Swift2でUIAlertControllerに追加したUITextFieldにバリデーションを実施する

• UIAlertController

let dialog: UIAlertController = UIAlertController(title: "ユーザ名登録", message: "名前を入力してください", preferredStyle:  UIAlertControllerStyle.Alert)

• AlertAction

let signUpAction: UIAlertAction = UIAlertAction(title: "OK", style: UIAlertActionStyle.Default, handler:{ (action: UIAlertAction!) -> Void in
    print(dialog.textFields![0].text)
    // any code
})
signUpAction.enabled = false

• UITextField for UserName

dialog.addTextFieldWithConfigurationHandler { (textFieldUser: UITextField!) in
    textFieldUser.placeholder = "1〜6文字"
    textFieldUser.keyboardType = .Default
}

• Validation

let textFieldValidationObserver: (NSNotification!) -> Void = { _ in
    let min = 1
    let max = 6
    let userName = dialog.textFields![0].text
    signUpAction.enabled = userName?.characters.count >= min && userName?.characters.count <= max
}

• Notifications for textField changes

NSNotificationCenter.defaultCenter().addObserverForName(UITextFieldTextDidChangeNotification, object: dialog.textFields![0], queue: NSOperationQueue.mainQueue(), usingBlock: textFieldValidationObserver)
dialog.addAction(signUpAction)
self.presentViewController(dialog, animated: true, completion: nil)