title: How to detect when a marker is found in AR.js
body: <p>I'm trying to detect when a marker if found/lost in ar.js, while using a-frame.</p>
  
  <p>From what I see in the <a href="https://github.com/jeromeetienne/AR.js/blob/master/aframe/build/aframe-ar.js" rel="nofollow noreferrer">source code</a>, when the marker is found, a 'getMarker' event should be fired, moreover artoolkit seems to dispatch a markerFound event.</p>
  
  <p>I tried to listen to those events on the <code>&lt;a-scene&gt;</code>, or on the <code>&lt;a-marker&gt;</code>, but it seems I'm either mistaken, or i need to get deeper to the <code>arController</code>, or <code>arToolkit</code> objects. </p>
  
  <p>When i log the scene, or the marker, i only get references to the attributes, which don't seem to have the above objects attached.(like <code>marker.arController</code>, or <code>marker.getAttribute('artoolkitmarker').arController</code>)</p>
  
  <p>Did anybody tried this and have any tips how to do this ?</p>