title: Enable UIAlertAction of UIAlertController only after input is validated
body: <p>I am using a <code>UIAlertController</code> to present a dialog with a <code>UITextField</code> and one <code>UIAlertAction</code> button labeled "Ok". How do I disable the button until a number of characters (say 5 characters) are input into the <code>UITextField</code>? </p>