改变Button的形状

        let btn = UIButton(type: .System)
        btn.frame =  CGRect(x: 100, y: 100, width: 100, height: 100)
        btn.backgroundColor = UIColor.redColor()
        self.view.addSubview(btn)
        
        //CoreAnimation
        //UIView拥有一个CALayer,控制显示功能
        //圆角半径
        btn.layer.cornerRadius = 50
        //边框颜色
        btn.layer.borderColor = UIColor.yellowColor().CGColor
        //边框宽度
        btn.layer.borderWidth = 2
        //设置阴影颜色
        btn.layer.shadowColor = UIColor.blackColor().CGColor
        //阴影的透明度默认是0, 看不见
        btn.layer.shadowOpacity = 1
        //width: 相对于原图形向右平移, 否者相反
        //height: 相对于原图形向下平移, 否者相反
        btn.layer.shadowOffset = CGSize(width: 10, height: 20)
        //阴影半径
        btn.layer.shadowRadius = 20