title: Detect when SKNode is added as child
body: <p>I have a Polygon class that is used as data container, but for debugging purposes I want to draw polygons by adding them as children to an SKNode. When I do this I want the Polygon object to add border sprite children to itself. Because of performance reasons I only want to add those sprites when the Polygon has been added as child to another node.</p>
  
  <p>Is there any way that the Polygon object itself can detect that it has been added to the scene, or do I need to tell it by making an extra <code>createSprites</code> call after it has been added to the scene? </p>
  
  <p>I guess I can poll the parent attribute, but I'm looking for something event driven.</p>