Singleton Pattern in Actionscript 3.0

AS3Singleton.as

package {
        public final class Singleton {
                /** 
                 * Storage for the singleton instance. 
                 */
                public static const instance:Singleton = new Singleton ( SingletonLock );
 
                /**
                 * Constructor
                 * 
                 * @param lock The Singleton lock class to pevent outside instantiation.
                 */
                public function Singleton(lock:Class) {
                    // Verify that the lock is the correct class reference.
                    if ( lock != SingletonLock )        {
                        throw new Error( "Invalid Singleton access.  Use Singleton.instance." );
                    }
                }
 
                public static function getInstance():Singleton {
                        return Singleton.instance;
                }
        }
}
 
/**
 * This is a private class declared outside of the package
 * that is only accessible to classes inside of the Singleton.as
 * file.  Because of that, no outside code is able to get a
 * reference to this class to pass to the constructor, which
 * enables us to prevent outside instantiation.
 */
internal class SingletonLock { }