Added support for Design.PreviewWith in resource dictionaries.

src/Avalonia.Controls/Design.cs

@@ -1,4 +1,5 @@
 
+using System.Collections.Generic;
 using System.Runtime.CompilerServices;
 using Avalonia.Styling;
 
@@ -6,6 +7,8 @@ namespace Avalonia.Controls
 {
     public static class Design
     {
+        private static Dictionary<object, Control?>? _previewWith;
+
         public static bool IsDesignMode { get; internal set; }
 
         public static readonly AttachedProperty<double> HeightProperty = AvaloniaProperty
@@ -47,19 +50,30 @@ namespace Avalonia.Controls
             return control.GetValue(DataContextProperty);
         }
         
-        public static readonly AttachedProperty<Control> PreviewWithProperty = AvaloniaProperty
-            .RegisterAttached<AvaloniaObject, Control>("PreviewWith", typeof (Design));
+        public static readonly AttachedProperty<Control?> PreviewWithProperty = AvaloniaProperty
+            .RegisterAttached<AvaloniaObject, Control?>("PreviewWith", typeof (Design));
 
-        public static void SetPreviewWith(AvaloniaObject target, Control control)
+        public static void SetPreviewWith(AvaloniaObject target, Control? control)
         {
             target.SetValue(PreviewWithProperty, control);
         }
 
-        public static Control GetPreviewWith(AvaloniaObject target)
+        public static void SetPreviewWith(ResourceDictionary target, Control? control)
+        {
+            _previewWith ??= new();
+            _previewWith[target] = control;
+        }
+
+        public static Control? GetPreviewWith(AvaloniaObject target)
         {
             return target.GetValue(PreviewWithProperty);
         }
 
+        public static Control? GetPreviewWith(ResourceDictionary target)
+        {
+            return _previewWith?[target];
+        }
+
         public static readonly AttachedProperty<IStyle> DesignStyleProperty = AvaloniaProperty
             .RegisterAttached<Control, IStyle>("DesignStyle", typeof(Design));
 

src/Avalonia.DesignerSupport/DesignWindowLoader.cs

@@ -37,6 +37,7 @@ namespace Avalonia.DesignerSupport
                 var localAsm = assemblyPath != null ? Assembly.LoadFile(Path.GetFullPath(assemblyPath)) : null;
                 var loaded = loader.Load(stream, localAsm, null, baseUri, true);
                 var style = loaded as IStyle;
+                var resources = loaded as ResourceDictionary;
                 if (style != null)
                 {
                     var substitute = Design.GetPreviewWith((AvaloniaObject)style);
@@ -58,6 +59,27 @@ namespace Avalonia.DesignerSupport
                             }
                         };
                 }
+                else if (resources != null)
+                {
+                    var substitute = Design.GetPreviewWith(resources);
+                    if (substitute != null)
+                    {
+                        substitute.Resources.MergedDictionaries.Add(resources);
+                        control = substitute;
+                    }
+                    else
+                        control = new StackPanel
+                        {
+                            Children =
+                            {
+                                new TextBlock {Text = "ResourceDictionaries can't be previewed without Design.PreviewWith. Add"},
+                                new TextBlock {Text = "<Design.PreviewWith>"},
+                                new TextBlock {Text = "    <Border Padding=20><!-- YOUR CONTROL FOR PREVIEW HERE --></Border>"},
+                                new TextBlock {Text = "</Design.PreviewWith>"},
+                                new TextBlock {Text = "in your resource dictionary"}
+                            }
+                        };
+                }
                 else if (loaded is Application)
                     control = new TextBlock {Text = "Application can't be previewed in design view"};
                 else