从 prop 事件处理程序设置时，React state 重置

Question

Cannot for the life of me figure out what is going on, but for some reason when "Click me" is clicked, the number increments as you'd expect. When a click is triggered by the Child component, it resets the state and ALWAYS prints 0.

function Child(props: {
    onClick?: (id: string) => void,
}) {
    const ref = useCallback((ref) => {
        ref.innerHTML = 'This Doesnt';
        ref.addEventListener('click',() => {
            props.onClick!('')
        })
    }, [])
    return (<div ref={ref}></div>)
}

function Parent() {
  const [number, setNumber] = useState(0);
  return <div>
            <div onClick={() => {
                setNumber(number + 1);
                console.log(number);
            }}>
                This Works
            </div>
            <Child
                onClick={(id) => {
                    setNumber(number + 1);
                    console.log(number);;
                }}
            />
        </div>
}

And here is a demonstration of the problem: https://jscomplete.com/playground/s333177

Answer 1

Both the onClick handlers in parent component are re-created on every render, rightly so as they have a closure on number state field.

The problem is, the onClick property sent to Child component is used in ref callback, which is reacted only during initial render due to empty dependency list. So onClick prop received by Child in subsequent renders does not get used at all.

Attempt to resolve this error, by either removing dependency param or sending props.onClick as in dependency list, we land into issue due to caveat mentioned in documentation. https://reactjs.org/docs/refs-and-the-dom.html

So you add null handling, and you see your updated callback now getting invoked, but... all earlier callbacks are also invoked as we have not removed those event listeners.

const ref = useCallback((ref) => {
    if(!ref) return;
    ref.innerHTML = 'This Doesnt';
    ref.addEventListener('click',() => {
        props.onClick!('')
    })
}, [props.onClick])

I believe this just an experiment being done as part of learning hooks, otherwise there is no need to go in roundabout way to invoke the onClick from ref callback. Just pass it on as prop to div .

Edit:
As per your comment, as this is not just an experiment but simplification of some genuine requirement where click handler needs to be set through addEventListener , here is a possible solution:

const ref = useRef(null);
useEffect(() => {
    if(!ref.current) return;
    ref.current.innerHTML = 'This Doesnt';
    const onClick = () => props.onClick!('');
    ref.current.addEventListener('click',onClick)

    // return the cleanup function to remove the click handler, which will be called before this effect is run next time.
    return () => {ref.current.removeEventListener("click", onClick)}
}, [ref.current, props.onClick]);

Basically, we need to use useEffect so that we get a chance to remove old listener before adding new one.

Hope this helps.

Answer 2

function Child(props: {
    onClick?: (id: string) => void,
}) {
    function handleClick() {
      props.onClick('')
    }
    const ref = useRef();
    useEffect(() => {
      ref.current.innerHTML = 'This Doesnt';
      ref.current.addEventListener('click', handleClick)
      return () => { ref.current.removeEventListener('click', handleClick); }
    }, [props.onClick])

    return (<div ref={ref}></div>)
}

@ckder almost works but console.log displayed all numbers from 0 to current number value. Issue was with event listener which has not been remove after Child component umount so to achive this I used useEffect and return function where I unmount listener.