测试轮询服务

我有一个Angular服务,其任务是每隔60秒轮询一次API,以验证客户端是否仍然连接:

export class AppService {

  get isOnline$(): Observable<{ online: boolean }> {
    return this.isOnlineSource.asObservable();
  }
  private isOnlineSource = new BehaviorSubject<{ online: boolean }>({ online: true });

  constructor(private http: HttpClient) {
    timer(0, 60000).switchMap(() =>
      this.http.get(`${environment.apiUrl}/api/assessments/amionline`, environment.httpOptions)
        .map(x => {
          return { online: true };
        })
        .catch((err, caught) => {
          return of({ online: false });
        })
    ).
      subscribe(data => this.isOnlineSource.next(data));
  }

}

如何测试此代码?我认为fakeasync块加上tick()就足以触发Observable,但httpmock没有收到任何请求。

测试代码:

 it('should return online equals false.', fakeAsync(() => {
   appService.isOnline$.subscribe(data => {
    expect(data.online).toEqual(false);
   });
   tick(70000);
   httpMock.expectOne(url).flush(undefined, mockErrorResponse);
   httpMock.verify();
  }));
 });

注:测试使用httpclienttestingmodule,httpmock为httptestingcontroller