Swift-拖动按钮从一个位置到另一个位置

我的代码 –

override func viewDidLoad() {
    super.viewDidLoad()
    // Do any additional setup after loading the view,typically from a nib.
     let btn_swap = UIButton.buttonWithType(UIButtonType.Custom) as UIButton!
    btn_swap .setTitle("Drag Me",forState: UIControlState.Normal)
    btn_swap.backgroundColor = UIColor.yellowColor()

    btn_swap.addTarget(self,action: "wasDragged:",forControlEvents: UIControlEvents.TouchDragInside)

    btn_swap.frame = CGRectMake((self.view.bounds.size.width - 100)/2.0,(self.view.bounds.size.height - 50)/2.0,100,50)

    self.view.addSubview(btn_swap)
    self.creation_of_btn()
 }

func wasDragged (buttn : UIButton,event :UIEvent)
{
    var touch : UITouch = event.touchesForView(buttn) . anyObject() as UITouch
    var previousLocation : CGPoint = touch .previousLocationInView(buttn)
     var location : CGPoint = touch .locationInView(buttn)
    var delta_x :CGFloat = location.x - previousLocation.x
      var delta_y :CGFloat = location.y - previousLocation.y
    buttn.center = CGPointMake(buttn.center.x + delta_x,buttn.center.y + delta_y);

}