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

在子视图中找到发际线的一个很好的简短Swift函数是:

 func findHairLineInImageViewUnder(view view: UIView) -> UIImageView? {
    if let hairLineView = view as? UIImageView where hairLineView.bounds.size.height <= 1.0 {
        return hairLineView
    }

    if let hairLineView = view.subviews.flatMap({self.findHairLineInImageViewUnder(view: $0)}).first {
      return hairLineView
    }

    return nil
}

pxpgraphics对Swift 3.0的回答。

import Foundation
import UIKit

extension UINavigationBar {

  func hideBottomHairline() {
    let navigationBarImageView = hairlineImageViewInNavigationBar(view: self)
    navigationBarImageView!.isHidden = true
  }

  func showBottomHairline() {
    let navigationBarImageView = hairlineImageViewInNavigationBar(view: self)
    navigationBarImageView!.isHidden = false
  }

  private func hairlineImageViewInNavigationBar(view: UIView) -> UIImageView? {
    if view is UIImageView && view.bounds.height <= 1.0 {
      return (view as! UIImageView)
    }

    let subviews = (view.subviews as [UIView])
    for subview: UIView in subviews {
      if let imageView: UIImageView = hairlineImageViewInNavigationBar(view: subview) {
        return imageView
      }
    }
    return nil
  }
}

extension UIToolbar {

  func hideHairline() {
    let navigationBarImageView = hairlineImageViewInToolbar(view: self)
    navigationBarImageView!.isHidden = true
  }

  func showHairline() {
    let navigationBarImageView = hairlineImageViewInToolbar(view: self)
    navigationBarImageView!.isHidden = false
  }

  private func hairlineImageViewInToolbar(view: UIView) -> UIImageView? {
    if view is UIImageView && view.bounds.height <= 1.0 {
      return (view as! UIImageView)
    }

    let subviews = (view.subviews as [UIView])
    for subview: UIView in subviews {
      if let imageView: UIImageView = hairlineImageViewInToolbar(view: subview) {
        return imageView
      }
    }
    return nil
  }
}

Swift 4测试 单线解决方案

在Viewdidload () 将导航控制器的用户默认值设置为true

override func viewDidLoad() {
    super.viewDidLoad()

    self.navigationController?.navigationBar.setValue(true, forKey: "hidesShadow")

}

在iOS8中，如果你设置UINavigationBar。你可以设置酒吧的背景为无边框的素色。

迅速:

UINavigationBar.appearance().translucent = false
UINavigationBar.appearance().barStyle = UIBarStyle.Black
UINavigationBar.appearance().barTintColor = UIColor.redColor()