addSubview to an SpringView

I use the Spring library (http://ift.tt/1tMhamB) to add iOS animations to my Swift project. I have a custom subclass ChooseButton of SpringView and I want to add a subview (mySubView in this case) to this view :

class ChooseButton: SpringView {
     enum TypeButton {
        case YES, NO
    }

    var type:TypeButton!
    var size:CGFloat = 90

    var enabled: Bool!
    var delegateButton: ChooseButtonDelegate?

    init (type: TypeButton) {
        super.init(frame: CGRectMake(0,0,size,size))
        self.type = type
        enabledButton()
        if type == TypeButton.YES {
            self.backgroundColor = CustomColor.Emeralde
            self.center = CGPointMake(gScreenSize.width*3/4, gScreenSize.height-self.frame.size.height)
        } else {
            self.backgroundColor = CustomColor.Alizarin
            self.center = CGPointMake(gScreenSize.width/4, gScreenSize.height-self.frame.size.height)
        }
        self.layer.cornerRadius = size/2

        var mySubView = UIView(frame: frame)
        mySubView.backgroundColor = UIColor.blackColor()
        addSubview(mySubView)
    }

But my Subview does not shows up... I can't see why.