WPF 水印TextBox WatermarkTextBox

//https://blog.csdn.net/puchitomato/article/details/12248691

转自以上链接，自己添加了Enter响应事件。
    public class WatermarkTextBox : TextBox
    {
        private string watermark = string.Empty;
        public string Watermark
        {
            get { return watermark; }
            set { watermark = value ?? string.Empty; }
        }

        public static readonly DependencyProperty WatermarkProperty =
            DependencyProperty.Register("Watermark", typeof(string), typeof(WatermarkTextBox));

        static WatermarkTextBox()
        {
            DefaultStyleKeyProperty.OverrideMetadata(typeof(WatermarkTextBox),
                new FrameworkPropertyMetadata(typeof(WatermarkTextBox)));  
        }
        public WatermarkTextBox() {
            this.KeyDown += WatermarkTextBox_KeyDown;
        }

        private void WatermarkTextBox_KeyDown(object sender, KeyEventArgs e)
        {
            if (e.Key == Key.Enter)
            {
                TraversalRequest request = new TraversalRequest(FocusNavigationDirection.Next);

                UIElement focusElement = Keyboard.FocusedElement as UIElement;
                if (focusElement != null)
                {
                    focusElement.MoveFocus(request);
                }
                e.Handled = true;
            }
        }

        protected override void OnTextChanged(TextChangedEventArgs e)
        {
            base.OnTextChanged(e);

            if (this.Text == string.Empty)
            {
                VisualStateManager.GoToState(this, "Empty", true);
            }
            else
            {
                VisualStateManager.GoToState(this, "NotEmpty", true);
            }
        }
    }
    public enum TextStates
    {
        /// <summary>
        /// 内容为空
        /// </summary>
        Empty,
        /// <summary>
        /// 内容不为空
        /// </summary>
        NotEmpty
    }

样例：