由于所有元素都继承自UIView，因此可以尝试

extension UIView {
 func addUnderline() {
   // create underline view add it with addSubview
 }
}

你可以从任何元素调用它

UIButton().addUnderline()
UILabel().addUnderline()

等等

如果你真的想使用Swift泛型，你可以写这样的代码：

func addUnderline<V>(inView view: V, withHeight height: CGFloat, andColor color: UIColor) where V: UIView {
    let underlineHeight: CGFloat = height
    let underlineView = UIView(frame: CGRect(x: 0, y: view.frame.height - underlineHeight, width: view.frame.width, height: underlineHeight))
    underlineView.backgroundColor = color
    view.addSubview(underlineView)
  }