我最喜欢的写法：

/*

 Copyright(c) 2010 Coamy - 2010.7

*/

package {

    /**

     * A Singleton.

     *

     */

    public class Singleton {

        // Singleton instance

        protected static var instance:Singleton;

        // Message Constants

        protected const SINGLETON_MSG:String = "Singleton Singleton already constructed!";

        /**

         * Constructor.

         *

         * This is a Singleton,

         * so you should not call the constructor directly,

         * but instead call the static Singleton Factory method getInstance().

         *

         * @throws Error Error if Singleton instance has already been constructed

         *

         */

        public function Singleton() {

            if(instance != null) throw Error(SINGLETON_MSG);

            instance = this;

        }

        public static function getInstance():Singleton {

            if(instance == null) instance = new Singleton();

            return instance;

        }

    }

}

最近再次看了一下AStar算法，并把理论转化成了 代码。后来在一个2.5D的格子上测试了一把，哈哈，很不错。