Select text in UIAlertController's text field

I have rewrote your code. Your class should conform to the UITextFieldDelegate protocol and implement the textFieldDidBeginEditing method, like this:

class ViewController: UIViewController, UITextFieldDelegate {

    override func viewDidLoad() {
        super.viewDidLoad()

        let ac = UIAlertController(title: "Rename", message: nil, preferredStyle: .Alert)
        ac.addTextFieldWithConfigurationHandler({
            [] (textField: UITextField) in
            textField.text = "filename.dat"
            textField.delegate = self

        })
        ac.addAction(UIAlertAction(title: "CANCEL", style: .Cancel, handler: nil))
        ac.addAction(UIAlertAction(title: "OK", style: .Default, handler: {
            [] Void in
            // do something
        }))
        dispatch_async(dispatch_get_main_queue(), {
            self.presentViewController(ac, animated: true, completion: nil)
        })

    }
    func textFieldDidBeginEditing(textField: UITextField) {
        textField.selectedTextRange = textField.textRangeFromPosition(textField.beginningOfDocument, toPosition: textField.endOfDocument)
        textField.becomeFirstResponder()
    }

}

A way to select all text without adding a delegate:

present(vc, animated: true) {
    vc.textFields?.first?.selectAll(nil)
}

Thanks, @ridvankucuk. Your solution works great.

But textfield delegate function can be simplified little bit:

func textFieldDidBeginEditing(_ textField: UITextField) {
    textField.selectAll(nil)
}