WP7 MVVM Panorama SelectedIndex Binding

The Panorama control doesn't support SelectedItem or SelectedIndex binding which is important for maintaining state on re-activation after tombstoning and may be required for navigation. To solve this problem, I've created a Panorama Behavior called TrackablePonaramaBehavior which can be attached to a panorama and offers a dependency property for binding to the selected index. The System.Windows.Interactivity library needs referencing to use it:

using System;
using Microsoft.Phone.Controls;
using System.Windows;
using System.Windows.Controls;
using System.Windows.Interactivity;

namespace Demo.Behaviors
{
    public class TrackablePanoramaBehavior : Behavior<Panorama>
    {
        private Panorama _panarama = null;
        private bool _updatedFromUI = false;

        // DP for binding index
        public static readonly DependencyProperty SelectedIndexProperty =
            DependencyProperty.Register("SelectedIndex", typeof(int), typeof(TrackablePanoramaBehavior),
            new PropertyMetadata(0, new PropertyChangedCallback(SelectedIndexPropertyChanged)));

        // Index changed by view model
        private static void SelectedIndexPropertyChanged(DependencyObject dpObj, DependencyPropertyChangedEventArgs change)
        {
            if(change.NewValue.GetType() != typeof(int) || dpObj.GetType() != typeof(TrackablePanoramaBehavior))
                return;

            TrackablePanoramaBehavior track = (TrackablePanoramaBehavior)dpObj;

            // If this flag is not checked, the panorama smooth transition is overridden
            if (!track._updatedFromUI)
            {
                Panorama pan = track._panarama;

                int index = (int)change.NewValue;

                if (pan.Items.Count > index)
                {
                    pan.DefaultItem = pan.Items[(int)change.NewValue];
                }
            }

            track._updatedFromUI = false;
        }

        public int SelectedIndex
        {
            get { return (int)GetValue(SelectedIndexProperty); }
            set { SetValue(SelectedIndexProperty, value); }
        }

        protected override void OnAttached()
        {
            base.OnAttached();

            this._panarama = base.AssociatedObject as Panorama;
            this._panarama.SelectionChanged += _panarama_SelectionChanged;
        }       

        protected override void OnDetaching()
        {
            base.OnDetaching();

            if(this._panarama != null)
                this._panarama.SelectionChanged += _panarama_SelectionChanged;
        }

        // Index changed by UI
        private void _panarama_SelectionChanged(object sender, SelectionChangedEventArgs e)
        {
            _updatedFromUI = true;
            SelectedIndex = _panarama.SelectedIndex;
        }
    }
}

To implement this in the View, the following XAML is used (I chopped most xmlns out to make it more concise):

<phone:PhoneApplicationPage

    xmlns:i="clr-namespace:System.Windows.Interactivity;assembly=System.Windows.Interactivity"

    DataContext="{Binding Main, Source={StaticResource Locator}}">
  
    <!--LayoutRoot contains the root grid where all other page content is placed-->
    <Grid x:Name="LayoutRoot">
        <controls:Panorama>
            <i:Interaction.Behaviors>
                <track:TrackablePanoramaBehavior SelectedIndex="{Binding Path=SelectedIndex, Mode=TwoWay}" />
            </i:Interaction.Behaviors>

And simply bind to the view model:

public int SelectedIndex
{
    get { return this._selectedIndex; }
    set
    {
        if (this._selectedIndex != value)
        {
            this._selectedIndex = value;

            base.RaisePropertyChanged("SelectedIndex");
        }
    }
}