[self.navigationController.navigationBar setBackgroundImage:[[UIImage alloc]init]
forBarMetrics:UIBarMetricsDefault];
[self.navigationController.navigationBar setShadowImage:[[UIImage alloc]init]];
方法二
self.navigationController.navigationBar.clipsToBounds = YES;
导航栏半透明
self.navigationController.navigationBar.translucent = YES;
方法三

• (void)viewWillAppear:(BOOL)animated
  {
  [super viewWillAppear:animated];

  _imageView.hidden = YES;
  }

  _imageView = [self findHairlineImageViewUnder:self.navigationController.navigationBar];

pragma mark - 隐藏导航栏下的横线

• (UIImageView *)findHairlineImageViewUnder:(UIView *)view {
  if ([view isKindOfClass:UIImageView.class] && view.bounds.size.height <= 1.0) {
  return (UIImageView *)view;
  }
  for (UIView *subview in view.subviews) {
  UIImageView *imageView = [self findHairlineImageViewUnder:subview];
  if (imageView) {
  return imageView;
  }
  }
  return nil;
  }