export type ResizeManagerEntryCallback = (entry: ResizeObserverEntry) => void;

/** A composition interface for ResizeObserver, allowing multiple observers per element. */
export class ResizeManager {
	#elementMap = new WeakMap<Element, Set<ResizeManagerEntryCallback>>();
	#elementEntry = new WeakMap<Element, ResizeObserverEntry>();

	#vo = new ResizeObserver((entries) => {
		for (const entry of entries) {
			this.#elementEntry.set(entry.target, entry);
			this.#elementMap.get(entry.target)?.forEach((callback) => callback(entry));
		}
	});

	observe(target: Element, callback: ResizeManagerEntryCallback): void {
		let callbacks = this.#elementMap.get(target);

		if (callbacks === undefined) {
			this.#vo.observe(target);
			this.#elementMap.set(target, (callbacks = new Set()));
		} else {
			const entry = this.#elementEntry.get(target);
			if (entry) {
				callback(entry);
			}
		}

		callbacks.add(callback);
	}

	unobserve(target: Element, callback: ResizeManagerEntryCallback): void {
		const callbacks = this.#elementMap.get(target);

		if (callbacks === undefined) return;

		callbacks.delete(callback);

		if (callbacks.size === 0) {
			this.#vo.unobserve(target);
			this.#elementMap.delete(target);
		}
	}
}