快速做这件事的方法:

UINavigationBar.appearance().setBackgroundImage(UIImage(), for: .any, barMetrics: .default)
UINavigationBar.appearance().shadowImage = UIImage()

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];
}

从iOS 13开始，就有一个系统API来设置或删除阴影

UIKit使用shadowImage和shadowColor属性来确定阴影的 外观。当shadowImage为nil时，该工具条显示默认的着色阴影 根据shadowColor属性中的值。如果shadowColor为nil或 包含clearColor颜色，条显示没有阴影。

    let appearance = UINavigationBarAppearance()
    appearance.shadowImage = nil
    appearance.shadowColor = nil
    navigationController.navigationBar.standardAppearance = appearance

https://developer.apple.com/documentation/uikit/uibarappearance/3198009-shadowimage

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)

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

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
    }

}