swift--如何设置子视图alpha不同于父视图？

//1.2加入商家标题评分容器
        let titleWarp=UIView(frame: CGRectMake(0, 150, screenObject.width, 50));
        titleWarp.backgroundColor=UIColor.blackColor().colorWithAlphaComponent(0.5);
//记住不要加下面这句代码
   //titleWarp.alpha=0.2;