对于任何想在Swift 2.x中这样做的人:

self.navigationController?.navigationBar.setBackgroundImage(UIImage(), forBarMetrics: .Default)
self.navigationController?.navigationBar.shadowImage = UIImage()
self.navigationController?.navigationBar.translucent = true

或Swift 3.x:

self.navigationController?.navigationBar.setBackgroundImage(UIImage(), for: .default)
self.navigationController?.navigationBar.shadowImage = UIImage()
self.navigationController?.navigationBar.isTranslucent = true

在iOS5中，你可以这样做，使导航栏透明:

nav.navigationBar.translucent = YES; // Setting this slides the view up, underneath the nav bar (otherwise it'll appear black)
const float colorMask[6] = {222, 255, 222, 255, 222, 255};
UIImage *img = [[UIImage alloc] init];
UIImage *maskedImage = [UIImage imageWithCGImage: CGImageCreateWithMaskingColors(img.CGImage, colorMask)];

[nav.navigationBar setBackgroundImage:maskedImage forBarMetrics:UIBarMetricsDefault]; 
[img release];

对于Swift 3.0:

override func viewDidLoad() {
    super.viewDidLoad()

    navigationController?.navigationBar.setBackgroundImage(UIImage(), for: .default)
    navigationController?.navigationBar.shadowImage = UIImage()
    navigationController?.navigationBar.isTranslucent = true
}

在Swift 4.2中

self.navigationController?.navigationBar.setBackgroundImage(UIImage(), for: .default)
self.navigationController?.navigationBar.shadowImage = UIImage()
self.navigationController?.navigationBar.isTranslucent = true

(在viewWillAppear中)然后在viewWillDisappear中，撤销它，put

self.navigationController?.navigationBar.shadowImage = nil
self.navigationController?.navigationBar.isTranslucent = false

解决方案- Swift 5 - iOS 13+

根据文档，在你的UIViewController子类中:

override func viewDidLoad()
{
    super.viewDidLoad()
    
    let appearance = UINavigationBarAppearance()
    appearance.configureWithTransparentBackground()
    //appearance.backgroundColor = UIColor.clear
    
    navigationItem.compactAppearance = appearance
    navigationItem.scrollEdgeAppearance = appearance
    navigationItem.standardAppearance = appearance
    
    //...
}

只是为了澄清，这使得UINavigationBar完全透明。栏按钮项仍然可见，并正常工作。

什么不起作用

override func viewDidLoad()
{
    super.viewDidLoad()
    
    navigationController?.navigationBar.isTranslucent = true
    navigationController?.navigationBar.isOpaque = false

    //...
}

这让我意识到我其实不知道透明RIP和半透明RIP之间的区别。

参考文献

https://developer.apple.com/documentation/uikit/uinavigationcontroller/customizing_your_app_s_navigation_bar

https://www.lexico.com/en/definition/transparent

https://www.lexico.com/en/definition/translucent

更新08/10/2021

在以我提供的方式设置外观后，更改navigationItem栏按钮将重置外观，您必须再次执行此操作。

这适用于Swift 5。

// Clear the background image.
navigationController?.navigationBar.setBackgroundImage(UIImage(), for: .default)

// Clear the shadow image.
navigationController?.navigationBar.shadowImage = UIImage()

// Ensure the navigation bar is translucent.
navigationController?.navigationBar.isTranslucent = true