Question 305659 in Javascript Question posted in
A very good W3school tutorial can be found here.

Javascript – React: Accessing Updated State from Custom Hook Inside Event Handler

In a React component, I’m facing a challenge with state synchronization between an event handler and a custom hook. The event handler updates a state variable, and I need to access the latest value of another state that is derived from a custom hook depending on this updated state.

We use a custom hook, and define a state variable and an event handler.

The event handler sets the value of state variable a to variable c (which depends on the event argument) and then needs to use the latest value of b, which is returned by a custom hook that depends on a.

function MyComponent(props) {
  const [a, setA] = useState(null);
  const b = useCustomHook(a);

  const eventHandler = useCallback(async (event) => {
    try {
      const c = event.data;

      setA(c);

      if (b) { // we need to use b here, but it is stale (one step behind)
        // logic
      }
      else {
        // more logic
      }
    } catch (e) {
      console.error(`Error:`, e);
    }

  }, [b]);
}

The issue is that b is always one step behind the event handler. When running the logic in the event handler, its value has not been updated yet.

How can one access the latest value of b (which depends on a, which depends on c, which depends on event) inside of the event handler?

I have tried using only setting the state variables inside of the event handler and use an effect for the rest of the logic, but the effect does not only run when the event handler is triggered.

What solution would be in line with the React paradigm? How can I restructure my code to solve this issue?

Addendum. I originally posted the simplified problem above which abstracted away the details of my use case, but I understand now that, sometimes, seeing the actual code can help provide a tailored answer to the problem. If you can think of a solution that is more React-friendly than the solution that I posted as an answer to this question, I would be glad to know about it!

Below is the code I had before I found a solution to my problem.

const [currentNode, setCurrentNode] = useState(null);
const dispatch = useCustomDispatch();

const isUITreeNodeOpen = useCustomSelector('isUITreeNodeOpenSelector', currentNode?.id);

const cellDoubleClickedListener = useCallback(async (event) => {
  try {
    const UITreeNode = event.data;
    setCurrentNode(UITreeNode); // setA(c)

    if (!UITreeNode.folder) {
      if (isUITreeNodeOpen) { // if(b)
        await serviceWorkerInterface.getAPI().switchToTab(UITreeNode);
      }
      else {
        await chrome.tabs.getCurrent()
        .then((currentTab) => {
          dispatch('openBookmarkThunk', {UITreeNode, currentTab})
        });
      }
    }
  } catch (e) {
    console.error(`Failed to open the bookmark or switch to it:`, e);
  }
}, [isUITreeNodeOpen]);
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2

Answers


  1. Chosen as BEST ANSWER
    0

    After a lot of brainstorming, code writing and debugging (with the help of the great ChatGPT), I finally came up with some code that seems to solve my problem. It may not be well-designed, simple, efficient, aligned with React's best practices, etc., but it does seem to work correctly in my use case.

    I would be glad to get feedback on this solution, and if you know of a more efficient, React-friendly way of handling this problem, I would love to know about it!

    Below is a revised version of my code that aims to provide a solution to the simplified problem (using a, b, and c) that I originally posted.

    Implementation:

    const useCustomHookArgs = ['foo', 'bar']

// useCustomHook needs to be adapted to take a flag based on which it may or may not reset its state.
// I reset its state when using it in conjunction with useEventDependentEffect, but otherwise
// pass shouldResetState = false in the rest of my code

const useCustomHook = (shouldResetState = false, ...useCustomHookArgs) => {
  const [b, setB] = useState(null);
  const [shouldReset, setShouldReset] = useState(false);

  // Use an effect to set b and an internal shouldReset flag based on the shouldResetState arg
  useEffect(() => {
    const fetchData = async () => {
      // Logic to fetch data based on args
      const fetchedData = /* fetch logic */;

      setB(fetchedData);

      if (shouldResetState) {
        setShouldReset(true); // Flag needed to reset b after it has been used
      }
    };

    unsubscribeFromStore = subscribeToStore();
    fetchData();

    return () => {
      unsubscribeFromStore();
    };
  }, [...useCustomHookArgs]);

  // Use an effect to reset b (and the shouldReset flag) after it has been used
  useEffect(() => {
    if (shouldReset) {
      setB(null);
      setShouldReset(false);
    }
  }, [shouldReset]);

  return b;
};

    // useEventDependentEffect is a custom hook that captures the logic needed to
// capture a at the time of the eventHandler call, and then compute b from a

const useEventDependentEffect = (eventHandler, effectFunction, aToCapture, computeB, computeBArgs) => {
  const [trigger, setTrigger] = useState(0);
  const capturedARef = useRef();
  const [capturedA, setCapturedA] = useState(null);
  const [computedB, setComputedB] = useState(null);


  const shouldResetState = true;

  const newComputedB = computeB(shouldResetState, ...computeBArgs, capturedA);


  useEffect(() => {
    if (newComputedB != null && newComputedB !== computedB) {
      setComputedB(newComputedB);
    }
  }, [newComputedB, computedB]);

  // Enhanced event handler to capture a and trigger the effect
  const enhancedEventHandler = useCallback((...args) => {
    eventHandler(...args);
    capturedARef.current = aToCapture.current;
    setTrigger(trigger + 1);  // Trigger the effect
  }, [eventHandler, trigger]);

  // Effect that runs the desired logic with the captured a
  useEffect(() => {
    if (trigger) {
      const newCapturedA = capturedARef.current;

      // Compare newCapturedA with the current capturedA
      if (newCapturedA !== capturedA) {
        setCapturedA(newCapturedA);
      }

      if(computedB != null) {
        // Call effectFunction with the computed b to run the logic
        // that needed to run below setA(c) in the original problem
        effectFunction(computedB, capturedARef.current);

        setTrigger(0);  // Reset the trigger
        setComputedB(null);
        setCapturedA(null);
      }
    }
  }, [trigger, effectFunction, capturedA, computeB, computedB]);

  return enhancedEventHandler;
};

    Usage in MyComponent:

    function MyComponent(props) {
  const [a, setA] = useState(null);
  const aRef = useRef(a);

  const eventHandler = (event) => {
    const c = event.data;

    setA(c);
    aRef.current = c;
  };

  const effectFunction = async (b, a) => {
    try {
      if (b) {
        // logic using a
      } else {
        // more logic using a
      }
    } catch (e) {
      console.error(`Error:`, e);
    }
  };

  const aToCapture = {
    a: aRef
  }

  const handleEvent = useEventDependentEffect(eventHandler, effectFunction, aToCapture, useCustomHook, useCustomHookArgs);

  // Render logic and event binding using handleEvent
}

  2. 0

    I ll suggest to use useEffect hook and useRef (to avoid rendering problems) :

    
    //Code

    const latestBRef = useRef(b);

    useEffect(() => {
      latestBRef.current = b;
    }, [b]);

    // inside your eventHandler , use latestBRef.current instead of b

    Remove b as a dependecy from useCallback.
    If this is not helping , please put that in a code sandbox, so we can try different solutions.

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