swift / UIViewを左右に揺らす

       var transform:CGAffineTransform = CGAffineTransformIdentity
        let duration:Double = 0.5
        transform = CGAffineTransformMakeRotation(CGFloat(0.25*M_PI))
        UIView.animateWithDuration(duration,animations:{()-> Void in
            sender.transform = transform
            })
            { (Bool)-> Void in
             UIView.animateWithDuration(duration,animations:{ () -> Void in
                sender.transform = CGAffineTransformIdentity
                }){ (Bool) -> Void in
            }
        }