对我来说，最简单的方法是创建一个具有所需颜色的png(它只需要一个像素乘以一个像素的尺寸)，然后设置backgroundImage和shadowImage:

let greenPixel = UIImage(named: "TheNameOfYourPng")
navigationBar.setBackgroundImage(greenPixel, forBarMetrics: UIBarMetrics.Default)
navigationBar.shadowImage = greenPixel

在Swift 3.0中

编辑你的AppDelegate.swift通过添加以下代码到你的应用程序函数:

// Override point for customization after application launch.

// Remove border in navigationBar
UINavigationBar.appearance().shadowImage = UIImage()
UINavigationBar.appearance().setBackgroundImage(UIImage(), for: .default)

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
    }
}

你应该在UISearchBar的底部添加一个视图

let rect = searchController.searchBar.frame;
let lineView : UIView = UIView.init(frame: CGRect.init(x: 0, y: rect.size.height-1, width: rect.size.width, height: 1))
lineView.backgroundColor = UIColor.init(hexString: "8CC73E")
searchController.searchBar.addSubview(lineView)

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

在Swift 3中，我们就是这样做的

对于任何视图控制器:

navigationBar.shadowImage = UIImage()
setBackgroundImage(UIImage(), for: .default)

对于整个应用程序:

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