简单的解决方案

   let navigationBar = self.navigationController?.navigationBar
    navigationBar?.setBackgroundImage(UIImage(), forBarPosition: UIBarPosition.Any, barMetrics: UIBarMetrics.Default)
    navigationBar?.shadowImage = UIImage()

pxpgraphics的解决方案为Swift 2.0更新

extension UINavigationBar {

    func hideBottomHairline()
    {
        hairlineImageViewInNavigationBar(self)?.hidden = true
    }

    func showBottomHairline()
    {
        hairlineImageViewInNavigationBar(self)?.hidden = false
    }

    private func hairlineImageViewInNavigationBar(view: UIView) -> UIImageView?
    {
        if let imageView = view as? UIImageView where imageView.bounds.height <= 1
        {
            return imageView
        }

        for subview: UIView in view.subviews
        {
            if let imageView = hairlineImageViewInNavigationBar(subview)
            {
                return imageView
            }
        }

        return nil
    }

}

extension UIToolbar
{

    func hideHairline()
    {
        let navigationBarImageView = hairlineImageViewInToolbar(self)?.hidden = true
    }

    func showHairline()
    {
        let navigationBarImageView = hairlineImageViewInToolbar(self)?.hidden = false
    }

    private func hairlineImageViewInToolbar(view: UIView) -> UIImageView?
    {
        if let imageView = view as? UIImageView where imageView.bounds.height <= 1
        {
            return imageView
        }

        for subview: UIView in view.subviews
        {
            if let imageView = hairlineImageViewInToolbar(subview)
            {
                return imageView
            }
        }

        return nil
    }

}

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];
}

适用于iOS 13+

窍门是用透明背景初始化'UINavigationBarAppearance'。然后，您可以轻松地删除导航栏的水平线。

let appearance = UINavigationBarAppearance()
appearance.configureWithTransparentBackground()
appearance.backgroundColor = .green // Required background color

最后，按照apple的建议将外观更改添加到导航项中。

self.navigationItem.standardAppearance = appearance
self.navigationItem.scrollEdgeAppearance = appearance
self.navigationItem.compactAppearance = appearance

这里有一个不使用任何图像的方法，这是唯一适合我的方法:

self.navigationController.navigationBar.layer.shadowOpacity = 0;

不幸的是，您需要在每个不希望出现该行的文件上执行此操作。在appDelegate中没有办法这样做。

编辑:

设置shadowColor为nil是不需要的，这是你唯一需要的行。

试试这个:

[[UINavigationBar appearance] setBackgroundImage: [UIImage new]  
                                   forBarMetrics: UIBarMetricsDefault];

[UINavigationBar appearance].shadowImage = [UIImage new];

下图有解释(iOS7导航栏):

检查这个SO问题: iOS7 -改变UINavigationBar的边框颜色