countdown.service.ts

import { Injectable } from '@angular/core';
import { BehaviorSubject, Subscription, Observable } from 'rxjs';
import { interval } 'rxjs/operators';

@Injectable()
export class IntervalService {
  private subject = new BehaviorSubject<number>(-1);
  private subscription: Subscription;
  readonly observable$ = this.subject.asObservable();

  start() {
    const countdown = this.limit();
    this.subscription = Observable.interval(1000).subscribe(value => {
      value = countdown - value;
      if (value >= 0)
        this.subject.next(value);
      else
        this.stop();
    });
  }

  stop() {
    if (this.subscription) {
      this.subscription.unsubscribe();
      this.subscription = undefined;
    }
  }

  restart() {
    this.stop();
    this.start();
  }

  limit() {
    return 120;
  }
}