设置背景图像的问题是它消除了模糊。您可以在不设置背景图像的情况下删除它。在这里看到我的答案。

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
    }
}

如果你想保持半透明，你不想子类化每个UINavigationController在你的应用:

#import <objc/runtime.h>

@implementation UINavigationController (NoShadow)

+ (void)load {
    Method original = class_getInstanceMethod(self, @selector(viewWillAppear:));
    Method swizzled = class_getInstanceMethod(self, @selector(swizzled_viewWillAppear:));
    method_exchangeImplementations(original, swizzled);
}

+ (UIImageView *)findHairlineImageViewUnder:(UIView *)view {
    if ([view isKindOfClass:UIImageView.class] && view.bounds.size.height <= 1.0) {
        return (UIImageView *)view;
    }

    for (UIView *subview in view.subviews) {
        UIImageView *imageView = [self findHairlineImageViewUnder:subview];
        if (imageView) {
            return imageView;
        }
    }

    return nil;
}

- (void)swizzled_viewWillAppear:(BOOL)animated {
    UIImageView *shadow = [UINavigationController findHairlineImageViewUnder:self.navigationBar];
    shadow.hidden = YES;

    [self swizzled_viewWillAppear:animated];
}

@end

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。

我使用UINavigationBar扩展，使我能够隐藏/显示使用UIAppearance API的阴影或选择哪个导航栏必须隐藏/显示使用故事板(或源代码)的阴影。这是扩展:

import UIKit

private var flatAssociatedObjectKey: UInt8 = 0

/*
  An extension that adds a "flat" field to UINavigationBar. This flag, when
  enabled, removes the shadow under the navigation bar.
 */
@IBDesignable extension UINavigationBar {
    @IBInspectable var flat: Bool {
        get {
            guard let obj = objc_getAssociatedObject(self, &flatAssociatedObjectKey) as? NSNumber else {
                return false
            }
            return obj.boolValue;
        }

        set {
            if (newValue) {
                let void = UIImage()
                setBackgroundImage(void, forBarPosition: .Any, barMetrics: .Default)
                shadowImage = void
            } else {
                setBackgroundImage(nil, forBarPosition: .Any, barMetrics: .Default)
                shadowImage = nil
            }
            objc_setAssociatedObject(self, &flatAssociatedObjectKey, NSNumber(bool: newValue),
                    objc_AssociationPolicy.OBJC_ASSOCIATION_RETAIN_NONATOMIC)
        }
    }
}

现在，要禁用所有导航条上的阴影，你必须使用:

UINavigationBar.appearance().flat = true

或者你可以使用故事板启用/禁用此行为:

Slightly Swift Solution 
func setGlobalAppearanceCharacteristics () {
    let navigationBarAppearace = UINavigationBar.appearance()
    navigationBarAppearace.tintColor = UIColor.white
    navigationBarAppearace.barTintColor = UIColor.blue
    navigationBarAppearace.setBackgroundImage(UIImage(), for: UIBarMetrics.default)
    navigationBarAppearace.shadowImage = UIImage()

}