private void TextKeyClicked(UIButton button)
        {
            var firstResponder = ResponderUtils.GetFirstResponder() as UITextField;

            if (firstResponder != null)
            {
                firstResponder.InsertText(button.Title(UIControlState.Normal));
            }
        }
        private void TextKeyClicked(UIButton sender)
        {
            var    firstResponder = ResponderUtils.GetFirstResponder() as UITextField;
            string key            = sender.TitleLabel.Text;

            if (int.TryParse(key, out int result))
            {
                firstResponder.InsertText(key);
            }
            else if (sender == btn11)
            {
                firstResponder.DeleteBackward();
            }
            else if (sender == btn12)
            {
                firstResponder.EndEditing(true);
            }
        }