以上问题的答案

//删除1px的导航条线

[[UINavigationBar appearance] setBackgroundImage:[[UIImage alloc]init] forBarMetrics:UIBarMetricsDefault];
[[UINavigationBar appearance] setShadowImage:[[UIImage alloc] init]];
[[UINavigationBar appearance] setTranslucent:NO];
[[UINavigationBar appearance] setTintColor:[UIColor yourColor]];

如果你只是想使用纯色导航条，并且已经在你的故事板中设置了它，在你的AppDelegate类中使用下面的代码通过外观代理删除1像素的边界:

[[UINavigationBar appearance] setBackgroundImage:[[UIImage alloc] init]
                                  forBarPosition:UIBarPositionAny
                                      barMetrics:UIBarMetricsDefault];

[[UINavigationBar appearance] setShadowImage:[[UIImage alloc] init]];

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()

下面是破解方法。因为它在关键路径上工作，将来可能会崩溃。但就目前而言，它的运作符合预期。

迅速:

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

Objective - C:

[self.navigationController.navigationBar setValue:@(YES) forKeyPath:@"hidesShadow"];

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 
        }
    }
}

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)