title: How to detect change in model for input field from directive in angular2
body: <p>I have this code for a textarea, it is working great with the ngModel and updating live, however i would like my-custom-directive to know when the model bound to this textarea is changing.</p>
  
  <p>How can i detect a model change for this textarea in the my-custom-directive?</p>
  
  <pre><code>  &lt;textarea my-custom-directive class="green" [(ngModel)]="customertext"&gt;&lt;/textarea&gt;
  </code></pre>