actionscript 3.0方向键控制移动

package  {
 import flash.display.Sprite;
 import flash.events.MouseEvent;
 import flash.text.TextField;
 import flash.geom.ColorTransform;
 import flash.events.KeyboardEvent;
 import flash.ui.Keyboard;

 public class hello extends Sprite{
  var sp:Sprite;
  var colorInfo:ColorTransform;
  public function hello() {
   sp = new Sprite();
   sp.graphics.beginFill(0xf00ff00);
   sp.graphics.drawRect(0,0,20,20);
   sp.graphics.endFill();
   sp.buttonMode = false;
   addChild(sp);
   colorInfo = sp.transform.colorTransform;
   stage.addEventListener(KeyboardEvent.KEY_DOWN,press);
  }
  public function press(evt:KeyboardEvent):void
  {
   switch(evt.keyCode)
   {
    case Keyboard.UP:
     sp.y-=2;break;
    case Keyboard.DOWN:
     sp.y+=2;break;
    case Keyboard.LEFT:
     sp.x-=2;break;
    case Keyboard.RIGHT:
     sp.x+=2;break;
    default:break;
   }
  }
 }
}