Swift 4.2解决方案:

private func removeHairlineFromNavbar() {
    UINavigationBar.appearance().setBackgroundImage(
        UIImage(),
        for: .any,
        barMetrics: .default)
    UINavigationBar.appearance().shadowImage = UIImage()
}

把这个函数放在第一个Viewcontroller，然后在viewdidload中调用它

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

    UIImage *emptyImage = [UIImage new];
    self.navigationController.navigationBar.shadowImage = emptyImage;
    [self.navigationController.navigationBar setBackgroundImage:emptyImage forBarMetrics:UIBarMetricsDefault];
}

在研究了Serhil的答案后，我创建了一个豆荚UINavigationBar+Addition，可以很容易地隐藏发际线。

#import "UINavigationBar+Addition.h"

- (void)viewDidLoad {
    [super viewDidLoad];

    UINavigationBar *navigationBar = self.navigationController.navigationBar;
    [navigationBar hideBottomHairline];
}

我的方法:

UINavigationBar.appearance().setBackgroundImage(
            UIImage(),
            forBarPosition: .Any,
            barMetrics: .Default)
    var _width:CGFloat! = self.navigationController?.navigationBar.layer.frame.width
            var _height:CGFloat! = self.navigationController?.navigationBar.layer.frame.height
            var navBarBg = UIView(frame:CGRectMake(0, 0, _width, _height))
            //solid color for bg
            navBarBg.backgroundColor = UIColor.orangeColor()
            view.addSubview(navBarBg)

以上问题的答案

//删除1px的导航条线

[[UINavigationBar appearance] setBackgroundImage:[[UIImage alloc]init] forBarMetrics:UIBarMetricsDefault];
[[UINavigationBar appearance] setShadowImage:[[UIImage alloc] init]];
[[UINavigationBar appearance] setTranslucent:NO];
[[UINavigationBar appearance] setTintColor:[UIColor yourColor]];

对于iOS 9用户，这对我来说很有效。只要加上这个:

UINavigationBar.appearance().shadowImage = UIImage()