title: Angular2 - Keyup require a clarification
body: <p>In my app, conditionally i am adding a class. and when the user enter something i am checking the value and accordingly i am adding the class name. it works fine.</p>
  
  <p>but it only updates on set of <code>(keyup)='0'</code> - setting some value on <code>keyup</code>. this is not like <code>angular 1</code> here. </p>
  
  <p>so any one explain me why do we set the <code>(keyup)=0</code> here? and what it do for us?</p>
  
  <p>here is my code :</p>
  
  <pre><code>import {Component} from "angular2/core"
  
  @Component({
  
      selector : 'my-component',
  
      template : `
                  &lt;h2&gt;My Name is: {{name}} 
                      &lt;span [class.is-awesome]="formReplay.value === 'yes' "&gt;So good&lt;/span&gt;
                  &lt;/h2&gt;
                  &lt;input type="text" #formReplay (keyup)="0" /&gt;
                  `,
  
      styles  : [`
  
          .is-awesome{
              color:green;
          }
  
      `]
  
  })
  
  export class MyComponent { 
      name = "My Name";   
  }
  </code></pre>