Add back Design.SetPreviewWith(AvaloniaObject, Control) overload (#21184)

* Add back Design.SetPreviewWith(AvaloniaObject, Control) overload

* Remove dead code

src/Avalonia.Controls/Design.cs

@@ -142,6 +142,40 @@ namespace Avalonia.Controls
             s_previewWith[target] = template;
         }
 
+        /// <summary>
+        /// Sets a preview control for the specified <see cref="AvaloniaObject"/> at design-time.
+        /// </summary>
+        /// <param name="target">The target object.</param>
+        /// <param name="control">The preview control.</param>
+        public static void SetPreviewWith(AvaloniaObject target, Control? control)
+        {
+            if (control is null)
+            {
+                s_previewWith[target] = null;
+                return;
+            }
+
+            switch (target)
+            {
+                case ResourceDictionary resourceDictionary:
+                    SetPreviewWith(resourceDictionary, control);
+                    break;
+                case IDataTemplate dataTemplate:
+                    SetPreviewWith(dataTemplate, control);
+                    break;
+                case IStyle style:
+                    SetPreviewWith(style, control);
+                    break;
+                case Visual:
+                    // Not a supported scenario without templates; causes stack overflows.
+                    s_previewWith[target] = null;
+                    break;
+                default:
+                    SetPreviewWith(target, new FuncTemplate<Control>(() => control));
+                    break;
+            }
+        }
+
         /// <summary>
         /// Sets a preview template for the specified <see cref="ResourceDictionary"/> at design-time.
         /// </summary>

tests/Avalonia.Markup.Xaml.UnitTests/Xaml/DesignModeTests.cs

@@ -33,23 +33,19 @@ public class DesignModeTests : XamlTestBase
     }
 
     [Fact]
-    public void Design_Mode_PreviewWith_Works_With_Control_Template()
+    public void Design_Mode_PreviewWith_Returns_Original_Control()
     {
         using (UnitTestApplication.Start(TestServices.MockWindowingPlatform))
         {
             var obj = (Control)AvaloniaRuntimeXamlLoader.Load(@"
 <Button xmlns='https://github.com/avaloniaui'>
     <Design.PreviewWith>
-        <Template>
+        <Border />
-            <Border>
-                <Button />
-            </Border>
-        </Template>
     </Design.PreviewWith>
 </Button>", designMode: true);
             var preview = Design.CreatePreviewWithControl(obj);
-            var previewBorder = Assert.IsType<Border>(preview);
+            // Should return the original control, not the preview, as this is not supported to avoid stack overflows.
-            Assert.IsType<Button>(previewBorder.Child);
+            Assert.Same(obj, preview);
         }
     }