index.ts

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

const DEFAULT_THRESHOLD = [0];
const DEFAULT_ROOT_MARGIN = '0px';

type IntersectionEntryCallback = (entry: IntersectionObserverEntry) => void;

type ObserverEntry = {
	observer: IntersectionObserver;
	observe: (target: Element, callback: IntersectionEntryCallback) => void;
	unobserve: (target: Element, callback: IntersectionEntryCallback) => void;
};

const observers = new Map<Element | Document, Map<string, ObserverEntry>>();

const getObserverEntry = (options: IntersectionObserverInit): ObserverEntry => {
	const root = options.root ?? document;

	let rootObservers = observers.get(root);

	if (!rootObservers) {
		rootObservers = new Map();
		observers.set(root, rootObservers);
	}

	const opt = JSON.stringify([options.rootMargin, options.threshold]);

	let observerEntry = rootObservers.get(opt);

	if (!observerEntry) {
		const callbacks = new Map<Element, Set<IntersectionEntryCallback>>();

		const observer = new IntersectionObserver((entries) => {
			for (const entry of entries) {
				const cbs = callbacks.get(entry.target);
				if (cbs === undefined || cbs.size === 0) {
					continue;
				}

				for (const cb of cbs) {
					setTimeout(() => {
						cb(entry);
					}, 0);
				}
			}
		}, options);

		observerEntry = {
			observer,
			observe(target, callback) {
				let cbs = callbacks.get(target);

				if (!cbs) {
					// If target has no observers yet - register it
					cbs = new Set();
					callbacks.set(target, cbs);
					observer.observe(target);
				}

				// As Set is duplicate-safe - simply add callback on each call
				cbs.add(callback);
			},
			unobserve(target, callback) {
				const cbs = callbacks.get(target);

				// Else branch should never occur in case of normal execution
				// because callbacks map is hidden in closure - it is impossible to
				// simulate situation with non-existent `cbs` Set
				if (cbs) {
					// Remove current observer
					cbs.delete(callback);

					if (cbs.size === 0) {
						// If no observers left unregister target completely
						callbacks.delete(target);
						observer.unobserve(target);

						// If not tracked elements left - disconnect observer
						if (callbacks.size === 0) {
							observer.disconnect();

							rootObservers.delete(opt);

							if (rootObservers.size === 0) {
								observers.delete(root);
							}
						}
					}
				}
			},
		};

		rootObservers.set(opt, observerEntry);
	}

	return observerEntry;
};

export type UseIntersectionObserverOptions = {
	/**
	 * An Element or Document object (or its react reference) which is an
	 * ancestor of the intended target, whose bounding rectangle will be
	 * considered the viewport. Any part of the target not visible in the visible
	 * area of the root is not considered visible.
	 */
	root?: RefObject<Element | Document | null> | Element | Document | null;
	/**
	 * A string which specifies a set of offsets to add to the root's bounding_box
	 * when calculating intersections, effectively shrinking or growing the root
	 * for calculation purposes. The syntax is approximately the same as that for
	 * the CSS margin property; The default is `0px`.
	 */
	rootMargin?: string;
	/**
	 * Array of numbers between 0.0 and 1.0, specifying a ratio of intersection
	 * area to total bounding box area for the observed target. A value of 0.0
	 * means that even a single visible pixel counts as the target being visible.
	 * 1.0 means that the entire target element is visible.
	 * The default is a threshold of `[0]`.
	 */
	threshold?: number[];
};

/**
 * Tracks intersection of a target element with an ancestor element or with a
 * top-level document's viewport.
 *
 * @param target React reference or Element to track.
 * @param options Like `IntersectionObserver` options but `root` can also be
 * react reference
 */
export function useIntersectionObserver<T extends Element>(
	target: RefObject<T | null> | T | null,
	{
		threshold = DEFAULT_THRESHOLD,
		root: r,
		rootMargin = DEFAULT_ROOT_MARGIN,
	}: UseIntersectionObserverOptions = {},
): IntersectionObserverEntry | undefined {
	const [state, setState] = useState<IntersectionObserverEntry>();

	useEffect(() => {
		const tgt = target && 'current' in target ? target.current : target;
		if (!tgt) {
			return;
		}

		let subscribed = true;
		const observerEntry = getObserverEntry({
			root: r && 'current' in r ? r.current : r,
			rootMargin,
			threshold,
		});

		const handler: IntersectionEntryCallback = (entry) => {
			// It is reinsurance for the highly asynchronous invocations, almost
			// impossible to achieve in tests, thus excluding from LOC
			if (subscribed) {
				setState(entry);
			}
		};

		observerEntry.observe(tgt, handler);

		return () => {
			subscribed = false;
			observerEntry.unobserve(tgt, handler);
		};
		// eslint-disable-next-line react-hooks/exhaustive-deps
	}, [target, r, rootMargin, ...threshold]);

	return state;
}