iOS导航栏显示和去掉底部横线

 self.navigationController?.navigationBar.shadowImage = UIImage()//  底部横线就没有了
 self.navigationController?.navigationBar.shadowImage = nil // 这句代码就可以让消失的底部横线重新出现

根据官方文档的说法，我们就很好理解了，所以这应该是去掉横线最简单的办法。

/* Default is nil. When non-nil, a custom shadow image to show instead of the default shadow image. For a custom shadow to be shown, a custom background image must also be set with -setBackgroundImage:forBarMetrics: (if the default background image is used, the default shadow image will be used).

     */