我尝试的代码获得了输出U所遇到的问题。

btnSort.backgroundColor = UIColor.redColor()->设置背景颜色并检查

 let btnSort   = UIButton(type: UIButtonType.System) as UIButton! //this is Swift2.0 in this place use your code
    btnSort.frame =  CGRectMake(2, 74, 140, 40)
    btnSort.tintColor = UIColor.whiteColor()
    btnSort.setImage(UIImage(named:"youtube16x16.png"), forState: UIControlState.Normal)
    btnSort.imageEdgeInsets = UIEdgeInsets(top: 6,left: 100,bottom: 6,right: 14)
    btnSort.titleEdgeInsets = UIEdgeInsets(top: 0,left: -30,bottom: 0,right: 34)
    btnSort.setTitle("SORT", forState: UIControlState.Normal)
    btnSort.layer.borderWidth = 1.0
    btnSort.backgroundColor = UIColor.redColor() --> set the background color and check 
    btnSort.layer.borderColor = UIColor.whiteColor().CGColor
    btnSort.addTarget(self, action: Selector("showSortTbl"), forControlEvents: UIControlEvents.TouchUpInside)
    self.view.addSubview(btnSort)

Swift3

 let btnSort   = UIButton(type: .system)
    btnSort.frame =  CGRect(x: 2, y: 74, width: 140, height: 40)
    btnSort.tintColor = UIColor.white
    btnSort.setImage(UIImage(named:"youtube16x16.png"), for: .normal)
    btnSort.imageEdgeInsets = UIEdgeInsets(top: 6,left: 100,bottom: 6,right: 14)
    btnSort.titleEdgeInsets = UIEdgeInsets(top: 0,left: -30,bottom: 0,right: 34)
    btnSort.setTitle("SORT", for: .normal)
    btnSort.layer.borderWidth = 1.0
    btnSort.backgroundColor = UIColor.red //--> set the background color and check
    btnSort.layer.borderColor = UIColor.white.cgColor
    btnSort.addTarget(self, action: #selector(ViewController.showSortTbl), for: UIControlEvents.touchUpInside)
    self.view.addSubview(btnSort)

and handle the action as

  func showSortTbl() {
    // do your stuff here

}