import { Injectable, NgZone } from '@angular/core';
import 'rxjs/add/observable/interval';
import 'rxjs/add/operator/merge';
import 'rxjs/add/operator/mergeMap';
import 'rxjs/add/operator/multicast';
import { Observable } from 'rxjs/Observable';
import { Observer } from 'rxjs/Observer';
import { Subject } from 'rxjs/Subject';
import { Subscription } from 'rxjs/Subscription';

@Injectable()
export class PoolingService {
  constructor(private zone: NgZone) {}

  // NOTE: Running the interval outside Angular ensures that e2e tests will not hang.
  execute<T>(
    operation: () => Observable<T>,
    frequency: number = 1000
  ): Observable<T> {
    const subject = new Subject();
    const source = Observable.create((observer: Observer<T>) => {
      let sub: Subscription;
      this.zone.runOutsideAngular(() => {
        const zone = this.zone;
        sub = Observable.interval(frequency).mergeMap(operation).subscribe({
          next(result) {
            zone.run(() => {
              observer.next(result);
            });
          },
          error(err) {
            zone.run(() => {
              observer.error(err);
            });
          }
        });
      });

      return () => {
        if (sub) {
          sub.unsubscribe();
        }
      };
    });

    return source.multicast(subject).refCount().merge(operation());
  }
}