将父组件注入构造函数时如何测试组件

Question

我正在尝试在Angular 8中测试子组件

子组件取决于其父组件功能，并且父组件不应该知道子组件，它可以是几种不同类型的子组件之一

在子组件中，我通过将其注入到构造函数中来获得对父组件的引用：

export class ChildComponent {

  constructor(
    @Inject(forwardRef(() => ParentComponent)) public parent: ParentComponent
  ) { }

}

问题是，当我尝试为子组件编写单元测试时，Karma会引发错误：No provider for ParentComponent

我在提供程序中添加了ParentComponent组件，该错误消失了，但是它仍然不起作用，例如致电childComponent.parent.ngOnInit()会得到TypeError: Cannot read property 'ngOnInit' of undefined

如何让父组件在子组件测试中工作？

import { ComponentFixture, TestBed } from '@angular/core/testing';

describe('Create Child Component', () => {
  let component: ChildComponent;
  let fixture: ComponentFixture<ChildComponent>;
  let componentElement: HTMLElement;

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


  beforeEach(() => {
    fixture = TestBed.createComponent(ChildComponent);
    component = fixture.componentInstance;
    componentElement = fixture.debugElement.nativeElement;

    fixture.detectChanges();
  });

  it('should create', () => {
    expect(component).toBeTruthy();
    expect(component.parent).toBeTruthy();
    
    component.parent.ngOnInit();
  });
});