void entityRepositoryObserver(EntityRepository repo)
        {
            var observer = repo.CreateObserver(
                Matcher.Position,
                EntityCollectionEventType.OnEntityAdded
                );

            var entities = observer.collectedEntities;

            foreach (var e in entities)
            {
                // do something
            }
            observer.ClearCollectedEntites();


            // ----------------------------
            observer.Deactivate();
        }