Lazily initialize DataTemplates.

Added an `IDataTemplateHost` interface with a `IsDataTemplatesInitialized` property to prevent the need for allocating empty `DataTemplates` collections for many controls.

src/Avalonia.Controls/Application.cs

@@ -66,11 +66,7 @@ namespace Avalonia
         /// <value>
         /// The application's global data templates.
         /// </value>
-        public DataTemplates DataTemplates
-        {
-            get { return _dataTemplates ?? (_dataTemplates = new DataTemplates()); }
-            set { _dataTemplates = value; }
-        }
+        public DataTemplates DataTemplates => _dataTemplates ?? (_dataTemplates = new DataTemplates());
 
         /// <summary>
         /// Gets the application's focus manager.
@@ -112,6 +108,9 @@ namespace Avalonia
         /// </remarks>
         public Styles Styles => _styles ?? (_styles = new Styles());
 
+        /// <inheritdoc/>
+        bool IDataTemplateHost.IsDataTemplatesInitialized => _dataTemplates != null;
+
         /// <summary>
         /// Gets the styling parent of the application, which is null.
         /// </summary>

src/Avalonia.Controls/Control.cs

@@ -243,11 +243,7 @@ namespace Avalonia.Controls
         /// Each control may define data templates which are applied to the control itself and its
         /// children.
         /// </remarks>
-        public DataTemplates DataTemplates
-        {
-            get { return _dataTemplates ?? (_dataTemplates = new DataTemplates()); }
-            set { _dataTemplates = value; }
-        }
+        public DataTemplates DataTemplates => _dataTemplates ?? (_dataTemplates = new DataTemplates());
 
         /// <summary>
         /// Gets a value that indicates whether the element has finished initialization.
@@ -300,6 +296,9 @@ namespace Avalonia.Controls
             internal set { SetValue(TemplatedParentProperty, value); }
         }
 
+        /// <inheritdoc/>
+        bool IDataTemplateHost.IsDataTemplatesInitialized => _dataTemplates != null;
+
         /// <summary>
         /// Gets a value indicating whether the element is attached to a rooted logical tree.
         /// </summary>

src/Avalonia.Controls/IControl.cs

@@ -14,7 +14,14 @@ namespace Avalonia.Controls
     /// <summary>
     /// Interface for Avalonia controls.
     /// </summary>
-    public interface IControl : IVisual, ILogical, ILayoutable, IInputElement, INamed, IStyleable, IStyleHost
+    public interface IControl : IVisual,
+        IDataTemplateHost,
+        ILogical,
+        ILayoutable,
+        IInputElement,
+        INamed,
+        IStyleable,
+        IStyleHost
     {
         /// <summary>
         /// Occurs when the control has finished initialization.
@@ -31,11 +38,6 @@ namespace Avalonia.Controls
         /// </summary>
         object DataContext { get; set; }
 
-        /// <summary>
-        /// Gets the data templates for the control.
-        /// </summary>
-        DataTemplates DataTemplates { get; }
-
         /// <summary>
         /// Gets a value that indicates whether the element has finished initialization.
         /// </summary>

src/Avalonia.Controls/IGlobalDataTemplates.cs

@@ -8,11 +8,7 @@ namespace Avalonia.Controls
     /// <summary>
     /// Defines the application-global data templates.
     /// </summary>
-    public interface IGlobalDataTemplates
+    public interface IGlobalDataTemplates : IDataTemplateHost
     {
-        /// <summary>
-        /// Gets the application-global data templates.
-        /// </summary>
-        DataTemplates DataTemplates { get; }
     }
 }

src/Avalonia.Controls/Templates/DataTemplateExtensions.cs

@@ -17,8 +17,8 @@ namespace Avalonia.Controls.Templates
         /// <param name="control">The control searching for the data template.</param>
         /// <param name="data">The data.</param>
         /// <param name="primary">
-        /// An optional primary template that can will be tried before the
-        /// <see cref="IControl.DataTemplates"/> in the tree are searched.
+        /// An optional primary template that can will be tried before the DataTemplates in the
+        /// tree are searched.
         /// </param>
         /// <returns>The data template or null if no matching data template was found.</returns>
         public static IDataTemplate FindDataTemplate(
@@ -31,13 +31,16 @@ namespace Avalonia.Controls.Templates
                 return primary;
             }
 
-            foreach (var i in control.GetSelfAndLogicalAncestors().OfType<IControl>())
+            foreach (var i in control.GetSelfAndLogicalAncestors().OfType<IDataTemplateHost>())
             {
-                foreach (IDataTemplate dt in i.DataTemplates)
+                if (i.IsDataTemplatesInitialized)
                 {
-                    if (dt.Match(data))
+                    foreach (IDataTemplate dt in i.DataTemplates)
                     {
-                        return dt;
+                        if (dt.Match(data))
+                        {
+                            return dt;
+                        }
                     }
                 }
             }
@@ -46,11 +49,14 @@ namespace Avalonia.Controls.Templates
 
             if (global != null)
             {
-                foreach (IDataTemplate dt in global.DataTemplates)
+                if (global.IsDataTemplatesInitialized)
                 {
-                    if (dt.Match(data))
+                    foreach (IDataTemplate dt in global.DataTemplates)
                     {
-                        return dt;
+                        if (dt.Match(data))
+                        {
+                            return dt;
+                        }
                     }
                 }
             }

src/Avalonia.Controls/Templates/IDataTemplateHost.cs

@@ -0,0 +1,27 @@
+// Copyright (c) The Avalonia Project. All rights reserved.
+// Licensed under the MIT license. See licence.md file in the project root for full license information.
+
+using System;
+
+namespace Avalonia.Controls.Templates
+{
+    /// <summary>
+    /// Defines an element that has a <see cref="DataTemplates"/> collection.
+    /// </summary>
+    public interface IDataTemplateHost
+    {
+        /// <summary>
+        /// Gets the data templates for the element.
+        /// </summary>
+        DataTemplates DataTemplates { get; }
+
+        /// <summary>
+        /// Gets a value indicating whether <see cref="DataTemplates"/> is initialized.
+        /// </summary>
+        /// <remarks>
+        /// The <see cref="DataTemplates"/> property may be lazily initialized, if so this property
+        /// indicates whether it has been initialized.
+        /// </remarks>
+        bool IsDataTemplatesInitialized { get; }
+    }
+}

src/Avalonia.Diagnostics/DevTools.xaml.cs

@@ -71,7 +71,7 @@ namespace Avalonia.Diagnostics
                         Width = 1024,
                         Height = 512,
                         Content = devTools,
-                        DataTemplates = new DataTemplates
+                        DataTemplates =
                         {
                             new ViewLocator<ViewModelBase>(),
                         }

tests/Avalonia.Controls.UnitTests/ItemsControlTests.cs

@@ -388,7 +388,7 @@ namespace Avalonia.Controls.UnitTests
             {
                 Template = GetTemplate(),
                 DataContext = "Base",
-                DataTemplates = new DataTemplates
+                DataTemplates =
                 {
                     new FuncDataTemplate<Item>(x => new Button { Content = x })
                 },

tests/Avalonia.Controls.UnitTests/ListBoxTests.cs

@@ -109,10 +109,10 @@ namespace Avalonia.Controls.UnitTests
                 {
                     Template = ListBoxTemplate(),
                     DataContext = "Base",
-                    DataTemplates = new DataTemplates
-                {
-                    new FuncDataTemplate<Item>(x => new Button { Content = x })
-                },
+                    DataTemplates =
+                    {
+                        new FuncDataTemplate<Item>(x => new Button { Content = x })
+                    },
                     Items = items,
                 };
 

tests/Avalonia.Controls.UnitTests/Presenters/ContentPresenterTests_Unrooted.cs

@@ -88,7 +88,7 @@ namespace Avalonia.Controls.UnitTests.Presenters
             root.Child = null;
             root = new TestRoot
             {
-                DataTemplates = new DataTemplates
+                DataTemplates =
                 {
                     new FuncDataTemplate<string>(x => new Decorator()),
                 },

tests/Avalonia.Controls.UnitTests/TabControlTests.cs

@@ -174,7 +174,7 @@ namespace Avalonia.Controls.UnitTests
             {
                 Template = new FuncControlTemplate<TabControl>(CreateTabControlTemplate),
                 DataContext = "Base",
-                DataTemplates = new DataTemplates
+                DataTemplates =
                 {
                     new FuncDataTemplate<Item>(x => new Button { Content = x })
                 },

tests/Avalonia.Controls.UnitTests/TreeViewTests.cs

@@ -25,9 +25,9 @@ namespace Avalonia.Controls.UnitTests
             {
                 Template = CreateTreeViewTemplate(),
                 Items = CreateTestTreeData(),
-                DataTemplates = CreateNodeDataTemplate(),
             };
 
+            CreateNodeDataTemplate(target);
             ApplyTemplates(target);
 
             Assert.Equal(new[] { "Root" }, ExtractItemHeader(target, 0));
@@ -69,9 +69,9 @@ namespace Avalonia.Controls.UnitTests
             {
                 Template = CreateTreeViewTemplate(),
                 Items = CreateTestTreeData(),
-                DataTemplates = CreateNodeDataTemplate(),
             };
 
+            CreateNodeDataTemplate(target);
             ApplyTemplates(target);
 
             var container = (TreeViewItem)target.ItemContainerGenerator.Containers.Single().ContainerControl;
@@ -87,7 +87,6 @@ namespace Avalonia.Controls.UnitTests
             {
                 Template = CreateTreeViewTemplate(),
                 Items = tree,
-                DataTemplates = CreateNodeDataTemplate(),
             };
 
             // For TreeViewItem to find its parent TreeView, OnAttachedToLogicalTree needs
@@ -95,6 +94,7 @@ namespace Avalonia.Controls.UnitTests
             var root = new TestRoot();
             root.Child = target;
 
+            CreateNodeDataTemplate(target);
             ApplyTemplates(target);
 
             var container = target.ItemContainerGenerator.Index.ContainerFromItem(
@@ -116,11 +116,12 @@ namespace Avalonia.Controls.UnitTests
             {
                 Template = CreateTreeViewTemplate(),
                 Items = tree,
-                DataTemplates = CreateNodeDataTemplate(),
             };
 
             var visualRoot = new TestRoot();
             visualRoot.Child = target;
+
+            CreateNodeDataTemplate(target);
             ApplyTemplates(target);
 
             var item = tree[0].Children[1].Children[0];
@@ -146,11 +147,12 @@ namespace Avalonia.Controls.UnitTests
             {
                 Template = CreateTreeViewTemplate(),
                 Items = tree,
-                DataTemplates = CreateNodeDataTemplate(),
             };
 
             var visualRoot = new TestRoot();
             visualRoot.Child = target;
+
+            CreateNodeDataTemplate(target);
             ApplyTemplates(target);
 
             var item = tree[0].Children[1].Children[0];
@@ -191,12 +193,13 @@ namespace Avalonia.Controls.UnitTests
             var target = new TreeView
             {
                 Template = CreateTreeViewTemplate(),
-                DataTemplates = CreateNodeDataTemplate(),
                 Items = tree,
             };
 
             var root = new TestRoot();
             root.Child = target;
+
+            CreateNodeDataTemplate(target);
             ApplyTemplates(target);
 
             Assert.Equal(5, target.ItemContainerGenerator.Index.Items.Count());
@@ -221,7 +224,7 @@ namespace Avalonia.Controls.UnitTests
             {
                 Template = CreateTreeViewTemplate(),
                 DataContext = "Base",
-                DataTemplates = new DataTemplates
+                DataTemplates =
                 {
                     new FuncDataTemplate<Node>(x => new Button { Content = x })
                 },
@@ -291,9 +294,9 @@ namespace Avalonia.Controls.UnitTests
             {
                 Template = CreateTreeViewTemplate(),
                 Items = data,
-                DataTemplates = CreateNodeDataTemplate(),
             };
 
+            CreateNodeDataTemplate(target);
             ApplyTemplates(target);
 
             Assert.Equal(new[] { "Root" }, ExtractItemHeader(target, 0));
@@ -328,7 +331,6 @@ namespace Avalonia.Controls.UnitTests
                 {
                     Template = CreateTreeViewTemplate(),
                     Items = data,
-                    DataTemplates = CreateNodeDataTemplate(),
                 };
 
                 var button = new Button();
@@ -341,6 +343,7 @@ namespace Avalonia.Controls.UnitTests
                     }
                 };
 
+                CreateNodeDataTemplate(target);
                 ApplyTemplates(target);
 
                 var item = data[0].Children[0];
@@ -411,12 +414,9 @@ namespace Avalonia.Controls.UnitTests
             };
         }
 
-        private DataTemplates CreateNodeDataTemplate()
+        private void CreateNodeDataTemplate(IControl control)
         {
-            return new DataTemplates
-            {
-                new TestTreeDataTemplate()
-            };
+            control.DataTemplates.Add(new TestTreeDataTemplate());
         }
 
         private IControlTemplate CreateTreeViewTemplate()

tests/Avalonia.LeakTests/ControlTests.cs

@@ -276,7 +276,7 @@ namespace Avalonia.LeakTests
                     {
                         Content = target = new TreeView
                         {
-                            DataTemplates = new DataTemplates
+                            DataTemplates =
                             {
                                 new FuncTreeDataTemplate<Node>(
                                     x => new TextBlock { Text = x.Name },