对于Swift 3.0:

override func viewDidLoad() {
    super.viewDidLoad()

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

IOS7:

self.navigationController.navigationBar.translucent = YES;
self.navigationController.navigationBar.shadowImage = [UIImage new];
self.navigationController.view.backgroundColor = [UIColor clearColor];
[self.navigationController.navigationBar setBackgroundImage:[UIImage new] forBarMetrics:UIBarMetricsDefault];
self.navigationController.navigationBar.backgroundColor = [UIColor clearColor];

对于Swift 3.0:

override func viewDidLoad() {
    super.viewDidLoad()

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

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。

如果有人想知道如何在iOS 7+中实现这一点，这里有一个解决方案(iOS 6也兼容)

在objective - c中

[self.navigationBar setBackgroundImage:[UIImage new]
                         forBarMetrics:UIBarMetricsDefault];
self.navigationBar.shadowImage = [UIImage new];
self.navigationBar.translucent = YES;

swift 3 (iOS 10)

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

在swift 2中

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

讨论

将导航栏上的半透明设置为YES就可以了，这是由于UINavigationBar文档中讨论的行为。我将在这里报告相关片段:

如果在具有不透明自定义背景图像的导航栏上将此属性设置为YES，则导航栏将对图像应用小于1.0的系统不透明度。

另一种方式，为我工作是子类UINavigationBar和留下drawRect方法空!!

@IBDesignable class MONavigationBar: UINavigationBar {


// Only override drawRect: if you perform custom drawing.
// An empty implementation adversely affects performance during animation.
override func drawRect(rect: CGRect) {
    // Drawing code
}}