iOS导航条外观问题，背景图片不能铺满

//swift
var backGroundImage = UIImage(named: "顶部Bar")
        backGroundImage = backGroundImage?.resizableImage(withCapInsets: UIEdgeInsets.zero, resizingMode: UIImageResizingMode.stretch)
        
        nav.navigationBar.setBackgroundImage(backGroundImage, for: .default)
　　　　　nav.navigationBar.titleTextAttributes = [NSAttributedStringKey.foregroundColor : UIColor.white]

//OC
UIImage *backGroundImage = [UIImage imageNamed:@"顶部Bar"];
    backGroundImage = [backGroundImage resizableImageWithCapInsets:UIEdgeInsetsZero resizingMode:UIImageResizingModeStretch];
    [self.navigationBar setBackgroundImage:backGroundImage forBarMetrics:UIBarMetricsDefault];