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src/Perspex.Controls/Generators/ItemContainerEventArgs.cs

@@ -12,6 +12,19 @@ namespace Perspex.Controls.Generators
     /// </summary>
     public class ItemContainerEventArgs : EventArgs
     {
+        /// <summary>
+        /// Initializes a new instance of the <see cref="ItemContainerEventArgs"/> class.
+        /// </summary>
+        /// <param name="startingIndex">The index of the first container in the source items.</param>
+        /// <param name="container">The container.</param>
+        public ItemContainerEventArgs(
+            int startingIndex,
+            ItemContainer container)
+        {
+            StartingIndex = startingIndex;
+            Containers = new[] { container };
+        }
+
         /// <summary>
         /// Initializes a new instance of the <see cref="ItemContainerEventArgs"/> class.
         /// </summary>

src/Perspex.Controls/Generators/TreeContainerIndex.cs

@@ -1,6 +1,7 @@
 // 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;
 using System.Collections.Generic;
 
 namespace Perspex.Controls.Generators
@@ -19,6 +20,16 @@ namespace Perspex.Controls.Generators
         private readonly Dictionary<object, IControl> _itemToContainer = new Dictionary<object, IControl>();
         private readonly Dictionary<IControl, object> _containerToItem = new Dictionary<IControl, object>();
 
+        /// <summary>
+        /// Signalled whenever new containers are materialized.
+        /// </summary>
+        public event EventHandler<ItemContainerEventArgs> Materialized;
+
+        /// <summary>
+        /// Event raised whenever containers are dematerialized.
+        /// </summary>
+        public event EventHandler<ItemContainerEventArgs> Dematerialized;
+
         /// <summary>
         /// Gets the currently materialized containers.
         /// </summary>
@@ -33,6 +44,10 @@ namespace Perspex.Controls.Generators
         {
             _itemToContainer.Add(item, container);
             _containerToItem.Add(container, item);
+
+            Materialized?.Invoke(
+                this, 
+                new ItemContainerEventArgs(0, new ItemContainer(container, item, 0)));
         }
 
         /// <summary>
@@ -44,6 +59,10 @@ namespace Perspex.Controls.Generators
             var item = _containerToItem[container];
             _containerToItem.Remove(container);
             _itemToContainer.Remove(item);
+
+            Dematerialized?.Invoke(
+                this, 
+                new ItemContainerEventArgs(0, new ItemContainer(container, item, 0)));
         }
 
         /// <summary>

src/Perspex.Controls/Primitives/HeaderedItemsControl.cs

@@ -51,8 +51,8 @@ namespace Perspex.Controls.Primitives
         /// <inheritdoc/>
         protected override void OnTemplateApplied(TemplateAppliedEventArgs e)
         {
-            base.OnTemplateApplied(e);
             HeaderPresenter = e.NameScope.Find<ContentPresenter>("PART_HeaderPresenter");
+            base.OnTemplateApplied(e);
         }
     }
 }

src/Perspex.Diagnostics/Views/TreePage.paml.cs

@@ -1,4 +1,5 @@
 using Perspex.Controls;
+using Perspex.Controls.Generators;
 using Perspex.Controls.Primitives;
 using Perspex.Controls.Shapes;
 using Perspex.Diagnostics.ViewModels;
@@ -11,10 +12,12 @@ namespace Perspex.Diagnostics.Views
     public class TreePageView : UserControl
     {
         private Control _adorner;
+        private TreeView _tree;
 
         public TreePageView()
         {
             this.InitializeComponent();
+            _tree.ItemContainerGenerator.Index.Materialized += TreeViewItemMaterialized;
         }
 
         protected void AddAdorner(object sender, PointerEventArgs e)
@@ -46,6 +49,22 @@ namespace Perspex.Diagnostics.Views
         private void InitializeComponent()
         {
             PerspexXamlLoader.Load(this);
+            _tree = this.FindControl<TreeView>("tree");
+        }
+
+        private void TreeViewItemMaterialized(object sender, ItemContainerEventArgs e)
+        {
+            var item = (TreeViewItem)e.Containers[0].ContainerControl;
+            item.TemplateApplied += TreeViewItemTemplateApplied;
+        }
+
+        private void TreeViewItemTemplateApplied(object sender, TemplateAppliedEventArgs e)
+        {
+            var item = (TreeViewItem)sender;
+            var header = item.HeaderPresenter.Child;
+            header.PointerEnter += AddAdorner;
+            header.PointerLeave += RemoveAdorner;
+            item.TemplateApplied -= TreeViewItemTemplateApplied;
         }
     }
 }