Angular 6 Subject Subscribe is not working

Question

I have an http interceptor which emits a 'string' whenever a request starts and end:

@Injectable({
  providedIn: 'root'
})
export class LoadingIndicatorService implements HttpInterceptor {

  private loadingIndicatorSource = new  Subject<string>();
  private loadingIndicator$ = this.loadingIndicatorSource.asObservable();

  intercept(req: HttpRequest<any>, next: HttpHandler): Observable<HttpEvent<any>> {
    this.updateVisibility('block');
    return next.handle(req)
      .pipe(
        finalize(
          () => {
            this.updateVisibility('none');
          }
        )
      )
      ;
  }

  updateVisibility(state: string) {
    this.loadingIndicatorSource.next(state);
  }

  getLoadingIndicator() {
    return this.loadingIndicator$;
  }
}

This is the component where I injected the service:

export class AppComponent {


  display = 'none';

  constructor(public  authenticationService: AuthenticationService,
              public loadingIndicatorService: LoadingIndicatorService) {
    this.loadingIndicatorService.getLoadingIndicator().subscribe(visibility => {
      console.log(visibility);
      this.display = visibility;
    });

    }
  }

Actually I am trying to show a loading indicator:

 <div [style.display]="display">
    <mat-progress-bar mode="indeterminate" color="warn" ></mat-progress-bar>
  </div>

I flowed this tutorial from the official Angular site.

But the subscribe method never executes.

Why subscribe method is not working?

Answer 1

What i would do is create a SpinnerService and a SpinnerInterceptor and not merge them together.

Make the spinner service allocate the amount of outstanding requests and show the spinner if the outstanding requests are greater than 0.

@Injectable()
export class SpinnerService {
    private requestAmount$ = new BehaviorSubject(0);

public showSpinner$ = this.requestAmount$.asObservable().pipe(map(r => r > 0));

requestStart() {
    this.requestAmount$.next(this.requestAmount$.getValue() + 1);
}

requestEnd() {
    this.requestAmount$.next(this.requestAmount$.getValue() - 1);
}
}

Inside your spinnerInterceptor you can inject the SpinnerService and update it based on each request.

@Injectable()
export class SpinnerInterceptor implements HttpInterceptor {
    constructor(private spinnerService: SpinnerService) {}

intercept(req: HttpRequest<any>, next: HttpHandler) {
        this.spinnerService.requestStart();

        return next.handle(req).pipe(
            finalize(() => {
                this.spinnerService.requestEnd();
            })
        );
    }
}

Inside your app component use the ngOnInit hook do not use the constructor it is against the style guide.

export class AppComponent implements OnInit{


  display = 'none';

  constructor(public  authenticationService: AuthenticationService,
              public spinnerService: SpinnerService) {}
ngOnInit(){
    this.spinnerService.showSpinner$.subscribe(visibility => {
      console.log(visibility);
      this.display = visibility ? 'block': 'none';
    });

    }
  }

Separation makes it easier to read and understand.

Hope this helped!