import { useEffect, useRef } from "react";

/**
 * Runs `run` once each time `value` leaves `watched`, however it leaves. Not on first render, and always with the
 * latest `run`, so a caller can pass a function it rebuilds every render.
 */
export const useRunWhenLeaving = <T,>(value: T, watched: T, run: () => void): void => {
  const previous = useRef(value);
  const latestRun = useRef(run);
  latestRun.current = run;

  useEffect(() => {
    if (previous.current === watched && value !== watched) latestRun.current();
    previous.current = value;
  }, [value, watched]);
};