在xcode 9中

let alert = UIAlertController(title: "Alert", message: "message", preferredStyle: UIAlertControllerStyle.alert)
alert.addAction(UIAlertAction(title: "Ok", style: UIAlertActionStyle.default, handler: nil))
self.present(alert, animated: true, completion: nil)

显示UIAlertView在swift语言:-

Protocol UIAlertViewDelegate

let alert = UIAlertView(title: "alertView", message: "This is alertView", delegate:self, cancelButtonTitle:"Cancel", otherButtonTitles: "Done", "Delete")
alert.show()

显示UIAlertViewController在swift语言:-

let alert = UIAlertController(title: "Error", message: "Enter data in Text fields", preferredStyle: UIAlertControllerStyle.Alert)
alert.addAction(UIAlertAction(title: "OK", style: UIAlertActionStyle.Default, handler: nil))
self.presentViewController(alert, animated: true, completion: nil)

你可以使用标准的构造函数创建一个UIAlert，但是“遗留的”构造函数似乎不起作用:

let alert = UIAlertView()
alert.title = "Alert"
alert.message = "Here's a message"
alert.addButtonWithTitle("Understood")
alert.show()

SWIFT 4:简单地为UIViewController创建一个扩展，如下所示:

extension  UIViewController {        
    func showSuccessAlert(withTitle title: String, andMessage message:String) {
        let alert = UIAlertController(title: title, message: message,
                                  preferredStyle: UIAlertController.Style.alert)
        alert.addAction(UIAlertAction(title: "OK".localized, style:
        UIAlertAction.Style.default, handler: nil))
        self.present(alert, animated: true, completion: nil)
    }
}

现在在你的ViewController中，直接调用上面的函数，就好像它们是由UIViewController提供的一样。

    yourViewController.showSuccessAlert(withTitle: 
      "YourTitle", andMessage: "YourCustomTitle")

从UIAlertView类:

// UIAlertView已弃用。使用UIAlertController 将UIAlertControllerStyleAlert改为preferredStyle

在iOS 8上，你可以这样做:

let alert = UIAlertController(title: "Alert", message: "Message", preferredStyle: UIAlertControllerStyle.Alert)
alert.addAction(UIAlertAction(title: "Click", style: UIAlertActionStyle.Default, handler: nil))
self.presentViewController(alert, animated: true, completion: nil)

UIAlertController是一个单独的类，用于创建和交互我们在ios8中知道的UIAlertViews和UIActionSheets。

编辑:处理动作:

alert.addAction(UIAlertAction(title: "OK", style: .Default, handler: { action in
    switch action.style{
    case .Default:
        print("default")
        
    case .Cancel:
        print("cancel")
        
    case .Destructive:
        print("destructive")
    }
}}))

为Swift 3编辑:

let alert = UIAlertController(title: "Alert", message: "Message", preferredStyle: UIAlertControllerStyle.alert)
alert.addAction(UIAlertAction(title: "Click", style: UIAlertActionStyle.default, handler: nil))
self.present(alert, animated: true, completion: nil)

为Swift 4.x编辑:

let alert = UIAlertController(title: "Alert", message: "Message", preferredStyle: .alert)
alert.addAction(UIAlertAction(title: "OK", style: .default, handler: { action in
    switch action.style{
        case .default:
        print("default")
        
        case .cancel:
        print("cancel")
        
        case .destructive:
        print("destructive")
        
    }
}))
self.present(alert, animated: true, completion: nil)

 let alertController = UIAlertController(title: "Select Photo", message: "Select atleast one photo", preferredStyle: .alert)
    let action1 = UIAlertAction(title: "From Photo", style: .default) { (action) in
        print("Default is pressed.....")
    }
    let action2 = UIAlertAction(title: "Cancel", style: .cancel) { (action) in
        print("Cancel is pressed......")
    }
    let action3 = UIAlertAction(title: "Click new", style: .default) { (action) in
        print("Destructive is pressed....")

    }
    alertController.addAction(action1)
    alertController.addAction(action2)
    alertController.addAction(action3)
    self.present(alertController, animated: true, completion: nil)

}