spyOn找不到我的服务方法

Question

我有一个LoaderService

export class LoaderService {

  private loaderSubject = new Subject<boolean>();
  loaderState = this.loaderSubject.asObservable();

  constructor() { }

  show(){
    console.log("showing progress bar");
    this.loaderSubject.next(true)
  }
  hide(){
    console.log("Hiding progress bar");
    this.loaderSubject.next(false);
  }
}

我正在从show中定义的LoaderService函数调用sendMessage的方法HelperService

sendMessage(url:string, body:any,httpOptions):Observable<HttpEvent<any>> {
    this.loaderService.show(); //HERE
    let observable:Observable<HttpEvent<any>> = this.http.post(url,body,{           headers: new HttpHeaders({ 'Content-Type': 'application/json' }),
      withCredentials: true,
      observe: 'events', //what post should observe (look for). Here it means events. This could be body as well
      responseType: 'json' //responseType tells post how to interpret the body
    });
    return observable.pipe( 
      tap((httpEvent:HttpEvent<any>) => {//tap transparently perform actions or side-effects, such as logging.

        if(httpEvent.type === HttpEventType.Response)
        {
          console.log('response from backend service:', httpEvent);

        }
        else {
          console.log("not an http response")
        }
        return httpEvent; 
      })         
      ,catchError(err=>this.handleError(err)) 
      ,finalize(()=> this.loaderService.hide())); 
  }

我为spec写了sendMessage

describe('sendMessage function test suite: ',()=> {

  let spy:any;
  const httpOptions = {
    headers: new HttpHeaders({'Content-Type': 'application/json'}),
    withCredentials: true, 
    observe: 'events', 
    responseType: 'json' 
  };


  beforeEach(() => {
    TestBed.configureTestingModule({
      providers: [HelperService, HttpClient, HttpHandler,LoaderService],
      imports: [ReactiveFormsModule, HttpClientModule, HttpClientTestingModule]
    });
  });

  fit('should return HttpEvent',
    inject([HttpTestingController,HelperService,HttpClient,LoaderService],(loader:LoaderService, httpClient:HttpClient,httpMock:HttpTestingController)=>{
      const responseData = { result: 'success', ['additional-info']: 'some data from server' };

      spyOn(loader,'show'); 

      let helperService = new HelperService(loader, httpClient);
      let result:Observable<HttpEvent<any>> = helperService.sendMessage('/someURL','someBody',httpOptions);
      let subscription = result.subscribe((event:HttpEvent<any>)=>{
        console.log('got response ',event);
        expect(event).toBeTruthy();
      });
      expect(loader.show).toHaveBeenCalled(); 
      const mockReq = httpMock.expectOne('/someURL');
      expect(mockReq.cancelled).toBeFalsy();
      expect(mockReq.request.responseType).toEqual('json');
      mockReq.flush(responseData);
      httpMock.verify();

  }));
});

运行规范时，出现错误 Error: <spyOn> : show() method does not exist Usage: spyOn(<object>, <methodName>)

我可能做错了什么？