需求：

有一个组件中有两个ref，一个f是orwardRef暴露出的ref，另一个是内部维护的ref

如何将这个两个ref都赋到一个element上？

const Element = forwardRef((props, ref) => {
    const insiderRef = useRef(null);

    return (
        <div></div>
    )
})

解决方案：

????????ref可以接收一个回调函数，这个回调函数在组件挂载或者卸载的时候被调用，函数参数为组件实例或DOM元素。

可以利用此机制，实现一个函数为两个ref赋值。

hooks实现如下：

import { MutableRefObject, Ref, useMemo } from 'react';

function setRef<T>(
  ref: MutableRefObject<T | null> | ((instance: T | null) => void) | null | undefined,
  value: T | null,
): void {
  if (typeof ref === 'function') {
    ref(value);
  } else if (ref) {
    ref.current = value;
  }
}

export default function useForkRef<InstanceA, InstanceB>(
  refA: Ref<InstanceA> | null | undefined,
  refB: Ref<InstanceB> | null | undefined,
): Ref<InstanceA & InstanceB> | null {
  /**
   * This will create a new function if the ref props change and are defined.
   * This means react will call the old forkRef with `null` and the new forkRef
   * with the ref. Cleanup naturally emerges from this behavior.
   */
  return useMemo(() => {
    if (refA == null && refB == null) {
      return null;
    }
    return (refValue) => {
      setRef(refA, refValue);
      setRef(refB, refValue);
    };
  }, [refA, refB]);
}