[iOS] 네비게이션 바 하단 그라데이션 (navigationBar)

extension UIViewController {
  func navigationBarShadow() {
//        self.navigationController?.navigationBar.layer.shadowColor = UIColor.blackColor().CGColor
        self.navigationController?.navigationBar.layer.borderColor = UIColor.whiteColor().CGColor
        self.navigationController?.navigationBar.layer.borderWidth = 6
        self.navigationController?.navigationBar.layer.shadowColor = UIColor ( red: 0.7741, green: 0.7741, blue: 0.7741, alpha: 1.0 ).CGColor
        self.navigationController?.navigationBar.layer.shadowOffset = CGSizeMake(1.0, 1.0)
        self.navigationController?.navigationBar.layer.shadowRadius = 3.0
        self.navigationController?.navigationBar.layer.shadowOpacity = 1.0
        self.navigationController?.navigationBar.layer.masksToBounds = false
   
        self.navigationController?.navigationBar.layer.shouldRasterize = true
  }
}