为什么我的视图总是顺时针旋转而不是逆时针旋转？

UIView.animateWithDuration(1.0, delay: 1, usingSpringWithDamping: 1, initialSpringVelocity: 1, options: UIViewAnimationOptions.AllowUserInteraction, animations: {
                var angle = degreesToRadians(180)
                self.imageView.transform = CGAffineTransformMakeRotation(-angle)
            }, completion: { (value: Bool) in })
UIView.animateWithDuration(1.0, delay: 1, using