我知道这是一个老话题，但我找到了一个非常有效的解决方案:

子类UINavigationBar。 在你的UINavigationBar子类中，用下面的代码重写didAddSubview:

- (void)didAddSubview:(UIView *)subview
{
    [super didAddSubview:subview];

    if ([subview isKindOfClass:[UIImageView class]]) {
        [subview setClipsToBounds:YES];
    }
}

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];
}

斯威夫特说

UINavigationBar.appearance().setBackgroundImage(UIImage(), forBarPosition: .Any, barMetrics: .Default)
UINavigationBar.appearance().shadowImage = UIImage()

in

func application(application: UIApplication, didFinishLaunchingWithOptions launchOptions: [NSObject: AnyObject]?) -> Bool

编写自己的初始化式:D

import Foundation
import UIKit

extension UINavigationController {
    convenience init(rootViewController : UIViewController, hidesShadow : Bool) {
        self.init(rootViewController : rootViewController)
        self.navigationBar.setValue(hidesShadow, forKey: "hidesShadow")
        if hidesShadow {
            self.extendedLayoutIncludesOpaqueBars = true
            self.navigationBar.isTranslucent = false 
        }
    }
}

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

想把Serhii回答的Swift版本加进去。我用下面的代码创建了一个uibareextension .swift:

import Foundation
import UIKit

extension UINavigationBar {
    func hideBottomHairline() {
        self.hairlineImageView?.isHidden = true
    }

    func showBottomHairline() {
        self.hairlineImageView?.isHidden = false
    }
}

extension UIToolbar {
    func hideBottomHairline() {
        self.hairlineImageView?.isHidden = true
    }

    func showBottomHairline() {
        self.hairlineImageView?.isHidden = false
    }
}

extension UIView {
    fileprivate var hairlineImageView: UIImageView? {
        return hairlineImageView(in: self)
    }

    fileprivate func hairlineImageView(in view: UIView) -> UIImageView? {
        if let imageView = view as? UIImageView, imageView.bounds.height <= 1.0 {
            return imageView
        }

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

        return nil
    }
}