ViewModelLocator.cs

 public class ViewModelLocator
    {
         public  ViewModelLocator()
        {
            ServiceLocator.SetLocatorProvider(() => SimpleIoc.Default);
           ....
            if (ViewModelBase.IsInDesignModeStatic)//if in desing time
            {
                SimpleIoc.Default.Register<IKhanAcademyService, DesignKhanAcademyService>();
            }else{
                SimpleIoc.Default.Register<IKhanAcademyService, KhanAcademyService>();
            }
            
            SimpleIoc.Default.Register<INavigationService, NavigationService>();
            SimpleIoc.Default.Register<MainViewModel>();
            SimpleIoc.Default.Register<TopicsViewModel>(true);
            SimpleIoc.Default.Register<PlayListViewModel>(true);
            SimpleIoc.Default.Register<VideoViewModel>(true);
        }

        public TopicsViewModel TopicsViewModel
        {
            get { return ServiceLocator.Current.GetInstance<TopicsViewModel>(); }
        }

        ............
        public VideoViewModel Video
        {
            get { return ServiceLocator.Current.GetInstance<VideoViewModel>(); }
        }
.....