vue3的Fragment实现原理

在 Vue 2 中，每个组件必须有一个根元素。而在 Vue 3 中，引入了 Fragment 特性，允许组件具有多个根节点。这为开发人员提供了更多灵活性，简化了组件模板的结构。

Vue3支持多个根节点，通过Fragment来实现，原理比较容易

我们用一个Fragment容器来渲染多个节点

分析

首先，在runtime-core模块中的vnode.ts里文件里新加一个类型Fragment

export const Fragment = Symbol.for('v-fgt') as any as {
  __isFragment: true
  new (): {
    $props: VNodeProps
  }
}

那么我在调runtime-core中的在我们调用render方法时，又会走到patch方法里，
如果老的虚拟节点不存在，那么就是创建操作，否则就走更新流程

  const render: RootRenderFunction = (vnode, container, namespace) => {
    if (vnode == null) {
      if (container._vnode) {
        unmount(container._vnode, null, null, true)
      }
    } else {
      patch(
        container._vnode || null,
        vnode,
        container,
        null,
        null,
        null,
        namespace,
      )
    }
    container._vnode = vnode
    if (!isFlushing) {
      isFlushing = true
      flushPreFlushCbs()
      flushPostFlushCbs()
      isFlushing = false
    }
  }

走到patch 方法里

  const patch: PatchFn = (
    n1,
    n2,
    container,
    anchor = null,
    parentComponent = null,
    parentSuspense = null,
    namespace = undefined,
    slotScopeIds = null,
    optimized = __DEV__ && isHmrUpdating ? false : !!n2.dynamicChildren,
  ) => {
    if (n1 === n2) {
      return
    }

    // patching & not same type, unmount old tree
    if (n1 && !isSameVNodeType(n1, n2)) {
      anchor = getNextHostNode(n1)
      unmount(n1, parentComponent, parentSuspense, true)
      n1 = null
    }

    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, namespace)
        } else if (__DEV__) {
          patchStaticNode(n1, n2, container, namespace)
        }
        break
      case Fragment:
        processFragment(
          n1,
          n2,
          container,
          anchor,
          parentComponent,
          parentSuspense,
          namespace,
          slotScopeIds,
          optimized,
        )
        break
      default:
        if (shapeFlag & ShapeFlags.ELEMENT) {
          processElement(
            n1,
            n2,
            container,
            anchor,
            parentComponent,
            parentSuspense,
            namespace,
            slotScopeIds,
            optimized,
          )
        } else if (shapeFlag & ShapeFlags.COMPONENT) {
          processComponent(
            n1,
            n2,
            container,
            anchor,
            parentComponent,
            parentSuspense,
            namespace,
            slotScopeIds,
            optimized,
          )
        } else if (shapeFlag & ShapeFlags.TELEPORT) {
          ;(type as typeof TeleportImpl).process(
            n1 as TeleportVNode,
            n2 as TeleportVNode,
            container,
            anchor,
            parentComponent,
            parentSuspense,
            namespace,
            slotScopeIds,
            optimized,
            internals,
          )
        } else if (__FEATURE_SUSPENSE__ && shapeFlag & ShapeFlags.SUSPENSE) {
          ;(type as typeof SuspenseImpl).process(
            n1,
            n2,
            container,
            anchor,
            parentComponent,
            parentSuspense,
            namespace,
            slotScopeIds,
            optimized,
            internals,
          )
        } else if (__DEV__) {
          warn('Invalid VNode type:', type, `(${typeof type})`)
        }
    }

    // set ref
    if (ref != null && parentComponent) {
      setRef(ref, n1 && n1.ref, parentSuspense, n2 || n1, !n2)
    }
  }

在patch里我们增加一个Fragment类型的判断命中Fragment类型后，我们写一个processFragment方法来进行处理

     case Fragment:
        processFragment(
          n1,
          n2,
          container,
          anchor,
          parentComponent,
          parentSuspense,
          namespace,
          slotScopeIds,
          optimized,
        )
        break

processFragment方法

  const processFragment = (
    n1: VNode | null,
    n2: VNode,
    container: RendererElement,
    anchor: RendererNode | null,
    parentComponent: ComponentInternalInstance | null,
    parentSuspense: SuspenseBoundary | null,
    namespace: ElementNamespace,
    slotScopeIds: string[] | null,
    optimized: boolean,
  ) => {
    const fragmentStartAnchor = (n2.el = n1 ? n1.el : hostCreateText(''))!
    const fragmentEndAnchor = (n2.anchor = n1 ? n1.anchor : hostCreateText(''))!

    let { patchFlag, dynamicChildren, slotScopeIds: fragmentSlotScopeIds } = n2

    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
    }

    // check if this is a slot fragment with :slotted scope ids
    if (fragmentSlotScopeIds) {
      slotScopeIds = slotScopeIds
        ? slotScopeIds.concat(fragmentSlotScopeIds)
        : fragmentSlotScopeIds
    }

    if (n1 == null) { //如果老的虚拟节点为null，那么我们就把新虚拟节点里的children挂载到容器中
      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(
        // #10007
        // such fragment like `<></>` will be compiled into
        // a fragment which doesn't have a children.
        // In this case fallback to an empty array
        (n2.children || []) as VNodeArrayChildren,
        container,
        fragmentEndAnchor,
        parentComponent,
        parentSuspense,
        namespace,
        slotScopeIds,
        optimized,
      )
    } else {
      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,
          namespace,
          slotScopeIds,
        )
        if (__DEV__) {
          // necessary for HMR
          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 /* shallow */)
        }
      } else {
        // 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,
          namespace,
          slotScopeIds,
          optimized,
        ) // ，走的是diff算法
      }
    }
  }

如果老的虚拟节点为null，那么我们就把新虚拟节点里的children挂载到容器中否则我们就对比新老虚拟节点的children，走的是diff算法

Fragment的原理比较简单，其实底层就是新增加了一个类型，用一个函数去处理初始挂载或diff更新。

在 Vue3 的编译过程中，如果组件的模板存在多个节点，Vue3 会将这些节点放入一个片段中，然后作为组件的根节点。

总结

Vue3 之所以能够支持多节点的 Fragment，是因为在内部采用了 Virtual DOM 和片段的机制。Vue3 会将多个节点放入一个片段中，然后作为组件的根节点进行 Virtual DOM 操作。使用片段的方式，可以方便地处理多节点的情况，并提高渲染效率

实际 DOM 更新时遵循单个根元素规则：虽然 Vue 3 允许组件具有多个根节点，但在实际的 DOM 结构中，每个组件仍然需要遵循单个根元素的规则。因此，在将虚拟 DOM 更新到实际的 DOM 时，Vue 会确保每个组件的多个根节点都被插入到正确的位置，且符合 DOM 规范。

Fragment 的优点

通过这种方式，Vue 3 的 Fragment 实现了对多个根节点的支持，同时保持了与 DOM 规范的兼容性。这使得开发人员能够编写更简洁、灵活的组件模板，提高了组件的可维护性和可读性。