eventEmitter.ts

/**
 * https://gist.github.com/kiprasmel/75de24e48c8c442e55d4771768d7ba49
 */

export type Cb = () => void;

export class EventEmitter<Event extends string> {
	listeners: Map<Event, Cb[]> = new Map();

	/**
	 * can be used directly inside useEffect's `return`,
	 * because returns the cleanup fn.
	 */
	addEventListener(event: Event, cb: Cb): Cb {
		const callbacks: Cb[] = this.listeners.get(event) || [];

		callbacks.push(cb);
		this.listeners.set(event, callbacks);

		const cleanup = () => this.removeEventListener(event, cb);
		return cleanup;
	}

	removeEventListener(event: Event, removableCb: Cb): void {
		const callbacks: Cb[] | undefined = this.listeners.get(event);
		if (!callbacks) return;

		const newCallbacks = callbacks.filter((cb) => cb !== removableCb);
		this.listeners.set(event, newCallbacks);
	}

	emit(event: Event): void {
		const cbs: Cb[] = this.listeners.get(event) || [];
		for (const cb of cbs) {
			cb();
		}
	}
}