angular2单元测试：无法读取componentInstance.method()未定义的

抛出错误：无法读取未定义的属性“ngOnInit”.

let myComponent: MyComponent;
let myService: MyService;

describe('myComponent',() => {
   beforeEach(() => {
    TestBed.configureTestingModule({
      declarations: [MyComponent],providers: [
      {provide: MyService,useClass: MockMyService} // **--passing Mock service**
  ]
}).compileComponents()
  .then(() => {
    myComponent = TestBed.createComponent(MyComponent).componentInstance;
    myService = TestBed.get(MyService);
    console.log(myService.getData());
  });
});

it('should get the mock data',() => {
   myComponent.ngOnInit(); //-----------> seems like myComponent is not defined at this place,how to resolve this error 
   expect(myComponent.data).toBe(DATA_OBJECT);
  });
});

下面是MyComponent：

@Component({
  selector: 'pm-catalogs',templateUrl: './catalog-list.component.html'
})

export class MyComponent implements OnInit {

 public data: IData[];

 constructor(private _myService: MyService) {

}

public ngOnInit(): void {
this._myService.getData()
  .subscribe(
    data => this.data = data
  //  error => this.errorMessage = <any>error
  );
 }
}

下面是模拟服务

export const DATA_OBJECT: IData[] = [
 {
   'Id': 1,'value': 'abc'
 },{
'Id': 2,'value': 'xyz'
 }];

@Injectable()
export class MockMyService {
 public getData(): Observable<IData[]> {
    return Observable.of(DATA_OBJECT);
  }
}

我是Angular2测试的新手,当myComponent.ngOnInit()在我的spec类中调用myService.getData()方法时,我希望myService.getData返回DATA_OBJECT
请帮助我实现这一目标.