How to execute store.unsubscribe from useEffect using React hooks and Redux

Question

I have a React stateless component using redux and hooks. I need to display the number of items on page load (useEffect) and update it every time I add or remove an item (store.subscribe)

useEffect(() => {
    setState({
        items: store.getState().items.length
    });
}, []);

store.subscribe(() => {
    setState({
        items: store.getState().items.length
    });
});

but this is causing the console to display the warning Can't perform a React state update on an unmounted component. This is a no-op, but it indicates a memory leak in your application. To fix, cancel all subscriptions and asynchronous tasks in a useEffect cleanup function. .

How can I unsubscribe from inside useEffect?

Answer 1

If you set the second parameter of useEffect call to [], the effect callback function would act as ComponentDidMount. If that callback function returns a function, this function will be called just before the component is unmounted(ComponentWillUnmount).

And I guess this setState should be replaced with setItems as below. Please try this code.

const [items, setItems] = useState([]);
useEffect(() => {
    setItems(store.getState().items.length);
    
    
    const unsubscribe = store.subscribe(() => {
        setItems(store.getState().items.length);
    });
    
    return unsubscribe;
}, []);

Answer 2

Return a function from useEffect to do cleanup. So the returned function will be called when the component gets unmounted.

store.subscribe return an unsubscribe function. Save its reference using useRef hook and return the same reference from the useEffect hook.

Read about it in the docs: https://reactjs.org/docs/hooks-effect.html#effects-with-cleanup.

const storeRef = useRef(() => {});
useEffect(() => {
  storeRef.current = store.subscribe(() => {
    setState({
      items: store.getState().items.length
    });
  });

  return storeRef.current;
}, []);

useEffect(() => {
  setState({
    items: store.getState().items.length
  });

  return storeRef.current;
}, []);