New Post: MVVM Event Aggregator and WCF Service. Not updating The UI

namespace Chameleon.Navigation.Views
{
    [Export("TopMenuView")]
    public partial class TopMenuView : UserControl, INavigationAware 
    {
        [ImportingConstructor]
        public TopMenuView(IEventAggregator evt, IRegionManager regionManager)
        {
            InitializeComponent();

            _regionManager = regionManager;
            _eventManager = evt;
        }

        [Import]
        public IRegionManager _regionManager;

        [Import]
        public IEventAggregator _eventManager;

        public bool IsNavigationTarget(NavigationContext navigationContext)
        {
            return true;
        }

        public void OnNavigatedFrom(NavigationContext navigationContext)
        {

        }

        public void OnNavigatedTo(NavigationContext navigationContext)
        {
            this.DataContext = new TopMenuViewModel(_eventManager, _regionManager, this.Dispatcher);
        }

        private void Button_Click_1(object sender, RoutedEventArgs e)
        {
            ((TopMenuViewModel)this.DataContext).OnLoggedIn(true);
        }
    }
}