Show scroll info in virt test app.

samples/VirtualizationTest/MainWindow.xaml

@@ -1,13 +1,25 @@
 <Window xmlns="https://github.com/avaloniaui"
         Title="Avalonia Virtualization Test">
     <DockPanel LastChildFill="True" Margin="16">
-        <StackPanel DockPanel.Dock="Right" Margin="16 0 0 0">
+        <StackPanel DockPanel.Dock="Right" 
+                    Margin="16 0 0 0" 
+                    MinWidth="150"
+                    Gap="4">
             <TextBox Watermark="Item Count"
                      UseFloatingWatermark="True"
                      Text="{Binding ItemCount}"/>
+            <TextBox Watermark="Extent"
+                     UseFloatingWatermark="True"
+                     Text="{Binding #listBox.Scroll.Extent, Mode=OneWay}"/>
+            <TextBox Watermark="Offset"
+                     UseFloatingWatermark="True"
+                     Text="{Binding #listBox.Scroll.Offset, Mode=OneWay}"/>
+            <TextBox Watermark="Viewport"
+                     UseFloatingWatermark="True"
+                     Text="{Binding #listBox.Scroll.Viewport, Mode=OneWay}"/>
         </StackPanel>
 
-        <ListBox Items="{Binding Items}">
+        <ListBox Name="listBox" Items="{Binding Items}">
             <ListBox.ItemTemplate>
                 <DataTemplate>
                     <TextBlock Text="{Binding Header}"/>

samples/VirtualizationTest/Program.cs

@@ -4,6 +4,8 @@
 using System;
 using Avalonia;
 using Avalonia.Controls;
+using Avalonia.Logging.Serilog;
+using Serilog;
 
 namespace VirtualizationTest
 {
@@ -11,10 +13,22 @@ namespace VirtualizationTest
     {
         static void Main(string[] args)
         {
+            InitializeLogging();
+
             AppBuilder.Configure<App>()
                .UseWin32()
                .UseDirect2D1()
                .Start<MainWindow>();
         }
+
+        private static void InitializeLogging()
+        {
+#if DEBUG
+            SerilogLogger.Initialize(new LoggerConfiguration()
+                .MinimumLevel.Warning()
+                .WriteTo.Trace(outputTemplate: "{Area}: {Message}")
+                .CreateLogger());
+#endif
+        }
     }
 }

samples/VirtualizationTest/ViewModels/ItemViewModel.cs

@@ -2,10 +2,11 @@
 // Licensed under the MIT license. See licence.md file in the project root for full license information.
 
 using System;
+using ReactiveUI;
 
 namespace VirtualizationTest.ViewModels
 {
-    internal class ItemViewModel
+    internal class ItemViewModel : ReactiveObject
     {
         private int _index;
 

samples/VirtualizationTest/VirtualizationTest.csproj

@@ -36,6 +36,14 @@
     <StartupObject />
   </PropertyGroup>
   <ItemGroup>
+    <Reference Include="Serilog, Version=1.5.0.0, Culture=neutral, PublicKeyToken=24c2f752a8e58a10, processorArchitecture=MSIL">
+      <HintPath>..\..\packages\Serilog.1.5.14\lib\net45\Serilog.dll</HintPath>
+      <Private>True</Private>
+    </Reference>
+    <Reference Include="Serilog.FullNetFx, Version=1.5.0.0, Culture=neutral, PublicKeyToken=24c2f752a8e58a10, processorArchitecture=MSIL">
+      <HintPath>..\..\packages\Serilog.1.5.14\lib\net45\Serilog.FullNetFx.dll</HintPath>
+      <Private>True</Private>
+    </Reference>
     <Reference Include="Splat, Version=1.6.2.0, Culture=neutral, processorArchitecture=MSIL">
       <HintPath>..\..\packages\Splat.1.6.2\lib\Net45\Splat.dll</HintPath>
       <Private>True</Private>
@@ -114,6 +122,10 @@
       <Project>{42472427-4774-4c81-8aff-9f27b8e31721}</Project>
       <Name>Avalonia.Layout</Name>
     </ProjectReference>
+    <ProjectReference Include="..\..\src\Avalonia.Logging.Serilog\Avalonia.Logging.Serilog.csproj">
+      <Project>{B61B66A3-B82D-4875-8001-89D3394FE0C9}</Project>
+      <Name>Avalonia.Logging.Serilog</Name>
+    </ProjectReference>
     <ProjectReference Include="..\..\src\Avalonia.ReactiveUI\Avalonia.ReactiveUI.csproj">
       <Project>{6417b24e-49c2-4985-8db2-3ab9d898ec91}</Project>
       <Name>Avalonia.ReactiveUI</Name>

samples/VirtualizationTest/packages.config

@@ -5,5 +5,6 @@
   <package id="Rx-Linq" version="2.2.5" targetFramework="net452" />
   <package id="Rx-Main" version="2.2.5" targetFramework="net452" />
   <package id="Rx-PlatformServices" version="2.2.5" targetFramework="net452" />
+  <package id="Serilog" version="1.5.14" targetFramework="net452" />
   <package id="Splat" version="1.6.2" targetFramework="net452" />
 </packages>

src/Avalonia.Controls/ListBox.cs

@@ -21,6 +21,12 @@ namespace Avalonia.Controls
         private static readonly FuncTemplate<IPanel> DefaultPanel =
             new FuncTemplate<IPanel>(() => new VirtualizingStackPanel());
 
+        /// <summary>
+        /// Defines the <see cref="Scroll"/> property.
+        /// </summary>
+        public static readonly DirectProperty<ListBox, IScrollable> ScrollProperty =
+            AvaloniaProperty.RegisterDirect<ListBox, IScrollable>(nameof(Scroll), o => o.Scroll);
+
         /// <summary>
         /// Defines the <see cref="SelectedItems"/> property.
         /// </summary>
@@ -39,6 +45,8 @@ namespace Avalonia.Controls
         public static readonly AvaloniaProperty<ItemVirtualizationMode> VirtualizationModeProperty =
             ItemsPresenter.VirtualizationModeProperty.AddOwner<ListBox>();
 
+        private IScrollable _scroll;
+
         /// <summary>
         /// Initializes static members of the <see cref="ItemsControl"/> class.
         /// </summary>
@@ -50,7 +58,11 @@ namespace Avalonia.Controls
         /// <summary>
         /// Gets the scroll information for the <see cref="ListBox"/>.
         /// </summary>
-        public IScrollable Scroll { get; set; }
+        public IScrollable Scroll
+        {
+            get { return _scroll; }
+            private set { SetAndRaise(ScrollProperty, ref _scroll, value); }
+        }
 
         /// <inheritdoc/>
         public new IList SelectedItems => base.SelectedItems;