IntersectionObserver with React & Hooks

Question

I'm trying to track element visibility with React/Hooks and the Intersection Observer API. However, I can't figure out how to set up observation with "useEffect". Does anybody have any idea how could I do that? Mine solution does not work...

function MainProvider({ children }) {
  const [targetToObserve, setTargetToObserve] = useState([]);

  window.addEventListener("load", () => {
    const findTarget = document.querySelector("#thirdItem");
    setTargetToObserve([findTarget]);
  });

  useEffect(() => {
    const observer = new IntersectionObserver(
      ([entry]) => {
        if (entry.intersectionRatio === 0.1) {
          console.log("It works!");
        }
      },
      {
        root: null,
        rootMargin: "0px",
        threshold: 0.1
      }
    );
    if (targetToObserve.current) {
      observer.observe(targetToObserve.current);
    }
  }, []);

  return (
    <main>
     <div className="Section-item" id="firstItem"></div>
     <div className="Section-item" id="secondItem"></div>
     <div className="Section-item" id="thirdItem"></div>
    </main>
  );
}

Answer 1

JavaScript

Hook

import { useEffect, useState, useRef } from 'react';

export function useOnScreen(ref) {
  const [isOnScreen, setIsOnScreen] = useState(false);
  const observerRef = useRef(null);

  useEffect(() => {
    observerRef.current = new IntersectionObserver(([entry]) =>
      setIsOnScreen(entry.isIntersecting)
    );
  }, []);

  useEffect(() => {
    observerRef.current.observe(ref.current);

    return () => {
      observerRef.current.disconnect();
    };
  }, [ref]);

  return isOnScreen;
}

Usage:

import { useRef } from 'react';
import useOnScreen from './useOnScreen';

function MyComponent() {
  const elementRef = useRef(null);
  const isOnScreen = useOnScreen(elementRef);

  console.log({isOnScreen});

  return (
    <div>
      <div style={{ paddingBottom: '140vh' }}>scroll to element...</div>
      <div ref={elementRef}>my element</div>
    </div>
  );
}

TypeScript

Hook

import { useEffect, useState, useRef, RefObject } from 'react';

export default function useOnScreen(ref: RefObject<HTMLElement>) {
  const observerRef = useRef<IntersectionObserver | null>(null);
  const [isOnScreen, setIsOnScreen] = useState(false);

  useEffect(() => {
    observerRef.current = new IntersectionObserver(([entry]) =>
      setIsOnScreen(entry.isIntersecting)
    );
  }, []);

  useEffect(() => {
    observerRef.current.observe(ref.current);

    return () => {
      observerRef.current.disconnect();
    };
  }, [ref]);

  return isOnScreen;
}

Usage:

import { useRef } from 'react';
import useOnScreen from './useOnScreen';

function MyComponent() {
  const elementRef = useRef<HTMLDivElement>(null);
  const isOnScreen = useOnScreen(elementRef);

  console.log({isOnScreen});

  return (
    <div>
      <div style={{ paddingBottom: '140vh' }}>scroll to element...</div>
      <div ref={elementRef}>my element</div>
    </div>
  );
}

https://codesandbox.io/s/useonscreen-uodb1?file=/src/useOnScreen.ts