在 React Native 中卸载组件期间，清理功能在 UseEffect Hook 中不起作用

Question

In react native app, there are two components A and B in App.js which toggle with k state change.

App.js

...

const App = () => {
      const [ k, setK ] = useState(false);

      const toggleK = () => setK(!k);

      if(k) {
            return <A />;
      }else {
            return <B toggleK={toggleK} />;
      }
};
...

In A , setInterval is initialized in useffect . It calls async function every 10 seconds. But when it unmounts on K state change in App.js , the cleaning function is not run (no A unmounting... is logged) and so does the clearInterval .

Any thing I'm doing wrong here?

...

const A = () => {
      const [ someState, setSomeState ] = useState(...);

      let timer;

      useEffect(() => {
            if(!timer) {
                  timer = setInterval(async () => await run_async_func(), 10000);
            }

            return () => {
                  console.log('A unmounting...');
                  clearInterval(timer);
            };
      }, [ someState ]);
};
...

Answer 1

Try this with useRef().

const A = () => {
 const timer = useRef()
  const [ someState, setSomeState ] = useState(...);


  useEffect(() => {
        if(!timer) {
              timer.current = setInterval(async () => await run_async_func(), 10000);
        }

        return () => {
              console.log('A unmounting...');
              clearInterval(timer.current);
        };
  }, [ someState ]);
};

Answer 2

Resolved the issue

• What was causing the issue?

A. Overlooked the someState in useEffect dependency array. Basically, cleaner function will only run if any variable in dependency array mutates. So, I used another useEffect without any dependencies (see below). Thanks @TayyabMazhar for pointing out this

...
const A = () => {
      const [ someState, setSomeState ] = useState(...);

      let timer;

      useEffect(() => {
            if(!timer) {
                  timer = setInterval(async () => await run_async_func(), 10000);
            }
      }, [ someState ]);

      useEffect(() => {
            return () => {
                 console.log('A unmounting...');
                 clearInterval(timer);
            };
      }, []);
};
...

Answer 3

you can use this hook i create this one

    import { useEffect, useRef } from 'react';

export const useInterval = (callback: () => void, delay: number): void => {
    const savedCallback = useRef(null);

    // Remember the latest callback.
    useEffect(() => {
        savedCallback.current = callback;
    }, [callback]);

    // Set up the interval.
    useEffect(() => {
        function tick() {
            savedCallback.current();
        }
        if (delay !== null) {
            const id = setInterval(tick, delay);
            return () => clearInterval(id);
        }
    }, [delay]);
};