这里有一个非常重要的注意事项——改变UIViewController的navigationItem的外观比直接改变navigationBar要灵活得多。

为什么这么问?

For the simple reason that the navigationItem is tied to a single UIViewController and represents the state of the navigationBar for that particular UIViewController. This is big, as you don't have to handle the navigation bar changes between different view controllers within viewWillAppear (or something similar), as you would if you mutated the navigationBar; which is, remember, shared between all view controllers of a given navigation stack (UINavigationController), and changing it in one place changes it for all view controllers up to the stack.

你只要为一个特定的视图控制器设置正确的UINavigationBarAppearance, UIKit就会正确地更新导航栏样式，这取决于当前哪个视图控制器是导航堆栈上的顶视图控制器。

navigationItem.standardAppearance` = `UINavigationBarAppearance()

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
  }
}

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

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)

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 
        }
    }
}