Angular 5 测试：如何获取对子组件的引用

我试图在一个Angular 应用程序中测试主组件和子组件之间的交互.我不知道如何在创建父组件时获取对子组件的引用.以下是设置:

child.component.spec.ts个

@Component({template: `<child [data]="model"></child>`})
class HostComponent {
  public model:any;
}

describe('ChildComponent', () => {
  let hostFixture: ComponentFixture<HostComponent>;
  let childFixture: ComponentFixture<ChildComponent>;

  beforeEach(async(() => {
    TestBed.configureTestingModule({
      declarations: [ChildComponent, HostComponent]
    });
  }));

  beforeEach(() => {
    // this creates the child component as well, but how to get to it?
    hostFixture = TestBed.createComponent(HostComponent);

    // this creates a NEW instance of child component, not the one from the Host template
    // so it's not the instance I actually want to test
    childFixture = TestBed.createComponent(ChildComponent);
  });
});

改变hostFixture.componentInstance中的model值实际上不会改变childFixture.componentInstance的data输入值；这就是我如何意识到有两个子组件的实例.

我的问题很简单，我怎样才能得到childFixture来引用HostComponent模板中的组件fixture ，而不是我目前拥有的另一个实例？

The docs美元没用.