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()

快速做这件事的方法:

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

我刚刚为这个创建了一个扩展…关于格式的问题很抱歉(这是我的第一个答案)。

用法:

  override func viewDidLoad() {
    super.viewDidLoad()
    self.navigationController?.hideShadow = true
}

扩展:

 UINavigationController.swift
//  Created by Ricardo López Rey on 16/7/15.

import Foundation


struct UINavigationControllerExtension {
    static var hideShadowKey : String = "HideShadow"
static let backColor = UIColor(red: 247/255, green: 247/255, blue: 248/255, alpha: 1.0)
}

extension UINavigationController {

    var hideShadow : Bool {
        get {
            if let ret =  objc_getAssociatedObject(self, &UINavigationControllerExtension.hideShadowKey) as? Bool {
                return ret
            } else {
                return false
            }


        }
        set {
            objc_setAssociatedObject(self,&UINavigationControllerExtension.hideShadowKey,newValue, objc_AssociationPolicy(OBJC_ASSOCIATION_RETAIN_NONATOMIC))

            if newValue {


            self.navigationBar.setBackgroundImage(solidImage(UINavigationControllerExtension.backColor), forBarMetrics: UIBarMetrics.Default)

                self.navigationBar.shadowImage = solidImage(UIColor.clearColor())
            } else {
                self.navigationBar.setBackgroundImage(nil, forBarMetrics: UIBarMetrics.Default)
            }
        }
    }

    private func solidImage(color: UIColor, size: CGSize = CGSize(width: 1,height: 1)) -> UIImage {
        var rect = CGRectMake(0, 0, size.width, size.height)
        UIGraphicsBeginImageContextWithOptions(size, false, 0)
        color.setFill()
        UIRectFill(rect)
        var image: UIImage = UIGraphicsGetImageFromCurrentImageContext()
        UIGraphicsEndImageContext()
        return image
    }


}

在Xamarin Forms中，这对我来说很管用。只需在AppDelegate.cs上添加这个:

UINavigationBar.Appearance.ShadowImage = new UIImage();

我也遇到了同样的问题，但没有一个答案真正令人满意。以下是我对Swift3的看法:

func hideNavigationBarLine() {
    navigationController?.navigationBar.setBackgroundImage(UIImage(), for: .default)
    navigationController?.navigationBar.shadowImage = UIImage()
}

只需在viewDidLoad()中调用它。

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