/// <summary>
 /// Preserves state associated with this page in case the application is suspended or the
 /// page is discarded from the navigation cache.  Values must conform to the serialization
 /// requirements of <see cref="SuspensionManager.SessionState"/>.
 /// </summary>
 /// <param name="sender">The source of the event; typically <see cref="NavigationHelper"/></param>
 /// <param name="e">Event data that provides an empty dictionary to be populated with
 /// serializable state.</param>
 private void navigationHelper_SaveState(object sender, SaveStateEventArgs e)
 {
 }
 void navigationHelper_SaveState(object sender, SaveStateEventArgs e)
 {
     if (_userViewModel != null)
         e.PageState["signInStatus"] = _userViewModel.SignedIn;
 }