反应钩。 为什么不设置功能改变状态？

Question

const Navbar = () => {
  const prevScrollY = React.useRef<number>();

  const [isHidden, setIsHidden] = React.useState(false);

  React.useEffect(() => {
    const onScroll = () => {
      const scrolledDown = window.scrollY > prevScrollY.current!;
      console.log(`is hidden ${isHidden}`);
      if (scrolledDown && !isHidden) {
        setIsHidden(true);
        console.log(`set hidden true`);
      } else if (!scrolledDown && isHidden) {
        console.log(`set hidden false. THIS NEVER HAPPENS`);
        setIsHidden(false);
      }

      prevScrollY.current = window.scrollY;
    };

    console.log("adding listener");

    window.addEventListener("scroll", onScroll);
    return () => {
      window.removeEventListener("scroll", onScroll);
    };
  }, []);

  return isHidden ? null : <div>navbar</div>;
};

Full example

The console.log(`is hidden ${isHidden}`); always prints false, and the setIsHidden(true) always gets triggered but never seems to change the state. Why? Basically the isHidden is never setto false, except after the useState initialization.

Answer 1

Basically what happens is that your useEffect runs only twice on mount and on unmount ( and that's apparently intentional ), however the unwanted side-effect of this is that the value of isHidden that you're checking against in the onScroll method gets closured at it's initial value (which is false ) - forever ( until the unmount that is ).

You could use functional form of the setter, where it receives the actual value of the state and put all the branching logic inside it. Something like:

  setIsHidden(isHidden => { // <- this will be the proper one
     const scrolledDown = window.scrollY > prevScrollY.current!;
     console.log(`is hidden ${isHidden}`);
     if (scrolledDown && !isHidden) {
        console.log(`set hidden true`);
        return true;  
      } else if (!scrolledDown && isHidden) {
        console.log(`set hidden false. THIS NEVER HAPPENS`);
        return false;
      } else {
        // ...