添加阴影

    self.layer.shadowColor = UIColor.lightGray.cgColor
    self.layer.shadowOffset = CGSize(width: 0, height: -5)
    self.layer.shadowOpacity = 0.3
    self.layer.masksToBounds = false // 防止阴影被裁减

阴影view一定要上所有view的最上层，否则会被覆盖