title: Angular 6 - ng-dirty and ng-invalid not working as expected
body: <p>I'm trying out some form validation and seeing strange behaviour from the classes that are applied to a form control.</p>
  
  <p><code>ng-dirty</code> is not being applied when I change input value from 'Rob' to 'Ro' but it does get applied when I change the value further to 'R'.</p>
  
  <p><code>ng-invalid</code> does not get applied when I clear out form field and required attribute is present. If I blur out though, the class gets applied.</p>
  
  <p>Link to the code - <a href="https://stackblitz.com/edit/angular-form-classes" rel="nofollow noreferrer">https://stackblitz.com/edit/angular-form-classes</a></p>
  
  <p>What am I doing wrong? Please let me know thanks.</p>