my VM

    using PropertyChanged;

    [ImplementPropertyChanged]
    public class SearchViewModel : ViewModelBase
    {
...

            IObservable<IEnumerable<SearchDisplay>> searchedStrings = SearchText.
                Where(x => !string.IsNullOrEmpty(x)).
                Throttle(TimeSpan.FromSeconds(1)).
                DistinctUntilChanged().
                Select(SearchForText).
                Switch();
            searchedStrings.Subscribe(matches =>
                {
                    ItemsSource.ClearOnScheduler();
                    ItemsSource.AddRangeOnScheduler(matches);
                    Application.Current.Dispatcher.BeginInvoke(DispatcherPriority.Background, new Action(() =>
                        {
                            if (ItemsSource.Count == 0)
                                ItemsSource.AddOnScheduler(new SearchDisplay(0L, SearchDisplayType.NotFound));
                        }
                    ));
                });
...
        public ReactiveProperty<string> SearchText { get; set; }
        public ReactiveCollection<SearchDisplay> ItemsSource { get; private set; }
...

        private async Task<IEnumerable<SearchDisplay>> SearchForText(string searchedText)
        {
            return await Task.FromResult(...);
        }


...