构造函数中的SpyOn服务函数调用

我遇到了一个问题,即试图监视构造函数中调用的服务函数调用。测试是基本的,只是验证函数调用是否实际被调用。

 beforeEach(async(() => {
    TestBed.configureTestingModule({
      declarations: [
        AppComponent
      ],
      providers: [TestService]
    }).compileComponents();
  }));

beforeEach(() => {

    fixture = TestBed.createComponent(AppComponent);
    let service = fixture.debugElement.injector.get(TestService);

    component = fixture.componentInstance;
    spyOn(service , "start").and.callThrough();

    fixture.detectChanges();
  });

 it('start gets called', () => {

    expect(service .start).toHaveBeenCalled();
  })

至于AppComponent,在构造函数中它只是调用服务。开始() 我认为问题在于,spyOn在创建组件后被调用,但在注入服务之前,我如何监视服务?ie:

fixture = TestBed.createComponent(AppComponent);
let service = fixture.debugElement.injector.get(TestService);