@Injectable({
  providedIn: 'root',
})
export class WorkerService {

  /** [...] **/

  private createWorkerObservableForTask(worker: Worker, task: BackgroundTask): Observable<BackgroundTaskMessage> {
    return Observable.create((observer: any) => {
      worker.addEventListener('message', event => {
        const message: BackgroundTaskMessage = event.data;
        if (message.taskId === task.id) {
          observer.next(message);
          if (message.taskStatus === 'TERMINATED') {
            observer.complete();
            worker.terminate();
          }
        }
      });
      worker.addEventListener('error', error => observer.error(error));
      this.notify(task.id, 'start', task);
    });
  }

  /** [...] **/

}