设置uinavigationbar导航条透明，并去除下面那条莫名其妙的线

For a custom shadow image to be shown, custom background image must also be set with the setBackgroundImage:forBarMetrics: method. If the default background image is used, then the default shadow image will be used regardless of the value of this property.

So the code is:

UINavigationBar *navigationBar = self.navigationController.navigationBar;

[navigationBar setBackgroundImage:[UIImage imageNamed:@"NavigationBarBackground"]
                   forBarPosition:UIBarPositionAny
                       barMetrics:UIBarMetricsDefault];

[navigationBar setShadowImage:[UIImage new]];

This code assumes you want the image named "NavigationBarBackground" as bar background. If it's not the case, you can make the background a solid color by setting backgroundImage to [UIImage new] and assigning navigationBar.backgroundColor to the color you like.

Above is the only "official" way to hide it. But unfortunately it removes bar's translucency.

How to keep bar translucent?

To keep translucency you need another approach, it looks like a hack but works well. The hairline we're trying to remove is UIImageView somewhere under UINavigationBar. So we have to find it and hide or show it when needed.

First – declare instance variable:

@implementation MyViewController {
    UIImageView *navBarHairlineImageView;
}

Then, in viewDidLoad do:

navBarHairlineImageView = [self findHairlineImageViewUnder:navigationBar];

Method which finds the image view we need:

- (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;
}

And this will do the rest of magic:

- (void)viewWillAppear:(BOOL)animated {
    [super viewWillAppear:animated];
    navBarHairlineImageView.hidden = YES;
}

- (void)viewWillDisappear:(BOOL)animated {
    [super viewWillDisappear:animated];
    navBarHairlineImageView.hidden = NO;
}

Same method should also work for UISearchBar hairline.