一、iOS11之后，更改系统导航栏的backBarButtonItem，系统返回按钮仍然显示，如下图：
导航栏
比较好的解决办法是隐藏backBarButtonItem，更改leftBarButtonItem。
navigationItem.hidesBackButton = true
let backItem = UIBarButtonItem.init(image: UIImage.init(named: "nav_back_black")?.withRenderingMode(.alwaysOriginal), style: .plain, target: self, action: #selector(backItemClick))
navigationItem.leftBarButtonItem = backItem