BindingExtension Source tests and sample

#1521

samples/BindingTest/MainWindow.xaml

@@ -1,4 +1,5 @@
 <Window xmlns="https://github.com/avaloniaui"
+        xmlns:x='http://schemas.microsoft.com/winfx/2006/xaml'
         xmlns:vm="clr-namespace:BindingTest.ViewModels;assembly=BindingTest"
         xmlns:local="clr-namespace:BindingTest;assembly=BindingTest">
   <Window.Styles>
@@ -6,6 +7,9 @@
       <Setter Property="FontSize" Value="18"/>
     </Style>
   </Window.Styles>
+  <Window.Resources>
+    <vm:TestItem x:Key="SharedItem" StringValue="shared" />
+  </Window.Resources>
   
   <TabControl>
     <TabItem Header="Basic">
@@ -40,6 +44,10 @@
             <TextBlock FontSize="16" Text="Binding Sources"/>
             <TextBox Watermark="Value of first TextBox" UseFloatingWatermark="True" 
                      Text="{Binding #first.Text, Mode=TwoWay}"/>
+            <TextBox Watermark="Value of SharedItem.StringValue" UseFloatingWatermark="True"
+                     Text="{Binding StringValue, Source={StaticResource SharedItem}, Mode=TwoWay}"/>
+            <TextBox Watermark="Value of SharedItem.StringValue (duplicate)" UseFloatingWatermark="True"
+                     Text="{Binding StringValue, Source={StaticResource SharedItem}, Mode=TwoWay}"/>
           </StackPanel>
           <StackPanel Margin="18" Gap="4" Width="200" HorizontalAlignment="Left">
             <TextBlock FontSize="16" Text="Scheduler"/>

tests/Avalonia.Markup.Xaml.UnitTests/MarkupExtensions/BindingExtensionTests.cs

@@ -0,0 +1,71 @@
+using System;
+using System.Collections.Generic;
+using System.Text;
+using Avalonia.Controls;
+using Avalonia.Controls.Presenters;
+using Avalonia.Controls.Templates;
+using Avalonia.Styling;
+using Avalonia.UnitTests;
+using Xunit;
+
+namespace Avalonia.Markup.Xaml.UnitTests.MarkupExtensions
+{
+    public class BindingExtensionTests
+    {
+
+        [Fact]
+        public void BindingExtension_Binds_To_Source()
+        {
+            using (StyledWindow())
+            {
+                var xaml = @"
+<Window xmlns='https://github.com/avaloniaui'
+             xmlns:x='http://schemas.microsoft.com/winfx/2006/xaml'>
+    <Window.Resources>
+        <x:String x:Key='text'>foobar</x:String>
+    </Window.Resources>
+
+    <TextBlock Name='textBlock' Text='{Binding Source={StaticResource text}}'/>
+</Window>";
+
+                var loader = new AvaloniaXamlLoader();
+                var window = (Window)loader.Load(xaml);
+                var textBlock = window.FindControl<TextBlock>("textBlock");
+
+                window.Show();
+
+                Assert.Equal("foobar", textBlock.Text);
+            }
+        }
+
+        private IDisposable StyledWindow(params (string, string)[] assets)
+        {
+            var services = TestServices.StyledWindow.With(
+                assetLoader: new MockAssetLoader(assets),
+                theme: () => new Styles
+                {
+                    WindowStyle(),
+                });
+
+            return UnitTestApplication.Start(services);
+        }
+
+        private Style WindowStyle()
+        {
+            return new Style(x => x.OfType<Window>())
+            {
+                Setters =
+                {
+                    new Setter(
+                        Window.TemplateProperty,
+                        new FuncControlTemplate<Window>(x =>
+                            new ContentPresenter
+                            {
+                                Name = "PART_ContentPresenter",
+                                [!ContentPresenter.ContentProperty] = x[!Window.ContentProperty],
+                            }))
+                }
+            };
+        }
+    }
+}