AlertController is not in the window hierarchy

If you're instancing your UIAlertController from a modal controller, you need to do it in viewDidAppear, not in viewDidLoad or you'll get an error.

Here's my code (Swift 4):

override func viewDidAppear(_ animated: Bool) {    super.viewDidAppear(animated)    let alertController = UIAlertController(title: "Foo", message: "Bar", preferredStyle: .alert)    alertController.addAction(UIAlertAction(title: "OK", style: .cancel, handler: nil))    present(alertController, animated: true, completion: nil)}

Let's look at your view hierarchy. You have a ViewController.Then you are creating an AlertController, you are not adding it to your hierarchy and you are calling an instance method on it, that attempts to use the AlertController as presenting controller to show just another controller (UIAlertController).

+ ViewController    + AlertController (not in hierarchy)        + UIAlertController (cannot be presented from AlertController)

To simplify your code

class ViewController: UIViewController {   override func viewDidLoad() {       super.viewDidLoad()     }   @IBAction func showAlertButton(sender: AnyObject) {       var alert = UIAlertController(title: "abc", message: "def", preferredStyle: .Alert)       self.presentViewController(alert, animated: true, completion: nil)   }}

This will work.

If you need the AlertController for something, you will have to add it to the hierarchy first, e.g. using addChildViewController or using another presentViewController call.

If you want the class to be just a helper for creating alert, it should look like this:

class AlertHelper {    func showAlert(fromController controller: UIViewController) {         var alert = UIAlertController(title: "abc", message: "def", preferredStyle: .Alert)        controller.presentViewController(alert, animated: true, completion: nil)    }}

called as

 var alert = AlertHelper() alert.showAlert(fromController: self)

You can use below function to call alert from any where just include these method in AnyClass

class func topMostController() -> UIViewController {        var topController: UIViewController? = UIApplication.shared.keyWindow?.rootViewController        while ((topController?.presentedViewController) != nil) {            topController = topController?.presentedViewController        }        return topController!    }    class func alert(message:String){        let alert=UIAlertController(title: "AppName", message: message, preferredStyle: .alert);        let cancelAction: UIAlertAction = UIAlertAction(title: "OK", style: .cancel) { action -> Void in        }        alert.addAction(cancelAction)        AnyClass.topMostController().present(alert, animated: true, completion: nil);    }

Then call

AnyClass.alert(message:"Your Message")