React: React callback ref cleanup function

[k15a]

At the time React added callback refs the main use case for them was to replace string refs. A lot of the callback refs looked like this:

<div ref={node => this.node = node} />

With the introduction of createRef and useRef this use case is pretty much replaced by these alternatives so the use case of callback refs will shift to advanced use cases like measuring DOM nodes.

It would be nice if you could return a cleanup function from the callback ref which is called instead of the callback with null. This way it will behave more like the useEffect API.

<div ref={node => {
  // Normal ref callback

  return () => {
    // Cleanup function which is called when the ref is removed
  }
}} />

This will be super helpful when you need to set up a Resize-, Intersection- or MutationObserver.

function useDimensions() {
  const [entry, setEntry] = useState()

  const targetRef = useCallback((node) => {
    const observer = new ResizeObserver(([entry]) => {
      setEntry(entry)
    })

    observer.observe(node)

    return () => {
      observer.disconnect()
    }
  }, [])

  return [entry, targetRef]
}

function Comp() {
  const [dimensions, targetRef] = useDimensions()

  return (
    <pre ref={targetRef}>
      {JSON.stringify(dimensions, null, 2)}
    </pre>
  )
}

Currently, if you want to implement something like this you need to save the observer into a ref and then if the callback ref is called with null you have to clean up the observer from the ref.

To be 99% backward compatible we could call both the callback ref with null and the cleanup function. The only case where it isn't backward compatible is if currently someone is returning a function and doesn't expect the function to be called.

function ref(node) {
  if (node === null) {
    return
  }

  // Do something

  return () => {
    // Cleanup something
  }
}

[bisubus]

In your example targetRef looks more like an effect, I'd expect it to be implemented with useRef and useEffect, which has cleanup function. As for callback ref that doesn't hold state, node === null seems like a legit condition to do a cleanup if necessary.

[k15a]

You can't use useEffect there as if you want to pass the ref down to a component useEffect is not necessarily triggered when the subtree updates.

[butchler]

I would also like a useEffect-style API for interacting with refs. It is particularly useful when you want to attach event listeners to an element and clean them up when it is unmounted.

Motivating example

As far as I can tell, this is what's required to safely do this with callback refs:

import React, { useRef, useCallback } from 'react';

export default function CallbackRefComponent() {
  const elementRef = useRef();
  // Can't safely use useCallback() to define event handler because the memoized
  // value may be freed, and we must make sure to pass the same value to
  // addEventListener and removeEventListener
  const onClickRef = useRef();
  const callbackRef = useCallback((node) => {
    // Cleanup event listener on old node
    if (elementRef.current && onClickRef.current) {
      elementRef.current.removeEventListener('click', onClickRef.current);
      onClickRef.current = null;
    }

    elementRef.current = node;

    if (elementRef.current) {
      onClickRef.current = () => console.log('clicked!');
      elementRef.current.addEventListener('click', onClickRef.current);
    }
  }, []);

  return <div ref={callbackRef}>test</div>;
}

On the other hand, with a useEffect-style API, we can define the event handler in the effect callback itself, and the code becomes much simpler:

import { useRefEffect } from '???';
export default function CallbackRefEffectComponent() {
  const callbackRef = useRefEffect((node) => {
    const onClick = () => console.log('clicked!');

    elementRef.current.addEventListener('click', onClick);

    return () => {
      elementRef.current.removeEventListener('click', onClick);
    };
  });

  return <div ref={callbackRef}>test</div>;
}

Attempted implementation

I've attempted implementing useRefEffect as follows, but I have doubts about the safety of this implementation:

import { useRef, useCallback } from 'react';

export default function useRefEffect(callback) {
  const currentCallback = useRef(callback);
  // WARNING: Mutating a ref's value as a side effect of the render function
  // being called is probably a big anti-pattern, and might cause bugs
  //
  // However, useEffect and useLayoutEffect (which we would normally use for
  // updating a ref to the latest value) get triggered before callback refs.
  // Just mutating it in the render function is the only way I could think of to
  // update the value before callback ref is triggered by React.
  currentCallback.current = callback;

  const cleanupRef = useRef(null);

  const callbackRef = useCallback(node => {
    if (node) {
      cleanupRef.current = currentCallback.current(node) || null;
    } else if (cleanupRef.current) {
      cleanupRef.current();
      cleanupRef.current = null;
    }
  }, []);

  return callbackRef;
}

In particular, I don't know if it's safe to update the value of currentCallback.current as a side effect of render in this case. On the one hand, this value is only used in a callback ref, which is triggered by React, so maybe we can assume that the most recent render had the correct value. On the other hand, causing side effects in render is an anti-pattern in general, and it's hard to be confident that this will always do the right thing even if running in concurrent mode and even if the return value of useRefEffect gets passed through multiple layers of components with React.memos/shouldComponentUpdates in between.

I would greatly appreciate it if someone from the React team (or someone who knows more about the internals of React) could judge my implementation of useRefEffect and tell me if it is making any unsafe assumptions.

[k15a]

@butchler I think what you want is exactly what I am proposing just without the need of a new hook.

export default function CallbackRefEffectComponent() {
  const callbackRef = useCallback((node) => {
    // For backwards compatibility you would need to check if the node is null

    const onClick = () => console.log('clicked!');

    node.addEventListener('click', onClick);

    return () => {
      node.removeEventListener('click', onClick);
    };
  }, []);

  return <div ref={callbackRef}>test</div>;
}

[butchler]

I think what you want is exactly what I am proposing just without the need of a new hook.

Basically 🙂

There is a small difference in behavior with my attempt at implementing it as a hook and your proposal: because you are using useCallback, if you need to use props/state/etc. inside the ref callback, it is necessary to pass them to useCallback's second argument, but this also has the side effect of re-triggering the callback ref, which may not always be desired. However, I realize now that this issue is not directly related to this proposal, but is a more general issue with callback refs and hooks, so I'll open a separate issue for that.

Here is another attempt at implementing it via a hook, that more closely matches your proposal:

import { useRef, useCallback } from 'react';

export default function useCallbackRef(rawCallback, deps) {
  const cleanupRef = useRef(null);
  const callback = useCallback((node) => {
    if (cleanupRef.current) {
      cleanupRef.current();
      cleanupRef.current = null;
    }

    if (node) {
      cleanupRef.current = rawCallback(node);
    }
  }, deps);

  return callback;
}

Then you could just use useCallbackRef in place of useCallback.

Unfortunately, this has a pretty big downside, which is that useCallbackRef will probably not be checked by the exhaustive-deps linting rule. This isn't an issue with my proposed useRefEffect implementation, because it does not require you to pass deps and only triggers the callback when the ref actually changes (but as a result has to use a potentially dirty hack to keep a reference to the latest version of the callback).

[butchler]

One reason that the original proposal (of adding new behavior to the ref prop) might not work is that callback refs get called with null when the element is unmounted or changed, but when returning a cleanup function, you probably only want it to to be called with an actual node once, and never get called with null (otherwise you'd have to manually check if the argument is null, which somewhat defeats the purpose).

React could try to detect if a cleanup function has been returned and change the behavior for that particular callback ref, but that seems quite confusing and would probably make it easy to introduce bugs.

It would be nice if it's possible to implement this as a custom hook outside of React itself, but I'm not sure how it can be implemented reliably.

[butchler]

Yet another attempt at implementing this as a custom hook:

import { useRef, useCallback } from 'react';

export default function useCallbackRef(rawCallback) {
  const cleanupRef = useRef(null);
  const callback = useCallback((node) => {
    if (cleanupRef.current) {
      cleanupRef.current();
      cleanupRef.current = null;
    }

    if (node) {
      cleanupRef.current = rawCallback(node);
    }
  }, [rawCallback]);

  return callback;
}

Usage:

const callback = useCallbackRef(useCallback((node) => {
  node.addEventListener(...);
  return () => {
    node.removeEventListener(...);
  };
}, []));

It's a bit more cumbersome to use, since you have to call both useCallback and useCallbackRef, but at least it allows the deps to be checked by the exhaustive-deps linting rule.

@k15a What do you think of this approach?

[vincerubinetti]

I'm very new to React hooks and still don't quite get how they work under the hood. That being said, here's something I came up with that seems to work:

export const useBbox = () => {
  const ref = useRef();
  const [bbox, setBbox] = useState({});

  const set = () =>
    setBbox(ref && ref.current ? ref.current.getBoundingClientRect() : {});

  useEffect(() => {
    set();
    window.addEventListener('resize', set);
    return () => window.removeEventListener('resize', set);
  }, []);

  return [bbox, ref];
};

Then to use it:

const SignIn = () => {
  const [bbox, ref] = useBbox();

  return (
    <>
      <Button ref={ref}>open popup</Button>
      <Popup anchorBbox={bbox}>popup content</Popup>
    </>
  );
};

This is modeled after this example in the React docs, but is modified to also update the bbox any time the window is resized (which may or may not be enough for your use case; use this with caution).

It seems to unmount the listener properly, but there could definitely be something I missed. Comments are welcome.

---

If anyone reading this is looking for the same functionality, here is another solution I found:
https://gist.github.com/morajabi/523d7a642d8c0a2f71fcfa0d8b3d2846

Also here's an example of a library using hooks to update positions of a popup, but with added debouncing and other stuff:
https://github.com/mui-org/material-ui/blob/master/packages/material-ui/src/Popover/Popover.js

[butchler]

@vincerubinetti Indeed, useRef+useEffect is a perfectly fine alternative to using a callback ref in most cases, and when using that approach you can just do your cleanup using useEffect. However, there are still some (rare) cases where it's necessary or useful to use a callback ref instead, in which case doing cleanup is a little more cumbersome compared to using useEffect.

So I'd say that is slightly out of scope of this issue on the React project itself, although it is still a good alternative to know about when writing code that uses React.

[bekliev]

I wrote simplest solution

👍 Pros

• too simple
• only 5 lines of code
• doesn't violate return statement (doesn't require the returning has to be function always as in solutions above)
• no refs (internally)
• not tied up to DOM-elements, it best fits for you if you:
  
  
  
  • have async recursion in your callbacks
  
  
  • subscribe to another module of your app (outside of React, like: Siebel OpenUI, Angular, Vue, Svelte, etc.)
  
  
• and yes - it can work with DOM-elements

👎 Cons

• still looking

💭 Thoughts

• It's a new way of thinking about hooks (pass baseCallback that returns array of [callback, cleanup]) - you have to prepare a bit for this

✔ Solution itself

import React from 'react';

/**
 * memoized callback (a.k.a. useCallback) with cleanup ability/feature
 * @param {Function} baseCallback callback that returns array with [callback, cleanup]
 *    callback - which will be passed to React.useCallback
 *    cleanup - which will be called in React.useEffect's cleanup function
 * @param {Array} deps standard hook dependencies
 * @return {Function} memoized callback by React.useCallback
 *
 * @example
 *   const callback = useCallbackCleanup(() => {
 *     let stop;
 *     return [
 *       function self(counter) => {
 *         // here goes your code
 * 
 *         if (someCondition) {
 *           setTimeout(() => !stop && self(counter + 1), 10 * 1000);
 *           return 'delayed';
 *         }
 *
 *         // and return whatever you have to (bool, string, array, etc.)
 *         return true;
 *       },
 *       function cleanup() {
 *         stop = true;
 *       },
 *     ]
 *   }, [a, b, c])
 *
 * @example
 *   const callbackRef = useCallbackCleanup(() => {
 *     let node;
 *     const listner = (e) => {};
 *
 *     return [
 *       (theNode) => {
 *         node = theNode;
 *         node.addEventListener('event', listner);
 *       },
 *       () => node.removeEventListener('event', listner)
 *     ]
 *   }, [a, b, c])
 *
 *   return <div ref={callbackRef}>Some Text</div>
 */
export default function useCallbackCleanup(baseCallback, deps) {
  const [callback, cleanup] = baseCallback();
  React.useEffect(() => () => typeof cleanup === 'function' && cleanup(), deps);
  return React.useCallback(callback, deps);
}

I've posted this solution as a gist too:
https://gist.github.com/bekliev/896709f1fffcde65a8dd55845e492ffa

[otakustay]

I made a useEffectRef in @huse/effect-ref to solve exactly the same problem

[butchler]

Given that updating the behavior of the ref prop would probably be tricky (see https://github.com/facebook/react/issues/15176#issuecomment-512682315), and that several people have been able to implement some version of this as custom hooks, it seems unlikely that this will be added to React and this issue can probably be closed.

That said, for anybody who wants to use the useCallback-based approach that people have mentioned in the above comments along with the this pattern for accessing often-changing values inside useCallback, be warned that you may run into a really weird edge case if you get really unlucky: https://github.com/facebook/react/issues/16154