import { Observable, Subscribable } from 'rxjs';

/**
 * Observable operator which calls the provided callbacks when observable
 * is subscribed to or unsubscribed from for the first time.
 *
 * Callbacks may be called multiple times if the last subscriber
 * is unsubscribed from it and then resubscribed again.
 *
 * @param onSubscribe - function which is called when observable
 *  is subscribed to for the first time
 * @param onUnsubscribe - function which is called when the last subscriber
 *  is unsubscribed from the observable
 */
export function onFirstSubscription<T>(
  onSubscribe: (() => void) | undefined,
  onUnsubscribe: (() => void) | undefined
) {
  return (source: Subscribable<T>) => {
    let count = 0;
    return new Observable<T>((observer) => {
      if (count === 0) {
        onSubscribe?.();
      }
      const sub = source.subscribe(observer);
      count++;
      return () => {
        sub.unsubscribe();
        count--;
        if (count === 0) {
          onUnsubscribe?.();
        }
      };
    });
  };
}