Simples alert for iOS 11 with Swift 4.

alert.swift

// 1. Creating the AlertController
let alert: UIAlertController = UIAlertController(title: "Teste",
                                                 message: "Aprendendo a fazer alertas",
                                                 preferredStyle: .alert)

// 1.1. Create the actions
let okAction = UIAlertAction(title: "OK", style: .default) { (okAction) in
    // Insert here the code to be executed when the user select the action
    print("O usuário escolheu: \(okAction.title!)")
}

let cancelAction = UIAlertAction(title: "Cancel", style: .cancel) { (cancelAction) in
    // Insert here the code to be executed when the user select the action
    print("O usuário escolheu: \(cancelAction.title!)")
}

// 1.2. Add the actions to the AlertController
alert.addAction(okAction)
alert.addAction(cancelAction)

// 2. Present the alert to the user
self.present(alert, animated: true, completion: {
    // Insert here the code to be executed when the alert closes
    print("Alerta completo!")
})