Present basic alert with UIViewController.

UIViewController+Alert.swift

import UIKit

extension UIViewController {
    func presentAlert(message: String) {
        if presentedViewController == nil {
            let alertController = UIAlertController(title: nil, message: message, preferredStyle: .Alert)
        
            let alertAction = UIAlertAction(title: "OK", style: .Default, handler: nil)
            alertController.addAction(alertAction)
        
            presentViewController(alertController, animated: true, completion: nil)
        }
    }
}