在IOS 9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.present(alert, animated: true, completion: nil)

老方法:UIAlertView

let alertView = UIAlertView(title: "Default Style", message: "A standard alert.", delegate: self, cancelButtonTitle: "Cancel", otherButtonTitles: "OK")
alertView.alertViewStyle = .Default
alertView.show()

// MARK: UIAlertViewDelegate

 func alertView(alertView: UIAlertView, clickedButtonAtIndex buttonIndex: Int) {
 switch buttonIndex {

    // ...
   }
  }

新方法:UIAlertController

let alertController = UIAlertController(title: "Default Style", message: "A standard alert.", preferredStyle: .Alert)

let cancelAction = UIAlertAction(title: "Cancel", style: .Cancel) { (action) in
// ...
 }
 alertController.addAction(cancelAction)

 let OKAction = UIAlertAction(title: "OK", style: .Default) { (action) in
// ...
 }
 alertController.addAction(OKAction)
 self.presentViewController(alertController, animated: true) {
 // ...
}

Swift 5上的SwiftUI。x和Xcode 11.x

import SwiftUI

struct ContentView: View {

    @State private var isShowingAlert = false

    var body: some View {
        VStack {
            Button("A Button") {

                self.isShowingAlert.toggle()
            }
            .alert(isPresented: $isShowingAlert) { () -> Alert in
                Alert(
                    title: Text("Alert"),
                    message: Text("This is an alert"),
                    dismissButton:
                        .default(
                            Text("OK"),
                            action: {
                                print("Dismissing alert")
                            }
                        )
                )
            }

        }
        .padding()
    }
}

struct ContentView_Previews: PreviewProvider {
    static var previews: some View {
        ContentView()
    }
}

  // UIAlertView is deprecated. Use UIAlertController 
  // title = title of the alert view.
  // message = Alert message you want to show.
  // By tap on "OK" , Alert view will dismiss.

 UIAlertView(title: "Alert", message: "Enter Message here.", delegate: nil, cancelButtonTitle: "OK").show()

 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)

}

我还有一招。假设您有5个类，其中要应用注销警报。尝试使用swift类扩展。

文件-新建- Swift类-命名。

增加如下内容:

public extension UIViewController
{

    func makeLogOutAlert()
    {
        var refreshAlert = UIAlertController(title: "Log Out", message: "Are You Sure to Log Out ? ", preferredStyle: UIAlertControllerStyle.Alert)

        refreshAlert.addAction(UIAlertAction(title: "Confirm", style: .Default, handler: { (action: UIAlertAction!) in
            self.navigationController?.popToRootViewControllerAnimated(true)
        }))

        refreshAlert.addAction(UIAlertAction(title: "Cancel", style: .Default, handler: { (action: UIAlertAction!) in
            refreshAlert .dismissViewControllerAnimated(true, completion: nil)
        }))

        presentViewController(refreshAlert, animated: true, completion: nil)
    }
}

实现使用:self.makeLogOutAlert()。希望能有所帮助。