一、iOS11之后，更改系統導航欄的backBarButtonItem，系統返回按鈕仍然顯示，如下圖：
導航欄
比較好的解決辦法是隱藏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