Added ResourceInclude.

src/Avalonia.Controls/Application.cs

@@ -149,7 +149,7 @@ namespace Avalonia
         IStyleHost IStyleHost.StylingParent => null;
 
         /// <inheritdoc/>
-        bool IResourceNode.HasResources => _resources?.Count > 0;
+        bool IResourceProvider.HasResources => _resources?.Count > 0;
 
         /// <inheritdoc/>
         IResourceNode IResourceNode.ResourceParent => null;
@@ -182,7 +182,7 @@ namespace Avalonia
         }
 
         /// <inheritdoc/>
-        bool IResourceNode.TryGetResource(string key, out object value)
+        bool IResourceProvider.TryGetResource(string key, out object value)
         {
             value = null;
             return (_resources?.TryGetResource(key, out value) ?? false) ||

src/Avalonia.Controls/Control.cs

@@ -401,7 +401,7 @@ namespace Avalonia.Controls
         IAvaloniaReadOnlyList<ILogical> ILogical.LogicalChildren => LogicalChildren;
 
         /// <inheritdoc/>
-        bool IResourceNode.HasResources => _resources?.Count > 0 || Styles.HasResources;
+        bool IResourceProvider.HasResources => _resources?.Count > 0 || Styles.HasResources;
 
         /// <inheritdoc/>
         IResourceNode IResourceNode.ResourceParent => ((IStyleHost)this).StylingParent as IResourceNode;
@@ -491,7 +491,7 @@ namespace Avalonia.Controls
         }
 
         /// <inheritdoc/>
-        bool IResourceNode.TryGetResource(string key, out object value)
+        bool IResourceProvider.TryGetResource(string key, out object value)
         {
             value = null;
             return (_resources?.TryGetResource(key, out value) ?? false) ||

src/Avalonia.Styling/Controls/IResourceDictionary.cs

@@ -9,28 +9,11 @@ namespace Avalonia.Controls
     /// <summary>
     /// An indexed dictionary of resources.
     /// </summary>
-    public interface IResourceDictionary : IDictionary<string, object>
+    public interface IResourceDictionary : IResourceProvider, IDictionary<string, object>
     {
-        /// <summary>
-        /// Raised when resources in the dictionary are changed.
-        /// </summary>
-        event EventHandler<ResourcesChangedEventArgs> ResourcesChanged;
-
         /// <summary>
         /// Gets a collection of child resource dictionaries.
         /// </summary>
-        IList<IResourceDictionary> MergedDictionaries { get; }
-
-        /// <summary>
-        /// Tries to find a resource within the dictionary.
-        /// </summary>
-        /// <param name="key">The resource key.</param>
-        /// <param name="value">
-        /// When this method returns, contains the value associated with the specified key,
-        /// if the key is found; otherwise, null
-        /// <returns>
-        /// True if the resource if found, otherwise false.
-        /// </returns>
-        bool TryGetResource(string key, out object value);
+        IList<IResourceProvider> MergedDictionaries { get; }
     }
 }

src/Avalonia.Styling/Controls/IResourceNode.cs

@@ -3,35 +3,13 @@
 namespace Avalonia.Controls
 {
     /// <summary>
-    /// Defines an element that can be queried for resources.
+    /// Represents resource provider in a tree.
     /// </summary>
-    public interface IResourceNode
+    public interface IResourceNode : IResourceProvider
     {
-        /// <summary>
-        /// Raised when resources in the element are changed.
-        /// </summary>
-        event EventHandler<ResourcesChangedEventArgs> ResourcesChanged;
-
-        /// <summary>
-        /// Gets a value indicating whether the node has resources.
-        /// </summary>
-        bool HasResources { get; }
-
         /// <summary>
         /// Gets the parent resource node, if any.
         /// </summary>
         IResourceNode ResourceParent { get; }
-
-        /// <summary>
-        /// Tries to find a resource within the element.
-        /// </summary>
-        /// <param name="key">The resource key.</param>
-        /// <param name="value">
-        /// When this method returns, contains the value associated with the specified key,
-        /// if the key is found; otherwise, null
-        /// <returns>
-        /// True if the resource if found, otherwise false.
-        /// </returns>
-        bool TryGetResource(string key, out object value);
     }
 }

src/Avalonia.Styling/Controls/IResourceProvider.cs

@@ -0,0 +1,32 @@
+using System;
+
+namespace Avalonia.Controls
+{
+    /// <summary>
+    /// Represents an object that can be queried for resources.
+    /// </summary>
+    public interface IResourceProvider
+    {
+        /// <summary>
+        /// Raised when resources in the provider are changed.
+        /// </summary>
+        event EventHandler<ResourcesChangedEventArgs> ResourcesChanged;
+
+        /// <summary>
+        /// Gets a value indicating whether the element has resources.
+        /// </summary>
+        bool HasResources { get; }
+
+        /// <summary>
+        /// Tries to find a resource within the provider.
+        /// </summary>
+        /// <param name="key">The resource key.</param>
+        /// <param name="value">
+        /// When this method returns, contains the value associated with the specified key,
+        /// if the key is found; otherwise, null
+        /// <returns>
+        /// True if the resource if found, otherwise false.
+        /// </returns>
+        bool TryGetResource(string key, out object value);
+    }
+}

src/Avalonia.Styling/Controls/ResourceDictionary.cs

@@ -4,6 +4,7 @@
 using System;
 using System.Collections.Generic;
 using System.Collections.Specialized;
+using System.Linq;
 using Avalonia.Collections;
 
 namespace Avalonia.Controls
@@ -13,7 +14,7 @@ namespace Avalonia.Controls
     /// </summary>
     public class ResourceDictionary : AvaloniaDictionary<string, object>, IResourceDictionary
     {
-        private AvaloniaList<IResourceDictionary> _mergedDictionaries;
+        private AvaloniaList<IResourceProvider> _mergedDictionaries;
 
         /// <summary>
         /// Initializes a new instance of the <see cref="ResourceDictionary"/> class.
@@ -27,18 +28,18 @@ namespace Avalonia.Controls
         public event EventHandler<ResourcesChangedEventArgs> ResourcesChanged;
 
         /// <inheritdoc/>
-        public IList<IResourceDictionary> MergedDictionaries
+        public IList<IResourceProvider> MergedDictionaries
         {
             get
             {
                 if (_mergedDictionaries == null)
                 {
-                    _mergedDictionaries = new AvaloniaList<IResourceDictionary>();
+                    _mergedDictionaries = new AvaloniaList<IResourceProvider>();
                     _mergedDictionaries.ResetBehavior = ResetBehavior.Remove;
                     _mergedDictionaries.ForEachItem(
                         x =>
                         {
-                            if (x.Count > 0)
+                            if (x.HasResources)
                             {
                                 OnResourcesChanged();
                             }
@@ -47,7 +48,7 @@ namespace Avalonia.Controls
                         },
                         x =>
                         {
-                            if (x.Count > 0)
+                            if (x.HasResources)
                             {
                                 OnResourcesChanged();
                             }
@@ -61,6 +62,12 @@ namespace Avalonia.Controls
             }
         }
 
+        /// <inheritdoc/>
+        bool IResourceProvider.HasResources
+        {
+            get => Count > 0 || (_mergedDictionaries?.Any(x => x.HasResources) ?? false);
+        }
+
         /// <inheritdoc/>
         public bool TryGetResource(string key, out object value)
         {

src/Avalonia.Styling/Styling/Style.cs

@@ -79,10 +79,10 @@ namespace Avalonia.Styling
         public IList<ISetter> Setters { get; set; } = new List<ISetter>();
 
         /// <inheritdoc/>
-        bool IResourceNode.HasResources => _resources?.Count > 0;
+        IResourceNode IResourceNode.ResourceParent => _parent;
 
         /// <inheritdoc/>
-        IResourceNode IResourceNode.ResourceParent => _parent;
+        bool IResourceProvider.HasResources => _resources?.Count > 0;
 
         /// <summary>
         /// Attaches the style to a control if the style's selector matches.

src/Markup/Avalonia.Markup.Xaml/Avalonia.Markup.Xaml.csproj

@@ -34,6 +34,7 @@
         <Compile Include="Converters\MatrixTypeConverter.cs" />
         <Compile Include="Converters\RectTypeConverter.cs" />
         <Compile Include="Converters\SetterValueTypeConverter.cs" />
+        <Compile Include="Data\ResourceInclude.cs" />
         <Compile Include="MarkupExtensions\DynamicResourceExtension.cs" />
         <Compile Include="MarkupExtensions\StaticResourceExtension.cs" />
         <Compile Include="MarkupExtensions\StyleIncludeExtension.cs" />

src/Markup/Avalonia.Markup.Xaml/Data/ResourceInclude.cs

@@ -0,0 +1,63 @@
+using System;
+using System.ComponentModel;
+using Avalonia.Controls;
+using Portable.Xaml.ComponentModel;
+using Portable.Xaml.Markup;
+
+namespace Avalonia.Markup.Xaml.Data
+{
+    /// <summary>
+    /// Loads a resource dictionary from a specified URL.
+    /// </summary>
+    public class ResourceInclude : MarkupExtension, IResourceProvider
+    {
+        private Uri _baseUri;
+        private IResourceDictionary _loaded;
+
+        public event EventHandler<ResourcesChangedEventArgs> ResourcesChanged;
+
+        /// <summary>
+        /// Gets the loaded resource dictionary.
+        /// </summary>
+        public IResourceDictionary Loaded
+        {
+            get
+            {
+                if (_loaded == null)
+                {
+                    var loader = new AvaloniaXamlLoader();
+                    _loaded = (IResourceDictionary)loader.Load(Source, _baseUri);
+
+                    if (_loaded.HasResources)
+                    {
+                        ResourcesChanged?.Invoke(this, new ResourcesChangedEventArgs());
+                    }
+                }
+
+                return _loaded;
+            }
+        }
+
+        /// <summary>
+        /// Gets or sets the source URL.
+        /// </summary>
+        public Uri Source { get; set; }
+
+        /// <inhertidoc/>
+        bool IResourceProvider.HasResources => Loaded.HasResources;
+
+        /// <inhertidoc/>
+        bool IResourceProvider.TryGetResource(string key, out object value)
+        {
+            return Loaded.TryGetResource(key, out value);
+        }
+
+        /// <inhertidoc/>
+        public override object ProvideValue(IServiceProvider serviceProvider)
+        {
+            var tdc = (ITypeDescriptorContext)serviceProvider;
+            _baseUri = tdc?.GetBaseUri();
+            return this;
+        }
+    }
+}

src/Markup/Avalonia.Markup.Xaml/Styling/StyleInclude.cs

@@ -52,7 +52,7 @@ namespace Avalonia.Markup.Xaml.Styling
         }
 
         /// <inheritdoc/>
-        bool IResourceNode.HasResources => Loaded.HasResources;
+        bool IResourceProvider.HasResources => Loaded.HasResources;
 
         /// <inheritdoc/>
         IResourceNode IResourceNode.ResourceParent => _parent;

tests/Avalonia.Markup.Xaml.UnitTests/Data/ResourceIncludeTests.cs

@@ -0,0 +1,55 @@
+using System;
+using Avalonia.Controls;
+using Avalonia.Media;
+using Avalonia.UnitTests;
+using Xunit;
+
+namespace Avalonia.Markup.Xaml.UnitTests.Data
+{
+    public class ResourceIncludeTests
+    {
+        public class StaticResourceExtensionTests
+        {
+            [Fact]
+            public void ResourceInclude_Loads_ResourceDictionary()
+            {
+                var includeXaml = @"
+<ResourceDictionary xmlns='https://github.com/avaloniaui'
+                    xmlns:x='http://schemas.microsoft.com/winfx/2006/xaml'>
+    <SolidColorBrush x:Key='brush'>#ff506070</SolidColorBrush>
+</ResourceDictionary>
+";
+                using (StartWithResources(("test:include.xaml", includeXaml)))
+                {
+                    var xaml = @"
+<UserControl xmlns='https://github.com/avaloniaui'
+             xmlns:x='http://schemas.microsoft.com/winfx/2006/xaml'>
+    <UserControl.Resources>
+        <ResourceDictionary>
+            <ResourceDictionary.MergedDictionaries>
+                <ResourceInclude Source='test:include.xaml'/>
+            </ResourceDictionary.MergedDictionaries>
+        </ResourceDictionary>
+    </UserControl.Resources>
+
+    <Border Name='border' Background='{StaticResource brush}'/>
+</UserControl>";
+
+                    var loader = new AvaloniaXamlLoader();
+                    var userControl = (UserControl)loader.Load(xaml);
+                    var border = userControl.FindControl<Border>("border");
+
+                    var brush = (SolidColorBrush)border.Background;
+                    Assert.Equal(0xff506070, brush.Color.ToUint32());
+                }
+            }
+
+            private IDisposable StartWithResources(params (string, string)[] assets)
+            {
+                var assetLoader = new MockAssetLoader(assets);
+                var services = new TestServices(assetLoader: assetLoader);
+                return UnitTestApplication.Start(services);
+            }
+        }
+    }
+}

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

@@ -485,7 +485,7 @@ namespace Avalonia.Markup.Xaml.UnitTests.MarkupExtensions
 
             Assert.Null(border.Background);
 
-            userControl.Resources.MergedDictionaries[0].Add("brush", new SolidColorBrush(0xff506070));
+            ((IResourceDictionary)userControl.Resources.MergedDictionaries[0]).Add("brush", new SolidColorBrush(0xff506070));
 
             var brush = (SolidColorBrush)border.Background;
             Assert.NotNull(brush);

tests/Avalonia.Styling.UnitTests/ResourceDictionaryTests.cs

@@ -167,7 +167,7 @@ namespace Avalonia.Styling.UnitTests
             var raised = false;
 
             target.ResourcesChanged += (_, __) => raised = true;
-            target.MergedDictionaries[0].Add("foo", "bar");
+            ((IResourceDictionary)target.MergedDictionaries[0]).Add("foo", "bar");
 
             Assert.True(raised);
         }