Select text in UIAlertController's text field

I need the text of the text field to be selected right after the UIAlertController is presented. However, the way I select text in a standard UITextField doesn't work here.

This is what I tried, but I can't seem to get it work.

let ac = UIAlertController(title: "Rename", message: nil, preferredStyle: .Alert)
ac.addTextFieldWithConfigurationHandler({
    [] (textField: UITextField) in
    textField.selectedTextRange = textField.textRangeFromPosition(textField.beginningOfDocument, toPosition: textField.endOfDocument)
    textField.text = "filename.dat"
    })
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)
})

Any ideas?