I am learning attribute directives in Angular 2. When using `@Input` alias in attribute directives, It will not work, why?

component
---------

```
<p appHighlight = "color">Hightlight Me</p>

```

directive
---------

```
export class HighlightDirective {

  @Input('appHighlight') highlightcolor: string;

  constructor(
    // ElementRef is a service that grants direct access to the DOM element through its nativeElement property.
    private el: ElementRef
  ) {
    // el.nativeElement.style.backgroundColor = 'yellow';
  }

  // @HostListener decorator lets you subscribe to events of the DOM element that hosts an attribute directive
  @HostListener('mouseenter') onMouseEnter() {
    this.highlight(this.highlightcolor || 'red');
  }

  @HostListener('mouseleave') onmouseleave() {
    this.highlight(null);
  };

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

}

```