一个MovieClip的移动操作

参考网上的一个例子，自己实现了相应的操作，具体看代码

package {

    import flash.display.MovieClip;
    import flash.display.Sprite;
    import flash.events.KeyboardEvent;
    import flash.text.TextField;
    
    public class MovieClipExample extends MovieClip {
        public var bl : Boolean;
        public var ball : MovieClip;
        
        public function MovieClipExample() {
            ball = new MovieClip();
            ball.graphics.beginFill( 0xCCFF00 );
            ball.graphics.drawCircle( 50, 60, 40 );
            ball.graphics.endFill();
            addChild( ball );
            
            stage.addEventListener( KeyboardEvent.KEY_DOWN, KeyDown );
            stage.addEventListener( KeyboardEvent.KEY_UP, KeyUp );
        }
        
        public function KeyDown( e : KeyboardEvent ) : void
        {
            if ( e.ctrlKey && e.keyLocation == 1 )
            {
                bl = true;
            }    
            
            if ( bl )
            {
                switch( e.keyCode )
                {
                case 37 :
                        ball.x -= 20;
                        break;
                case 38:
                        ball.y -= 20;
                        break;
                case 39:
                        ball.x += 20;
                        break;
                case 40 :
                        ball.y += 20;
                        break;
                }
            }
        }
        
        public function KeyUp( e : KeyboardEvent ) : void
        {
            if ( e.ctrlKey == false )
            {
                bl = false;
            }
        }
    }
}