title: Directive need to be refreshed when input change
body: <p>I made the following directive :</p>
  
  <pre><code>import { Directive, ElementRef, HostListener, Input, OnInit } from '@angular/core';
  import { Observable, Subscription } from 'rxjs/Rx';
  
  @Directive({
      selector: '[myDisabled]'
  })
  
  export class DisableDirective implements OnInit {
      private el: HTMLElement;
  
      @Input('myDisabled') isDisable: boolean;
  
      constructor(el: ElementRef) { this.el = el.nativeElement;}
  
      ngOnInit() {
          this.disable();
      }
  
      private disable() {
          this.isDisable ? this.el.style.opacity = '0.65' : this.el.style.opacity = '1';
      }
  }
  </code></pre>
  
  <p>This directive set the opacity of a button depend on an input : isDesable. This setting need to be refreshed when the input change, however, I don't know how to do it.</p>
  
  <p>I use this directive like this :</p>
  
  <pre><code>&lt;button class="btn" [myDisabled]="!sharedDetails.isEnabled"&gt;A button !&lt;/button&gt;
  </code></pre>