UIButton 继承自UIControl，所以可以对UIControlContentHorizontalAlignment进行设置

        btn.setImage(UIImage.init(named: "unSelected"), for: UIControl.State.normal)
        btn.setImage(UIImage.init(named: "selected"), for: UIControl.State.selected)
        btn.contentHorizontalAlignment = .right

　　

https://blog.csdn.net/timtian008/article/details/53611744