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
UIButton设置UIControlContentHorizontalAlignment调整文字对齐方式
原文：https://www.cnblogs.com/qingzZ/p/10570070.html