void rootEntities_CollectionChanged(object sender, TrackingCollectionChangedEventArgs e)
{
switch (e.Action)
{
case NotifyCollectionChangedAction.Add:
EntitySystem.Add((Entity)e.Item);
break;
case NotifyCollectionChangedAction.Remove:
EntitySystem.Remove((Entity)e.Item);
break;
default:
throw new NotSupportedException();
}
}