title: Conditional @HostBinding depending on @Input()
body: <p>I'm trying to bind the CSS class <code>foo</code> to my host component, using <code>@HostBinding</code> depending on a test I do on a <strong>dynamic</strong> variable. 
  But can't manage to make it work properly.<br>
  Here's what I already tried:</p>
  
  <pre><code>export class MyComponent {
      @Input()
      public input: string;
  
      @HostBinding('class.foo')
      public isFoo: Boolean = this.inputIsCorrect();
  
      constructor() {
      }
  
      private inputIsCorrect(){
          return (this.input != 'not correct');
      }
  }
  </code></pre>
  
  <p>How I could make it work ? I was maybe thinking of way to listen to <code>input</code> changes ?</p>