public class PageBase : PhoneApplicationPage
  {

     ...
    private List<Object> m_viewModels;
    public static Func<IViewModelResolver> ViewModelManager
    {
      get;
      set;
    }



    protected override void OnNavigatedFrom(NavigationEventArgs e)
    {

      base.OnNavigatedFrom(e);
      SaveState();
      handleAllDeactivated();

    }

    private T tryGetTypedViewModel<T>(object obj)
              where T: class
    {
      return obj as T;
    }

    protected virtual void SaveState()
    {

      var statebag = new KnownTypesDictionary();
      var rootPair = getRootpair();
      saveTransientState(rootPair, statebag, this, null);
      string stateKey = GetType().FullName;
      State[stateKey] = statebag;
      LastSavedTransientState = statebag;

    }

    private ElementIndexPair getRootpair()
    {
      var rootPair = new ElementIndexPair
      {
        Element = this,
        Parent = null,
        Index = 0
      };
      return rootPair;
    }

    private void saveTransientState(ElementIndexPair pair, Dictionary<string, Object> stateBag, FrameworkElement root, Object lastDataContext)
    {
      if (ViewModelManager().IsView(pair.Element))
      {

        var stateManager = getStateManager(pair.Element.DataContext);

        if (stateManager != null && stateManager != lastDataContext)
        {
          lastDataContext = stateManager;
          saveStateInner(pair, stateBag, root, stateManager);
        }
      }

      UIHelper.GetChildren(pair.Element)
              .ToList()
              .ForEach(elementPair =>
                               {
                                 elementPair.Parent = pair;
                                 saveTransientState(elementPair, stateBag, root, lastDataContext);
                               });

    }

    private static void saveStateInner(ElementIndexPair pair, Dictionary<string, Object> stateBag, FrameworkElement root, ITransientStateManager stateManager)
    {
      var state = stateManager.SaveState();
      if (state != null)
      {
        var stateKey = UIHelper.GetTransientStateKey(pair, root);
        stateBag[stateKey] = state;
      }
    }


    private ITransientStateManager getStateManager(Object obj)
    {
      var stateManager = obj as ITransientStateManager;
      return stateManager;
    }

    private void handleAllDeactivated()
    {
      m_viewModels.ForEach(notifyDeactivated);
    }


    private void notifyDeactivated(object obj)
    {
      var deactivatedItem = tryGetTypedViewModel<IDeactivated>(obj);
      if (deactivatedItem != null)
      {
        deactivatedItem.Deactivate();
      }
    }
  }
     ...
}