Angular 带输入变量的主机绑定

我试图有条件地设置CSS样式，比如当更改为true时，背景为红色，当更改为false时，背景为黑色。但我不知道如何通过检查更改标志让hostbinding知道这一点。演示代码如下所示：

 import { Component, HostBinding } from '@angular/core';

@Component({
  selector: 'app-root',
  templateUrl: './app.component.html',
  styleUrls: ['./app.component.css'],
})
export class AppComponent {
  title: string;
  change: boolean;
  @HostBinding('class.someChange') isChanged() {
    this.change = true;
    return this.change;
  }

  constructor() {
    this.title = 'Tour of Heroes';
  }
}

<h1>{{title}}</h1>

<nav>
  <a routerLink = "/dashboard"> Dashboard </a>
  <a routerLink = "/heroes"> Heroes </a>
</nav>
<router-outlet></router-outlet>
<app-messages></app-messages>

对于HTML文件，如下所示：

 import { Component, HostBinding } from '@angular/core';

@Component({
  selector: 'app-root',
  templateUrl: './app.component.html',
  styleUrls: ['./app.component.css'],
})
export class AppComponent {
  title: string;
  change: boolean;
  @HostBinding('class.someChange') isChanged() {
    this.change = true;
    return this.change;
  }

  constructor() {
    this.title = 'Tour of Heroes';
  }
}

<h1>{{title}}</h1>

<nav>
  <a routerLink = "/dashboard"> Dashboard </a>
  <a routerLink = "/heroes"> Heroes </a>
</nav>
<router-outlet></router-outlet>
<app-messages></app-messages>