对字符串进行阴影处理,需要先将字符串转换为Attribute形式
shadow = NSShadow.init()
shadow.shadowColor = UIColor.black
shadow.shadowBlurRadius = 4.0
shadow.shadowOffset = CGSize.init(width: 1, height: 2)
let timeStr = NSMutableAttributedString.init(string: radNavBar.timeLabel.text!)
timeStr.addAttribute(NSShadowAttributeName, value: shadow, range: NSMakeRange(0, timeStr.length))