Example to call Alert from different ViewControllers

Alert.swift

//
//  Alert.swift
//
//  Created by Vasiliy Savchuk on 16.12.16.
//  Copyright © All rights reserved.
//
// Usage this snippet:
// to call - displayAlert(title: "Oops!", message: "Error", viewController: self)

import Foundation
import UIKit

func displayAlert(title: String, message: String, viewController: UIViewController) {
    
    let alert = UIAlertController(title: title, message: message, preferredStyle: UIAlertControllerStyle.alert)
    let action = UIAlertAction(title: "Ok", style: UIAlertActionStyle.default, handler: nil)
    let actionRetry = UIAlertAction(title: "Retry", style: UIAlertActionStyle.cancel) {action in 
    //execute some code when this option is selected
    }
    
    alert.addAction(action)
    alert.addAction(actionRetry)
    viewController.present(alert, animated: true, completion: nil)
}