Fix loading of UserControls.

Also added a UserControl test, though it turns out it was unrelated to
the problem at hand.

src/Markup/Perspex.Markup.Xaml/PerspexXamlLoader.cs

@@ -67,21 +67,26 @@ namespace Perspex.Markup.Xaml
             // in certain situations, so we try to load .xaml and if that's not found we try .paml.
             // Ideally we'd be able to use .xaml everywhere
             var assetLocator = PerspexLocator.Current.GetService<IAssetLoader>();
+
             if (assetLocator == null)
             {
                 throw new InvalidOperationException(
                     "Could not create IAssetLoader : maybe Application.RegisterServices() wasn't called?");
             }
+
             foreach (var uri in GetUrisFor(type))
             {
                 if (assetLocator.Exists(uri))
                 {
                     using (var stream = assetLocator.Open(uri))
                     {
+                        var initialize = rootInstance as ISupportInitialize;
+                        initialize?.BeginInit();
                         return Load(stream, rootInstance);
                     }
                 }
             }
+
             throw new FileNotFoundException("Unable to find view for " + type.FullName);
         }
 

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

@@ -11,6 +11,8 @@ namespace Perspex.Markup.Xaml.Styling
     /// </summary>
     public class StyleInclude : IStyle
     {
+        private IStyle _loaded;
+
         /// <summary>
         /// Gets or sets the source URL.
         /// </summary>
@@ -19,19 +21,25 @@ namespace Perspex.Markup.Xaml.Styling
         /// <summary>
         /// Gets the loaded style.
         /// </summary>
-        public IStyle Loaded { get; private set; }
-
-        /// <inheritdoc/>
-        public void Attach(IStyleable control, IStyleHost container)
+        public IStyle Loaded
         {
-            if (Source != null)
+            get
             {
-                if (Loaded == null)
+                if (_loaded == null)
                 {
                     var loader = new PerspexXamlLoader();
-                    Loaded = (IStyle)loader.Load(Source);
+                    _loaded = (IStyle)loader.Load(Source);
                 }
 
+                return _loaded;
+            }
+        }
+
+        /// <inheritdoc/>
+        public void Attach(IStyleable control, IStyleHost container)
+        {
+            if (Source != null)
+            {
                 Loaded.Attach(control, container);
             }
         }

src/Perspex.Controls/Control.cs

@@ -323,11 +323,10 @@ namespace Perspex.Controls
         {
             if (_initCount == 0)
             {
-                ++_initCount;
-                //throw new InvalidOperationException("BeginInit was not called.");
+                throw new InvalidOperationException("BeginInit was not called.");
             }
 
-            if (--_initCount == 0 && !_styled)
+            if (--_initCount == 0 && _isAttachedToLogicalTree && !_styled)
             {
                 RegisterWithNameScope();
                 ApplyStyling();

tests/Perspex.Controls.UnitTests/Perspex.Controls.UnitTests.csproj

@@ -90,6 +90,7 @@
   </Choose>
   <ItemGroup>
     <Compile Include="ClassesTests.cs" />
+    <Compile Include="UserControlTests.cs" />
     <Compile Include="DockPanelTests.cs" />
     <Compile Include="EnumerableExtensions.cs" />
     <Compile Include="GridSplitterTests.cs" />

tests/Perspex.Controls.UnitTests/UserControlTests.cs

@@ -0,0 +1,57 @@
+// Copyright (c) The Perspex Project. All rights reserved.
+// Licensed under the MIT license. See licence.md file in the project root for full license information.
+
+using System.Linq;
+using Perspex.Controls.Presenters;
+using Perspex.Controls.Primitives;
+using Perspex.Controls.Templates;
+using Perspex.Styling;
+using Perspex.UnitTests;
+using Xunit;
+
+namespace Perspex.Controls.UnitTests
+{
+    public class UserControlTests
+    {
+        [Fact]
+        public void Should_Be_Styled_As_UserControl()
+        {
+            using (UnitTestApplication.Start(TestServices.RealStyler))
+            {
+                var target = new UserControl();
+                var root = new TestRoot
+                {
+                    Styles = new Styles
+                    {
+                        new Style(x => x.OfType<ContentControl>())
+                        {
+                            Setters = new[]
+                            {
+                                new Setter(TemplatedControl.TemplateProperty, GetTemplate())
+                            }
+                        }
+                    },
+                    Child = target,
+                };
+
+                Assert.NotNull(target.Template);
+            }
+        }
+
+        private FuncControlTemplate GetTemplate()
+        {
+            return new FuncControlTemplate<ContentControl>(parent =>
+            {
+                return new Border
+                {
+                    Background = new Media.SolidColorBrush(0xffffffff),
+                    Child = new ContentPresenter
+                    {
+                        Name = "PART_ContentPresenter",
+                        [~ContentPresenter.ContentProperty] = parent[~ContentControl.ContentProperty],
+                    }
+                };
+            });
+        }
+    }
+}