Rocket.Chat/client/providers/createReactiveSubscriptionFactory.ts

import { Tracker } from 'meteor/tracker';
import { Subscription, Unsubscribe } from 'use-subscription';

interface ISubscriptionFactory<T> {
	(...args: any[]): Subscription<T>;
}

export const createReactiveSubscriptionFactory = <T>(
	computeCurrentValueWith: (...args: any[]) => T,
): ISubscriptionFactory<T> => (...args: any[]): Subscription<T> => {
	const computeCurrentValue = (): T => computeCurrentValueWith(...args);

	const callbacks = new Set<Unsubscribe>();

	let currentValue = computeCurrentValue();

	let computation: Tracker.Computation | undefined;
	const timeout = setTimeout(() => {
		computation = Tracker.autorun(() => {
			currentValue = computeCurrentValue();
			callbacks.forEach((callback) => {
				callback();
			});
		});
	}, 0);

	return {
		getCurrentValue: (): T => currentValue,
		subscribe: (callback): Unsubscribe => {
			callbacks.add(callback);

			return (): void => {
				clearTimeout(timeout);

				callbacks.delete(callback);

				if (callbacks.size === 0) {
					computation && computation.stop();
				}
			};
		},
	};
};