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import { RefObject, useEffect, useRef, useState } from 'react';
/**
* Returns whether an element has touched/crossed the viewport's upper boundary,
* plus or minus a given vertical offset.
* @param offsetY A vertical offset (in pixels) to add or subtract
* when determining if the observed element has crossed
* the viewport's upper boundary
* @returns A tuple with a reference to the observed element,
* and a boolean indicating whether it's crossed
* the viewport's upper boundary
*/
const useDetectWindowBoundary = (
offsetY = 0
): [ RefObject< HTMLDivElement | undefined >, boolean | undefined ] => {
const elementRef = useRef< HTMLDivElement >();
const observerRef = useRef< IntersectionObserver >();
const [ borderCrossed, setBorderCrossed ] = useState< boolean | undefined >( undefined );
useEffect( () => {
// We can't do anything without a valid reference to an element on the page
if ( ! elementRef.current ) {
return;
}
// If the observer is already defined, no need to continue
if ( observerRef.current ) {
return;
}
const handler = ( entries: IntersectionObserverEntry[] ) => {
if ( ! entries.length ) {
return;
}
// When the observed element crosses out of the top of the viewport,
// its bounding rectangle will always have a Y coordinate
// smaller than the value of our offsetY parameter.
setBorderCrossed( entries[ 0 ].boundingClientRect.y < offsetY );
};
// Only trigger the handler when the observed element's
// intersection ratio becomes 1.0 (fully visible), or
// when it becomes less than 1.0 (not fully visible)
observerRef.current = new IntersectionObserver( handler, {
rootMargin: `${ -1 * offsetY }px 0px 0px 0px`,
threshold: [ 1 ],
} );
observerRef.current.observe( elementRef.current );
// Stop observing when this hook is unmounted
return () => observerRef.current?.disconnect?.();
}, [ offsetY ] );
return [ elementRef, borderCrossed ];
};
export default useDetectWindowBoundary;
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