javascript - 从 React Hook useEffect 内部对 'timeInterval' 变量的赋值将在每次渲染后丢失

Question

我需要每 6 秒自动重新渲染一次，我将组件定义如下。

const KioskPage = () => {

  const [time, setTime] = useState(Date.now())

  useEffect(() => {

    timeInterval = setInterval(() => setTime(Date.now()), 60000)

    return () => {
      clearInterval(timeInterval)
    }
  }, [])
}

但我收到了通知:

Assignments to the 'timeInterval' variable from inside React Hook useEffect will be lost after each render. To preserve the value over time, store it in a useRef Hook and keep the mutable value in the '.current' property. Otherwise, you can move this variable directly inside useEffect react-hooks/exhaustive-deps

为什么会这样？以及如何解决这个问题？

问候

Answer 1

Assignments to the 'timeInterval' variable from inside React Hook useEffect will be lost after each render.

举例说明:

const KioskPage = () => {

  const [time, setTime] = useState(Date.now())

  let timeInterval;
  // ^^^^^ this piece of code gets run on each render, when state/prop changes.
  // this will, in practice, clear the `timeInterval` value set by your effect below, when the component re-renders after being mounted.

  useEffect(() => {

    timeInterval = setInterval(() => setTime(Date.now()), 60000)
    // ^^^^ this assignment gets run once when the component mounts

    return () => {
      clearInterval(timeInterval)
    }
  }, []);
  
  return (/* render something*/);
}

您可以按照建议通过“将此变量直接移动到 useEffect 中”来解决此问题。

const KioskPage = () => {
  const [time, setTime] = useState(Date.now())

  useEffect(() => {
    const timeInterval = setInterval(() => setTime(Date.now()), 6000);
    // ^^^^^^^^^^^^^^^

    return () => {
      console.log('clearing!');
      clearInterval(timeInterval)
    }
  }, []);

  const formatted = new Date(time).toLocaleTimeString();
  return (
    <h1>Time: {formatted}</h1>
  );
}