public override void LayoutSubviews()
{
// layout the stock UIAlertView control
base.LayoutSubviews ();
// build out the text field
_tf = ComposeTextFieldControl(_secureTextEntry);
// add the text field to the alert view
this.AddSubview(_tf);
// shift the contents of the alert view around to accomodate the extra text field
AdjustControlSize();
}