如何在窗口事件监听器中访问无状态组件状态？

Question

I have a stateless component which needs to listen on keyboard event. It adds keydown listener when the component is mounted and remove it when the component is unmounted. There is a state test is boolean value. It is set to true when the component is mounted. But in the keydown event listener, its value always false . It looks like the listener doesn't take the state reference. What's wrong with my code?

const { useEffect, useState } = React;


const Comp = () => {
  const [test, setTest] = useState(false);
  const keyPressHandler = (e) => {
    setTest(!test);
    console.log(test);
  }
  useEffect(() => {
    setTest(true);
    window.addEventListener('keydown', keyPressHandler);
    return () => {
      window.removeEventListener('keydown', keyPressHandler);
    };
  }, []);

  return (
    <div className="test">
      hello {test + ""}
    </div>
  );
};

Answer 1

You can re-run useEffect when test change which will make the listener always have the current value.

  useEffect(() => {
    setTest(true);
    window.addEventListener('keydown', keyPressHandler);
    return () => {
      window.removeEventListener('keydown', keyPressHandler);
    };
  }, [test]);

and you should remove the setTest from the useEffect since it will keep changing test to true whenever you use setTest() .