[ActionScript 3.0] Singleton , 싱글톤 페턴

package {
    public class Singleton {
        private static var instance:Singleton;
        private static const checker:Object = {};
 
        public function Singleton(initObj:Object) {
            if(initObj != checker) {
                throw new Error("Private constructor!");
            }
        }
 
        public static function getInstance():Singleton {
            if(instance == null) {
                instance = new Singleton(checker);
            }
            return instance;
        }
    }
}