export class LoadingService {
  isLoading$: Observable<boolean>;
  private loadingCount$ = new BehaviorSubject(0);

  constructor() {
    this.isLoading$ = this.loadingCount$
      .pipe(
        map(count => count > 0),
        distinctUntilChanged()  
      );
  }

  inc() {
    this.updateCount(c => c + 1);
  }

  dec() {
    this.updateCount(c => (c > 0
      ? c - 1
      : 0));
  }

  public track<T>(destroyed$: Observable<void>) {
    return (target: Observable<T>) =>
      Observable.create((observer: Observer<T>) => {
        this.inc();

        const sub = target
          .pipe(takeUntil(destroyed$))
          .subscribe(observer);

        return () => {
          this.dec();
          sub.unsubscribe();
        };
      });
  }

  private updateCount(fn: (count: number) => number) {
    this.loadingCount$.next(fn(this.loadingCount$.value));
  }
}