍历移除子节点
      ;(vnode.children as VNode[]).forEach(child => {
        if (child.type === Comment) {
          hostRemove(child.el!)
        } else {
          remove(child)
        }
      })
    } else {
      // 移除片段
      removeFragment(el!, anchor!)
    }
    return
  }
  // 移除静态节点
  if (type === Static) {
    removeStaticNode(vnode)
    return
  }
  
  const performRemove = () => {
    // 移除el
    hostRemove(el!)
    if (transition && !transition.persisted && transition.afterLeave) {
      // 动画的 afterLeave 钩子
      transition.afterLeave()
    }
  }
  if (
    vnode.shapeFlag & ShapeFlags.ELEMENT &&
    transition &&
    !transition.persisted
  ) {
    const { leave, delayLeave } = transition
    const performLeave = () => leave(el!, performRemove)
    // 推迟 leave 动画
    if (delayLeave) {
      delayLeave(vnode.el!, performRemove, performLeave)
    } else {
      performLeave()
    }
  } else {
    // 执行
    performRemove()
  }
}

const removeFragment = (cur: RendererNode, end: RendererNode) => {
  // For fragments, directly remove all contained DOM nodes.
  // (fragment child nodes cannot have transition)
  let next
  while (cur !== end) {
    next = hostNextSibling(cur)!
    hostRemove(cur)
    cur = next
  }
  hostRemove(end)
}

const unmountComponent = (
  instance: ComponentInternalInstance,
  parentSuspense: SuspenseBoundary | null,
  doRemove?: boolean
) => {
  if (__DEV__ && instance.type.__hmrId) {
    unregisterHMR(instance)
  }
  const { bum, scope, update, subTree, um } = instance
  // 调用 beforeUnmounted 钩子
  // beforeUnmount hook
  if (bum) {
    invokeArrayFns(bum)
  }
  if (
    __COMPAT__ &&
    isCompatEnabled(DeprecationTypes.INSTANCE_EVENT_HOOKS, instance)
  ) {
    instance.emit('hook:beforeDestroy')
  }
  // 停止副作用
  // stop effects in component scope
  scope.stop()
  // 关闭 update，卸载子组件树
  // update may be null if a component is unmounted before its async
  // setup has resolved.
  if (update) {
    // so that scheduler will no longer invoke it
    update.active = false
    unmount(subTree, instance, parentSuspense, doRemove)
  }
  // 调用unmounted钩子
  // unmounted hook
  if (um) {
    queuePostRenderEffect(um, parentSuspense)
  }
  // 向后兼容：destroyed 钩子
  if (
    __COMPAT__ &&
    isCompatEnabled(DeprecationTypes.INSTANCE_EVENT_HOOKS, instance)
  ) {
    queuePostRenderEffect(
      () => instance.emit('hook:destroyed'),
      parentSuspense
    )
  }
  // 更改状态为已卸载
  queuePostRenderEffect(() => {
    instance.isUnmounted = true
  }, parentSuspense)
  // 处理<Suspense>
  // A component with async dep inside a pending suspense is unmounted before
  // its async dep resolves. This should remove the dep from the suspense, and
  // cause the suspense to resolve immediately if that was the last dep.
  if (
    __FEATURE_SUSPENSE__ &&
    parentSuspense &&
    parentSuspense.pendingBranch &&
    !parentSuspense.isUnmounted &&
    instance.asyncDep &&
    !instance.asyncResolved &&
    instance.suspenseId === parentSuspense.pendingId
  ) {
    parentSuspense.deps--
    if (parentSuspense.deps === 0) {
      parentSuspense.resolve()
    }
  }
  if (__DEV__ || __FEATURE_PROD_DEVTOOLS__) {
    devtoolsComponentRemoved(instance)
  }
}

const unmountChildren: UnmountChildrenFn = (
  children,
  parentComponent,
  parentSuspense,
  doRemove = false,
  optimized = false,
  start = 0
) => {
  for (let i = start; i < children.length; i++) {
    unmount(children[i], parentComponent, parentSuspense, doRemove, optimized)
  }
}