UINavigationBar 在 AppDelegate.swift 中设置自定义阴影

我想在整个应用程序的 UINavigationBar 底部设置一些阴影。这是我尝试过但不起作用的方法：

func application(application: UIApplication, didFinishLaunchingWithOptions launchOptions: [NSObject: AnyObject]?) -> Bool {
    UINavigationBar.appearance().layer.shadowOffset = CGSizeMake(0, 3)
    UINavigationBar.appearance().layer.shadowRadius = 3.0
    UINavigationBar.appearance().layer.shadowColor = UIColor.yellowColor().CGColor
    UINavigationBar.appearance().layer.shadowOpacity = 0.7
}

请告诉我该怎么做？

UPDATE:通过从 UINavigationController 子类化来解决

import UIKit

class ShadowUINavigationController: UINavigationController {

    override func viewWillAppear(animated: Bool) {
        let darkColor: CGColorRef = UIColor(hex: 0x212121).CGColor
        let lightColor: CGColorRef = UIColor.clearColor().CGColor
        let navigationBarBottom: CGFloat = self.navigationBar.frame.origin.y + self.navigationBar.frame.size.height + 20
        println(self.navigationBar.frame.origin.y)
        println(self.navigationBar.frame.size.height)
        println(navigationBarBottom)

        let newShadow: CAGradientLayer = CAGradientLayer()
        newShadow.frame = CGRectMake(0, navigationBarBottom, self.view.frame.size.width, 1)
        newShadow.colors = [darkColor, lightColor]
        self.view.layer.addSublayer(newShadow)
        super.viewWillAppear(animated)
    }
}

仍然使用外观并且不需要您子类化 UINavigationBar 并向每个导航控制器添加代码的更好解决方案是：

扩展 UINavigationBar

extension UINavigationBar {

  var castShadow : String {
    get { return "anything fake" }
    set {
        self.layer.shadowOffset = CGSizeMake(0, 3)
        self.layer.shadowRadius = 3.0
        self.layer.shadowColor = UIColor.yellowColor().CGColor
        self.layer.shadowOpacity = 0.7

    }
  }

}

并添加应用程序广泛的外观规则（例如在 appdelegate“didFinishLaunchingWithOptions”内）

UINavigationBar.appearance().castShadow = ""