Angular 2+中带有参数的单元测试指令

我有一个指令，可以在鼠标悬停时更改元素的背景颜色，如下所示。

import {Directive, ElementRef, HostListener, Input} from '@angular/core';

@Directive({
  selector: '[appHighlightme]'
})
export class HighlightmeDirective {

  @Input('appHighlightme') color : string
  constructor(private el:ElementRef) { }

  @HostListener('mouseenter') onMouseEnter(){
    this.highlight(this.color || 'yellow');
  }

  @HostListener('mouseleave') onMouseLeave(){
    this.highlight('inherit');
  }

  highlight(color){
    this.el.nativeElement.style.backgroundColor = color;
  }
}

我正在尝试为此指令编写单元测试用例，如下所示

import { HighlightmeDirective } from './highlightme.directive';
import {ChangeDetectionStrategy, Component, DebugElement, ViewChild} from '@angular/core';
import {async, ComponentFixture, TestBed} from '@angular/core/testing';
import {By} from '@angular/platform-browser';

@Component({
  selector: 'my-test-component',
  template: '<a [appHighlightme]="color" >test</a>'
})
export class TestComponent {
  color:string = 'blue';
}

describe('HighlightmeDirective', () => {

  let component: TestComponent;
  let fixture: ComponentFixture<TestComponent>;
  let inputEl: DebugElement;

  beforeEach(() => {
    TestBed.configureTestingModule({
      declarations: [
        TestComponent,
        HighlightmeDirective
      ]
    })

    fixture = TestBed.createComponent(TestComponent);
    component = fixture.componentInstance;
    inputEl = fixture.debugElement.query(By.directive(HighlightmeDirective));

  });

  it('detect hover changes', () => {
    inputEl.triggerEventHandler('mouseenter', {});
    fixture.detectChanges();
    expect(inputEl.nativeElement.style.backgroundColor).toBe(component.color);
    inputEl.triggerEventHandler('mouseleave', {});
    fixture.detectChanges();
    expect(inputEl.nativeElement.style.backgroundColor).toBe('inherit');
  });

  it('should create an instance', () => {
    const directive = new HighlightmeDirective(inputEl);
    expect(directive).toBeTruthy();
  });
});

该指令在其他组件中运行良好。它接受ts文件中定义的color参数变量，并在将元素悬停时用作bg颜色。但是，当我尝试对其进行单元测试时，伪指令未检测到从TestComponent传递的color参数，并且测试用例失败并显示以下错误消息。

错误：预期“黄色”为“蓝色”。-由于将黄色设置为默认的悬停颜色