title: Why could not be found 'IVirtualButtonEventHandle' on unity/vuforia?
body: <p>I use vuforia SDK on Unity. When add VirtualButton, i see this problem. </p>
  
  <p><code>Assets/VirtualButtonEventHandle.cs(4,56): error CS0246: The type or namespace name 'IVirtualButtonEventHandle' could not be found. Are you missing a using directive or an assembly reference?</code></p>
  
  <p>VirtualButtonEventHandle.cs :</p>
  
  <pre><code>------------------
  using UnityEngine;
  using System.Collections;
  
  public class VirtualButtonEventHandle : MonoBehaviour **IVirtualButtonEventHandle**{
  ---------------
  </code></pre>