core/packages/compiler-dom/src/transforms/Transition.ts

import {
  type CompilerError,
  type ComponentNode,
  ElementTypes,
  type IfBranchNode,
  type NodeTransform,
  NodeTypes,
  isCommentOrWhitespace,
} from '@vue/compiler-core'
import { TRANSITION } from '../runtimeHelpers'
import { DOMErrorCodes, createDOMCompilerError } from '../errors'

export const transformTransition: NodeTransform = (node, context) => {
  if (
    node.type === NodeTypes.ELEMENT &&
    node.tagType === ElementTypes.COMPONENT
  ) {
    const component = context.isBuiltInComponent(node.tag)
    if (component === TRANSITION) {
      return postTransformTransition(node, context.onError)
    }
  }
}

export function postTransformTransition(
  node: ComponentNode,
  onError: (error: CompilerError) => void,
  hasMultipleChildren: (
    node: ComponentNode,
  ) => boolean = defaultHasMultipleChildren,
): () => void {
  return () => {
    if (!node.children.length) {
      return
    }

    if (hasMultipleChildren(node)) {
      onError(
        createDOMCompilerError(DOMErrorCodes.X_TRANSITION_INVALID_CHILDREN, {
          start: node.children[0].loc.start,
          end: node.children[node.children.length - 1].loc.end,
          source: '',
        }),
      )
    }

    // check if it's a single child w/ v-show
    // if yes, inject "persisted: true" to the transition props
    const child = node.children[0]
    if (child.type === NodeTypes.ELEMENT) {
      for (const p of child.props) {
        if (p.type === NodeTypes.DIRECTIVE && p.name === 'show') {
          node.props.push({
            type: NodeTypes.ATTRIBUTE,
            name: 'persisted',
            nameLoc: node.loc,
            value: undefined,
            loc: node.loc,
          })
        }
      }
    }
  }
}

function defaultHasMultipleChildren(
  node: ComponentNode | IfBranchNode,
): boolean {
  // filter out potential comment nodes (#1352) and whitespace (#4637)
  const children = (node.children = node.children.filter(
    c => !isCommentOrWhitespace(c),
  ))
  const child = children[0]
  return (
    children.length !== 1 ||
    child.type === NodeTypes.FOR ||
    (child.type === NodeTypes.IF &&
      child.branches.some(defaultHasMultipleChildren))
  )
}