iOS 统一设置返回按钮图片 返回按钮标题不显示

 func application(_ application: UIApplication, didFinishLaunchingWithOptions launchOptions: [UIApplicationLaunchOptionsKey: Any]?) -> Bool {
        let backImg: UIImage = UIImage(named: "256")!
        UIBarButtonItem.appearance().setBackButtonBackgroundImage(backImg, for: .normal, barMetrics: .default)
        UIBarButtonItem.appearance().setBackButtonTitlePositionAdjustment(UIOffsetMake(0, -80.0), for: .default)
        return true
    }