RxUI Search example

App.cs

public partial class App : Application
    {
        protected override void OnStartup(StartupEventArgs e)
        {
            base.OnStartup(e);

            //ghetto bootstrap - IS THIS OK?
            var searchView = new SearchView();
            searchView.ViewModel = new SearchViewModel();
            searchView.DataContext = searchView.ViewModel;
            var mw = new AppShell(searchView);

            mw.Show();

        }
    }
    
    public AppShell(SearchView searchView)
        {
            InitializeComponent();
            this.MainPanel.Children.Add(searchView);
        }

SearchView.xaml

<UserControl x:Class="FeedManagement.WPF.Search.SearchView"
             xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
             xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
             xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006" 
             xmlns:d="http://schemas.microsoft.com/expression/blend/2008" 
             xmlns:v="clr-namespace:FeedManagement.WPF.Search"
             mc:Ignorable="d" 
             d:DesignHeight="300" d:DesignWidth="300">
    <UserControl.Resources>
        <DataTemplate DataType="{x:Type v:SearchViewModel}">
            <v:SearchView ViewModel="{Binding}"/>
        </DataTemplate>
    </UserControl.Resources>
    <Grid>
        <Grid.ColumnDefinitions>
            <ColumnDefinition/>
        </Grid.ColumnDefinitions>
        <Grid.RowDefinitions>
            <RowDefinition Height="1*"/>
            <RowDefinition Height="8*"/>
        </Grid.RowDefinitions>
        <Grid Grid.Row="0">
            <Grid.ColumnDefinitions>
                <ColumnDefinition Width="Auto"/>
                <ColumnDefinition/>
                <ColumnDefinition Width="Auto"/>
            </Grid.ColumnDefinitions>
            <Label>Search</Label>
            <TextBox x:Name="SearchText" Grid.Column="1"></TextBox>
            <Button x:Name="SearchButton" Grid.Column="2" Content="{StaticResource Icons.Search}" ></Button>
        </Grid>
        <DockPanel Grid.Row="1">
            <ScrollViewer VerticalScrollBarVisibility="Auto" HorizontalScrollBarVisibility="Auto">
                <ListBox x:Name="TweetTiles" >
                    
                </ListBox>
            </ScrollViewer>
        </DockPanel>
    </Grid>
</UserControl>

SearchView.xaml.cs

public partial class SearchView : UserControl , IViewFor<SearchViewModel>
    {
        public SearchView()
        {
            InitializeComponent();
            //2 way bind between the view and view model of the search text
            this.Bind(ViewModel, vm => vm.SearchText, v=>v.SearchText.Text);
            //one way bind from the view model prop TweetTiles to the view TweetTiles.ItemsSource
            this.OneWayBind(ViewModel, vm => vm.TweetTiles, v => v.TweetTiles.ItemsSource);
            //binds the search command, defined on viewmodel, to the view SearchButton
            this.BindCommand(ViewModel, vm => vm.SearchCommand, v => v.SearchButton);
        }

        //The interface implementation needs to be a DependencyProperty, so that RUI
        //can pick up on view model changes. make sure that the viewmodel type matches
        //the type declared in IViewFor<xxx> (e.g. is it an interface?)
        public static readonly DependencyProperty ViewModelProperty =
            DependencyProperty.Register("ViewModel", typeof(SearchViewModel), typeof(SearchView), new PropertyMetadata(null));

        public SearchViewModel ViewModel
        {
            get { return (SearchViewModel)GetValue(ViewModelProperty); }
            set { SetValue(ViewModelProperty, value); }
        }

        object IViewFor.ViewModel
        {
            get { return ViewModel; }
            set { ViewModel = (SearchViewModel)value; }
        }
    }

SearchViewModel.cs

public class SearchViewModel : ReactiveObject, ISearchViewModel
    {
        private Twitter _twitterAPI = new Twitter();

        public ReactiveCommand SearchCommand { get; set; }

        private ObservableAsPropertyHelper<List<TwitterTileViewModel>> _TweetTiles;
        public List<TwitterTileViewModel> TweetTiles
        {
            get
            {
                return _TweetTiles.Value;
            }
        }

        public SearchViewModel()
        {

            SearchCommand = new ReactiveCommand(null, false, null);

            var results = SearchCommand.RegisterAsyncTask(t => Task.Factory.StartNew(() => _twitterAPI.SearchFor(SearchText)))
                .Select(x => ConvertTweets(x));
                

            SearchCommand.ThrownExceptions
                .Subscribe(ex => this.Log().WarnException("Twitter Search Failed", ex));

            _TweetTiles = new ObservableAsPropertyHelper<List<TwitterTileViewModel>>(results, vm => { this.RaisePropertyChanged("TweetTiles"); });
        }

        List<TwitterTileViewModel> ConvertTweets(List<Tweet> tweets)
        {
            var tempTiles = new List<TwitterTileViewModel>();
            tweets.ForEach(tweet => tempTiles.Add(new TwitterTileViewModel(tweet)));
            return new List<TwitterTileViewModel>(tempTiles);
        }

        private string _searchText = "";
        public string SearchText
        {
            get { return _searchText; }
            set { _searchText = value; }
        }
    }

StackTrace.txt

System.NullReferenceException was unhandled
  HResult=-2147467261
  Message=Object reference not set to an instance of an object.
  Source=ReactiveUI.Xaml
  StackTrace:
       at ReactiveUI.Xaml.ViewModelViewHost.<.ctor>b__5(<>f__AnonymousType2`2 x)
       at System.Reactive.AnonymousSafeObserver`1.OnNext(T value)
       at System.Reactive.Linq.Observαble.CombineLatest`3._.F.OnNext(TFirst value)
       at System.Reactive.Linq.Observαble.Select`2._.OnNext(TSource value)
       at System.Reactive.Linq.Observαble.DistinctUntilChanged`2._.OnNext(TSource value)
       at System.Reactive.Linq.Observαble.Select`2._.OnNext(TSource value)
       at System.Reactive.Linq.Observαble.Where`1._.OnNext(TSource value)
       at System.Reactive.Linq.Observαble.Switch`1._.ι.OnNext(TSource value)
       at System.Reactive.Linq.Observαble.Concat`1._.OnNext(TSource value)
       at System.Reactive.Linq.Observαble.Select`2._.OnNext(TSource value)
       at System.Reactive.AutoDetachObserver`1.OnNextCore(T value)
       at System.Reactive.ObserverBase`1.OnNext(T value)
       at ReactiveUI.Xaml.DependencyObjectObservableForProperty.<>c__DisplayClass4.<>c__DisplayClass6.<GetNotificationForProperty>b__2(Object o, EventArgs e)
       at MS.Internal.ComponentModel.PropertyChangeTracker.OnPropertyInvalidation(DependencyObject d, DependencyPropertyChangedEventArgs args)
       at System.Windows.DependentList.InvalidateDependents(DependencyObject source, DependencyPropertyChangedEventArgs sourceArgs)
       at System.Windows.DependencyObject.NotifyPropertyChange(DependencyPropertyChangedEventArgs args)
       at System.Windows.DependencyObject.UpdateEffectiveValue(EntryIndex entryIndex, DependencyProperty dp, PropertyMetadata metadata, EffectiveValueEntry oldEntry, EffectiveValueEntry& newEntry, Boolean coerceWithDeferredReference, Boolean coerceWithCurrentValue, OperationType operationType)
       at System.Windows.StyleHelper.ApplyTemplatedParentValue(DependencyObject container, FrameworkObject child, Int32 childIndex, FrugalStructList`1& childRecordFromChildIndex, DependencyProperty dp, FrameworkElementFactory templateRoot)
       at System.Windows.StyleHelper.InvalidatePropertiesOnTemplateNode(DependencyObject container, FrameworkObject child, Int32 childIndex, FrugalStructList`1& childRecordFromChildIndex, Boolean isDetach, FrameworkElementFactory templateRoot)
       at System.Windows.FrameworkTemplate.InvalidatePropertiesOnTemplate(DependencyObject container, Object currentObject)
       at System.Windows.FrameworkTemplate.HandleBeforeProperties(Object createdObject, DependencyObject& rootObject, DependencyObject container, FrameworkElement feContainer, INameScope nameScope)
       at System.Windows.FrameworkTemplate.<>c__DisplayClass6.<LoadOptimizedTemplateContent>b__3(Object sender, XamlObjectEventArgs args)
       at System.Xaml.XamlObjectWriter.OnBeforeProperties(Object value)
       at System.Xaml.XamlObjectWriter.Logic_CreateAndAssignToParentStart(ObjectWriterContext ctx)
       at System.Xaml.XamlObjectWriter.WriteEndObject()
       at System.Xaml.XamlWriter.WriteNode(XamlReader reader)
       at System.Windows.FrameworkTemplate.LoadTemplateXaml(XamlReader templateReader, XamlObjectWriter currentWriter)
       at System.Windows.FrameworkTemplate.LoadTemplateXaml(XamlObjectWriter objectWriter)
       at System.Windows.FrameworkTemplate.LoadOptimizedTemplateContent(DependencyObject container, IComponentConnector componentConnector, IStyleConnector styleConnector, List`1 affectedChildren, UncommonField`1 templatedNonFeChildrenField)
       at System.Windows.FrameworkTemplate.LoadContent(DependencyObject container, List`1 affectedChildren)
       at System.Windows.StyleHelper.ApplyTemplateContent(UncommonField`1 dataField, DependencyObject container, FrameworkElementFactory templateRoot, Int32 lastChildIndex, HybridDictionary childIndexFromChildID, FrameworkTemplate frameworkTemplate)
       at System.Windows.FrameworkTemplate.ApplyTemplateContent(UncommonField`1 templateDataField, FrameworkElement container)
       at System.Windows.FrameworkElement.ApplyTemplate()
       at System.Windows.FrameworkElement.MeasureCore(Size availableSize)
       at System.Windows.UIElement.Measure(Size availableSize)
       at System.Windows.Controls.Border.MeasureOverride(Size constraint)
       at System.Windows.FrameworkElement.MeasureCore(Size availableSize)
       at System.Windows.UIElement.Measure(Size availableSize)
       at System.Windows.Controls.Control.MeasureOverride(Size constraint)
       at System.Windows.FrameworkElement.MeasureCore(Size availableSize)
       at System.Windows.UIElement.Measure(Size availableSize)
       at System.Windows.Controls.VirtualizingStackPanel.MeasureChild(IItemContainerGenerator& generator, IContainItemStorage& itemStorageProvider, IContainItemStorage& parentItemStorageProvider, Object& parentItem, Boolean& hasUniformOrAverageContainerSizeBeenSet, Double& computedUniformOrAverageContainerSize, Boolean& computedAreContainersUniformlySized, IList& items, Object& item, IList& children, Int32& childIndex, Boolean& visualOrderChanged, Boolean& isHorizontal, Size& childConstraint, Rect& viewport, VirtualizationCacheLength& cacheSize, VirtualizationCacheLengthUnit& cacheUnit, Boolean& foundFirstItemInViewport, Double& firstItemInViewportOffset, Size& stackPixelSize, Size& stackPixelSizeInViewport, Size& stackPixelSizeInCacheBeforeViewport, Size& stackPixelSizeInCacheAfterViewport, Size& stackLogicalSize, Size& stackLogicalSizeInViewport, Size& stackLogicalSizeInCacheBeforeViewport, Size& stackLogicalSizeInCacheAfterViewport, Boolean& mustDisableVirtualization, Boolean isBeforeFirstItem, Boolean isAfterFirstItem, Boolean isAfterLastItem, Boolean skipActualMeasure, Boolean skipGeneration, Boolean& hasBringIntoViewContainerBeenMeasured, Boolean& hasVirtualizingChildren)
       at System.Windows.Controls.VirtualizingStackPanel.MeasureOverrideImpl(Size constraint, Nullable`1& lastPageSafeOffset, List`1& previouslyMeasuredOffsets, Boolean remeasure)
       at System.Windows.Controls.VirtualizingStackPanel.MeasureOverride(Size constraint)
       at System.Windows.FrameworkElement.MeasureCore(Size availableSize)
       at System.Windows.UIElement.Measure(Size availableSize)
       at System.Windows.ContextLayoutManager.UpdateLayout()
       at System.Windows.ContextLayoutManager.UpdateLayoutCallback(Object arg)
       at System.Windows.Media.MediaContext.InvokeOnRenderCallback.DoWork()
       at System.Windows.Media.MediaContext.FireInvokeOnRenderCallbacks()
       at System.Windows.Media.MediaContext.RenderMessageHandlerCore(Object resizedCompositionTarget)
       at System.Windows.Media.MediaContext.AnimatedRenderMessageHandler(Object resizedCompositionTarget)
       at System.Windows.Threading.ExceptionWrapper.InternalRealCall(Delegate callback, Object args, Int32 numArgs)
       at MS.Internal.Threading.ExceptionFilterHelper.TryCatchWhen(Object source, Delegate method, Object args, Int32 numArgs, Delegate catchHandler)
       at System.Windows.Threading.DispatcherOperation.InvokeImpl()
       at System.Windows.Threading.DispatcherOperation.InvokeInSecurityContext(Object state)
       at System.Threading.ExecutionContext.RunInternal(ExecutionContext executionContext, ContextCallback callback, Object state, Boolean preserveSyncCtx)
       at System.Threading.ExecutionContext.Run(ExecutionContext executionContext, ContextCallback callback, Object state, Boolean preserveSyncCtx)
       at System.Threading.ExecutionContext.Run(ExecutionContext executionContext, ContextCallback callback, Object state)
       at System.Windows.Threading.DispatcherOperation.Invoke()
       at System.Windows.Threading.Dispatcher.ProcessQueue()
       at System.Windows.Threading.Dispatcher.WndProcHook(IntPtr hwnd, Int32 msg, IntPtr wParam, IntPtr lParam, Boolean& handled)
       at MS.Win32.HwndWrapper.WndProc(IntPtr hwnd, Int32 msg, IntPtr wParam, IntPtr lParam, Boolean& handled)
       at MS.Win32.HwndSubclass.DispatcherCallbackOperation(Object o)
       at System.Windows.Threading.ExceptionWrapper.InternalRealCall(Delegate callback, Object args, Int32 numArgs)
       at MS.Internal.Threading.ExceptionFilterHelper.TryCatchWhen(Object source, Delegate method, Object args, Int32 numArgs, Delegate catchHandler)
       at System.Windows.Threading.Dispatcher.LegacyInvokeImpl(DispatcherPriority priority, TimeSpan timeout, Delegate method, Object args, Int32 numArgs)
       at MS.Win32.HwndSubclass.SubclassWndProc(IntPtr hwnd, Int32 msg, IntPtr wParam, IntPtr lParam)
       at MS.Win32.UnsafeNativeMethods.DispatchMessage(MSG& msg)
       at System.Windows.Threading.Dispatcher.PushFrameImpl(DispatcherFrame frame)
       at System.Windows.Threading.Dispatcher.PushFrame(DispatcherFrame frame)
       at System.Windows.Threading.Dispatcher.Run()
       at System.Windows.Application.RunDispatcher(Object ignore)
       at System.Windows.Application.RunInternal(Window window)
       at System.Windows.Application.Run(Window window)
       at System.Windows.Application.Run()
       at FeedManagement.WPF.App.Main() in c:\Users\phil.SONOCINE\Documents\Visual Studio 2013\Projects\FeedReaders\FeedManagement.WPF\obj\Debug\App.g.cs:line 0
       at System.AppDomain._nExecuteAssembly(RuntimeAssembly assembly, String[] args)
       at System.AppDomain.ExecuteAssembly(String assemblyFile, Evidence assemblySecurity, String[] args)
       at Microsoft.VisualStudio.HostingProcess.HostProc.RunUsersAssembly()
       at System.Threading.ThreadHelper.ThreadStart_Context(Object state)
       at System.Threading.ExecutionContext.RunInternal(ExecutionContext executionContext, ContextCallback callback, Object state, Boolean preserveSyncCtx)
       at System.Threading.ExecutionContext.Run(ExecutionContext executionContext, ContextCallback callback, Object state, Boolean preserveSyncCtx)
       at System.Threading.ExecutionContext.Run(ExecutionContext executionContext, ContextCallback callback, Object state)
       at System.Threading.ThreadHelper.ThreadStart()
  InnerException: 

On line 19 of SearchViewModel, you're setting SearchText on a non-UI thread. Instead, write it this way:

var results = SearchCommand.RegisterAsyncTask(t =>Task.Factory.StartNew(() => twitterAPI.SearchFor(SearchText));
results.Subscribe(x => SearchResult = x);

Also, the property setter on line 42 should be written as:

set { this.RaiseAndSetIfChanged(ref _SearchResult, value); }

As the name implies, RaiseAndSetIfChanged only does something if the property changes. But since you updated the field yourself, the property is always equal according to RaiseAndSetIfChanged