React DOM 片段引用共享

// 比较空片段的文档位置
export function compareDocumentPositionForEmptyFragment<TPublicInstance>(
  fragmentFiber: Fiber,
  parentHostInstance: TPublicInstance,
  otherNode: TPublicInstance,
  getPublicInstance: (fiber: Fiber) => TPublicInstance,
): number {
  let result;
  // If the fragment has no children, we can use the parent and
  // siblings to determine a position.
  // 如果片段没有子元素，我们可以使用父元素和兄弟元素来确定位置。
  const parentResult = parentHostInstance.compareDocumentPosition(otherNode);
  result = parentResult;
  if (parentHostInstance === otherNode) {
    result = Node.DOCUMENT_POSITION_CONTAINS;
  } else {
    if (parentResult & Node.DOCUMENT_POSITION_CONTAINED_BY) {
      // otherNode is one of the fragment's siblings. Use the next
      // sibling to determine if its preceding or following.
      // otherNode 是片段的其中一个兄弟节点。使用下一个兄弟节点来判断它是前一个还是后一个。
      const nextSiblingFiber = getNextSiblingHostFiber(fragmentFiber);
      if (nextSiblingFiber === null) {
        result = Node.DOCUMENT_POSITION_PRECEDING;
      } else {
        const nextSiblingInstance = getPublicInstance(nextSiblingFiber);
        const nextSiblingResult =
          nextSiblingInstance.compareDocumentPosition(otherNode);
        if (
          nextSiblingResult === 0 ||
          nextSiblingResult & Node.DOCUMENT_POSITION_FOLLOWING
        ) {
          result = Node.DOCUMENT_POSITION_FOLLOWING;
        } else {
          result = Node.DOCUMENT_POSITION_PRECEDING;
        }
      }
    }
  }

  result |= Node.DOCUMENT_POSITION_IMPLEMENTATION_SPECIFIC;
  return result;
}