如何去隐藏系统导航栏默认的下面的线条

• 代码一句 简单粗暴

 -(void)viewWillAppear:(BOOL)animated{
  [super viewWillAppear:animated];
  [self.navigationController.navigationBar setBackgroundImage:[UIImageimageWithColor:RGB(255,255,255)]forBarMetrics:UIBarMetricsDefault];
  [self.navigationController.navigationBar setShadowImage:[UIImagenew]];
}