SettingsBehavior to save Roamed Settings (attach to TextBox) in WinRT

SettingsBehavior

[Microsoft.Xaml.Interactivity.TypeConstraint(typeof(Windows.UI.Xaml.Controls.TextBox))]
    class SettingsBehavior : DependencyObject, IBehavior
    {
        #region IBehavior Members

        public Windows.UI.Xaml.DependencyObject AssociatedObject { get; set; }


        public string SettingsIndex
        {
            get { return (string)GetValue(SettingsIndexProperty); }
            set { SetValue(SettingsIndexProperty, value); }
        }

        // Using a DependencyProperty as the backing store for SettingsIndex.  This enables animation, styling, binding, etc...
        public static readonly DependencyProperty SettingsIndexProperty =
            DependencyProperty.Register("SettingsIndex", typeof(string), typeof(SettingsBehavior), new PropertyMetadata(string.Empty));



        public void Attach(Windows.UI.Xaml.DependencyObject associatedObject)
        {
            this.AssociatedObject = associatedObject;

            TextBox txt = null;

            if (this.AssociatedObject is TextBox)
            {
                txt = this.AssociatedObject as TextBox;
                txt.TextChanged += txt_TextChanged;
                txt.Loaded += txt_Loaded;
            }
        }

        void txt_Loaded(object sender, RoutedEventArgs e)
        {
            var txt = sender as TextBox;

            if (ApplicationData.Current.RoamingSettings.Values.ContainsKey(this.SettingsIndex))
            {
                txt.Text = ApplicationData.Current.RoamingSettings.Values[this.SettingsIndex] as string;
            }

        }

        void txt_TextChanged(object sender, TextChangedEventArgs e)
        {

            var txt = sender as TextBox;
            ApplicationData.Current.RoamingSettings.Values[this.SettingsIndex] = txt.Text;

        }

        public void Detach()
        {
            if (this.AssociatedObject != null)
            {
                TextBox txt = null;

                if (this.AssociatedObject is TextBox)
                {
                    txt = this.AssociatedObject as TextBox;
                    txt.TextChanged -= txt_TextChanged;
                    txt.Loaded -= txt_Loaded;
                }
            }
        }

        #endregion
    }