导航栏细线

UIImageView * _navBarHairlineImageView;

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

-(void)viewWillAppear:(BOOL)animated
{
    _navBarHairlineImageView.hidden = YES;
}
-(void)viewDidDisappear:(BOOL)animated
{
    _navBarHairlineImageView.hidden = NO;
}

//通过一个方法来找到这个黑线(findHairlineImageViewUnder):
- (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;
}

https://www.jianshu.com/p/effa4a48f1e3