创建根节点的代码生成节点

createRootCodegen

• createRootCodegen 目的就是 为 root 这个虚拟的 ast 根节点创建一个代码生成节点
• 如果 root 的子节点 children 是单个元素节点 就将 root 转换成一个 block 节点 把这个 child 的 codegenNode 赋值给 root 的 codegenNode
• 如果 root 的子节点 children 是多个节点 返回一个 fragement 的代码生成节点 并赋值给 root 的 codegenNode
• createRootCodegen 完成之后 把转换 ast 节点的过程中创建的一些上下文数据赋值给 root 节点对应的属性

function createRootCodegen(root: RootNode, context: TransformContext) {
  const { helper } = context
  const { children } = root
  if (children.length === 1) {
    const child = children[0]
    // if the single child is an element, turn it into a block.
    if (isSingleElementRoot(root, child) && child.codegenNode) {
      // single element root is never hoisted so codegenNode will never be
      // SimpleExpressionNode
      const codegenNode = child.codegenNode
      if (codegenNode.type === NodeTypes.VNODE_CALL) {
        makeBlock(codegenNode, context)
      }
      root.codegenNode = codegenNode
    } else {
      // - single <slot/>, IfNode, ForNode: already blocks.
      // - single text node: always patched.
      // root codegen falls through via genNode()
      root.codegenNode = child
    }
  } else if (children.length > 1) {
    // root has multiple nodes - return a fragment block.
    let patchFlag = PatchFlags.STABLE_FRAGMENT
    let patchFlagText = PatchFlagNames[PatchFlags.STABLE_FRAGMENT]
    // check if the fragment actually contains a single valid child with
    // the rest being comments
    if (
      __DEV__ &&
      children.filter(c => c.type !== NodeTypes.COMMENT).length === 1
    ) {
      patchFlag |= PatchFlags.DEV_ROOT_FRAGMENT
      patchFlagText += `, ${PatchFlagNames[PatchFlags.DEV_ROOT_FRAGMENT]}`
    }
    root.codegenNode = createVNodeCall(
      context,
      helper(FRAGMENT),
      undefined,
      root.children,
      patchFlag + (__DEV__ ? ` /* ${patchFlagText} */` : ``),
      undefined,
      undefined,
      true,
      undefined,
      false /* isComponent */
    )
  } else {
    // no children = noop. codegen will return null.
  }
}