Swfit--取消导航栏自带一条线

let nav = UINavigationBar.appearance()
nav.barTintColor = StaticData.navApperanceColor
nav.tintColor = StaticData.navTitleColor

    //取消nav下的边框线
    nav.setBackgroundImage(UIImage(), forBarMetrics:UIBarMetrics.Default)
    nav.shadowImage = UIImage()