React 深度学习：ReactChildFiber

packages/react-reconciler/src/ReactChildFiber.js

ReactChildFiber 实现了 React 对子元素的协调算法

reconcileSingleElement

reconcileSingleElement 方法用于协调元素类型的 fiber，它接收四个参数:

• returnFiber：父 fiber
• currentFirstChild：当前 fiber
• element: 是新的单个子元素
• expirationTime：过期时间

reconcileSingleElement 算法逻辑：

• returnFiber 可能有多个子 fiber
• 遍历 returnFiber 的子元素，删除 key 与最新的 element.key 不相同的元素。

function reconcileSingleElement(
    returnFiber: Fiber,
    currentFirstChild: Fiber | null,
    element: ReactElement,
    expirationTime: ExpirationTime,
  ): Fiber {
    const key = element.key;
    let child = currentFirstChild;
    while (child !== null) {
      // TODO: If key === null and child.key === null, then this only applies to
      // the first item in the list.
      if (child.key === key) {
        if (
          child.tag === Fragment
            ? element.type === REACT_FRAGMENT_TYPE
            : child.elementType === element.type
        ) {
          deleteRemainingChildren(returnFiber, child.sibling);
          const existing = useFiber(
            child,
            element.type === REACT_FRAGMENT_TYPE
              ? element.props.children
              : element.props,
            expirationTime,
          );
          existing.ref = coerceRef(returnFiber, child, element);
          existing.return = returnFiber;
          if (__DEV__) {
            existing._debugSource = element._source;
            existing._debugOwner = element._owner;
          }
          return existing;
        } else {
          deleteRemainingChildren(returnFiber, child);
          break;
        }
      } else {
        deleteChild(returnFiber, child);
      }
      child = child.sibling;
    }

    if (element.type === REACT_FRAGMENT_TYPE) {
      const created = createFiberFromFragment(
        element.props.children,
        returnFiber.mode,
        expirationTime,
        element.key,
      );
      created.return = returnFiber;
      return created;
    } else {
      const created = createFiberFromElement(
        element,
        returnFiber.mode,
        expirationTime,
      );
      created.ref = coerceRef(returnFiber, currentFirstChild, element);
      created.return = returnFiber;
      return created;
    }
  }