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@gragland
Last active February 6, 2019 22:06
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import { useState, useEffect, useRef } from 'react';
// Usage
function App() {
// Ref for the element that we want to detect whether on screen
const ref = useRef();
// Call the hook passing in ref and root margin
// In this case it would only be considered onScreen if more ...
// ... than 300px of element is visible.
const onScreen = useOnScreen(ref, '-300px');
return (
<div>
<div style={{ height: '100vh' }}>
<h1>Scroll down to next section 👇</h1>
</div>
<div
ref={ref}
style={{
height: '100vh',
backgroundColor: onScreen ? '#23cebd' : '#efefef'
}}
>
{onScreen ? (
<div>
<h1>Hey I'm on the screen</h1>
<img src="https://i.giphy.com/media/ASd0Ukj0y3qMM/giphy.gif" />
</div>
) : (
<h1>Scroll down 300px from the top of this section 👇</h1>
)}
</div>
</div>
);
}
// Hook
function useOnScreen(ref, rootMargin = '0px') {
// State and setter for storing whether element is visible
const [isIntersecting, setIntersecting] = useState(false);
useEffect(() => {
const observer = new IntersectionObserver(
([entry]) => {
// Update our state when observer callback fires
setIntersecting(entry.isIntersecting);
},
{
rootMargin
}
);
if (ref.current) {
observer.observe(ref.current);
}
return () => {
observer.unobserve(ref.current);
};
}, []); // Empty array ensures that effect is only run on mount and unmount
return isIntersecting;
}
@ianobermiller
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I made a couple small changes:

https://gist.github.com/ianobermiller/1146d469e22561f88a9b4d81ea477e4c

  1. You can remove root entirely, since it defaults to the viewport anyway (also document.querySelector('body').current is always undefined, could be document.body but isn't needed anyway)
  2. Rename margin to rootMargin for clarity and conciseness

@Andarist
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If you pass [] as inputs you should cache ref.current in a local variable in the effect - otherwise you are risking a leak because the ref is mutable and can change over time - therefore u might call unobserve with the wrong element

@gragland
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@ianobermiller Good call, updated!

@gragland
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@mattfysh and @Andarist: Good points! So I'm thinking it would make sense to pass ref.current in input array so that effect is recalled if it changes and then wrap entire effect function body in if (ref.current) { ... } so that it does nothing if ref.current is falsy. Thoughts?

@QuentinRoy
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I agree. I do believe ref.current should be passed as argument of useEffect line 58. Currently if the ref changes, nothing happens and the previous is leaked.

@QuentinRoy
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You may also consider returning the ref from the hook instead of passing it as argument, c.f. https://usehooks.com/useHover/.

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