Storyboard中按住ctrl键拖动至代码区，没有自动弹出@IBAOutlet以及选择和命名的窗口。

class ViewController: UIViewController{
    
    override func viewDidLoad() {
        super.viewDidLoad()
        // Do any additional setup after loading the view, typically from a nib.
    }
    
    //设置导航栏图片
 self.navigationController?.navigationBar.setBackgroundImage(UIImage(named: "Background"), for: UIBarMetrics.default)
    
     override func didReceiveMemoryWarning() {
        super.didReceiveMemoryWarning()
        // Dispose of any resources that can be recreated.
    }

}