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HTML

              
                <div id="root"></div>
              
            
!

CSS

              
                
              
            
!

JS

              
                import { useEffect, useState, useRef, useCallback } from "https://cdn.skypack.dev/react";
import ReactDOM from "https://cdn.skypack.dev/react-dom";

function debounce(func, wait) {
	var timeout;
	return function() {
		var context = this, args = arguments;
		var later = function() {
			timeout = null;
			func.apply(context, args);
		};
		var callNow = immediate && !timeout;
		clearTimeout(timeout);
		timeout = setTimeout(later, wait);
		if (callNow) func.apply(context, args);
	};
}

const DEFAULT_OPTIONS = {
  config: { attributes: true, childList: true, subtree: true },
  debounceTime: 0
};
/**
 * This custom hooks abstracts the usage of the Mutation Observer with React components.
 * Watch for changes being made to the DOM tree and trigger a custom callback.
 * @param {Element} targetEl DOM element to be observed
 * @param {Function} cb callback that will run when there's a change in targetEl or any
 * child element (depending on the provided options)
 * @param {Object} options
 * @param {Object} options.config check [options](https://developer.mozilla.org/en-US/docs/Web/API/MutationObserver/observe)
 * @param {number} [options.debounceTime=0] a number that represents the amount of time in ms
 * that you which to debounce the call to the provided callback function
 */
function useMutationObservable(targetEl, cb, options = DEFAULT_OPTIONS) {
  const [observer, setObserver] = useState(null);

  useEffect(() => {
    if (!cb || typeof cb !== "function") {
      console.warn(
        `You must provide a valida callback function, instead you've provided ${cb}`
      );
      return;
    }
    const { debounceTime } = options;
    const obs = new MutationObserver(
      debounceTime > 0 ? debounce(cb, debounceTime) : cb
    );
    setObserver(obs);
  }, [cb, options, setObserver]);

  useEffect(() => {
    if (!observer) return;

    if (!targetEl) {
      console.warn(
        `You must provide a valid DOM element to observe, instead you've provided ${targetEl}`
      );
    }

    const { config } = options;

    try {
      observer.observe(targetEl, config);
    } catch (e) {
      console.error(e);
    }

    return () => {
      if (observer) {
        observer.disconnect();
      }
    };
  }, [observer, targetEl, options]);
}

function App() {
  const listRef = useRef();
  const [count, setCount] = useState(2);
  const [fruits, setFruits] = useState(["apple", "peach"]);
  const onListMutation = useCallback(
    (mutationList) => {
      setCount(mutationList[0].target.children.length);
    },
    [setCount]
  );
  useMutationObservable(listRef.current, onListMutation);

  return (
    <div>
      <span>{`Added ${count} fruits`}</span>
      <br />
      <button
        onClick={() => setFruits([...fruits, `random fruit ${fruits.length}`])}
      >
        Add random fruit
      </button>
      <ul ref={listRef}>
        {fruits.map((f) => (
          <li key={f}>{f}</li>
        ))}
      </ul>
    </div>
  );
}

const el = document.querySelector("#root");
ReactDOM.render(<App/>, el);
              
            
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999px

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