title: Adding validators for existing formgroup
body: <p>I have a form with currently one field <code>description</code> and I'm building that form from a model as given below</p>
  
  <p><strong>view-model.ts</strong></p>
  
  <pre><code>export class ViewModel {
    description = '';
  }
  </code></pre>
  
  <p><strong>view-action.component.ts</strong></p>
  
  <pre><code>export class ViewActionComponent {
    constructor(public fb: FormBuilder) {
      this.groupForm = this.fb.group(new ViewModel());
    }
  ...
  }
  </code></pre>
  
  <p>The form works perfectly fine. Now the problem is I need to add validator for that description field (like entering only text). Is there any way to add a validator for description in <code>view-model.ts</code> file so that I wont modify <code>view-action.component.ts</code> file?</p>
  
  <p>So the basic idea is, If I need to add another field with validator, I can do it in the ViewModel itself.</p>