title: Angular 2- change class from one component to another
body: <p>I have 2 components- A,B
  And want from A change B's class.for example change ng-valid to ng-invalid</p>
  
  <p><code>&lt;A  #a&gt;&lt;/A &gt;
  &lt;B&gt; &lt;/B&gt;</code></p>
  
  <p>From B I want give class to A,or change A's ng-valid class to ng-invalid.</p>