Scroll newly selected tree view node into view.

src/Perspex.Base/Threading/DispatcherTimer.cs

@@ -146,6 +146,33 @@ namespace Perspex.Threading
             return Disposable.Create(() => timer.Stop());
         }
 
+        /// <summary>
+        /// Runs a method once, after the specified interval.
+        /// </summary>
+        /// <param name="action">
+        /// The method to call after the interval has elapsed.
+        /// </param>
+        /// <param name="interval">The interval after which to call the method.</param>
+        /// <param name="priority">The priority to use.</param>
+        /// <returns>An <see cref="IDisposable"/> used to cancel the timer.</returns>
+        public static IDisposable RunOnce(
+            Action action,
+            TimeSpan interval,
+            DispatcherPriority priority = DispatcherPriority.Normal)
+        {
+            var timer = new DispatcherTimer(priority) { Interval = interval };
+
+            timer.Tick += (s, e) =>
+            {
+                action();
+                timer.Stop();
+            };
+
+            timer.Start();
+
+            return Disposable.Create(() => timer.Stop());
+        }
+
         /// <summary>
         /// Starts the timer.
         /// </summary>

src/Perspex.Controls/TreeView.cs

@@ -8,6 +8,7 @@ using Perspex.Controls.Primitives;
 using Perspex.Input;
 using Perspex.Interactivity;
 using Perspex.Styling;
+using Perspex.Threading;
 using Perspex.VisualTree;
 
 namespace Perspex.Controls
@@ -17,6 +18,14 @@ namespace Perspex.Controls
     /// </summary>
     public class TreeView : ItemsControl
     {
+        /// <summary>
+        /// Defines the <see cref="AutoScrollToSelectedItem"/> property.
+        /// </summary>
+        public static readonly PerspexProperty<bool> AutoScrollToSelectedItemProperty =
+            PerspexProperty.Register<TreeView, bool>(
+                nameof(AutoScrollToSelectedItem),
+                defaultValue: true);
+
         /// <summary>
         /// Defines the <see cref="SelectedItem"/> property.
         /// </summary>
@@ -41,6 +50,15 @@ namespace Perspex.Controls
         public new ITreeItemContainerGenerator ItemContainerGenerator => 
             (ITreeItemContainerGenerator)base.ItemContainerGenerator;
 
+        /// <summary>
+        /// Gets or sets a value indicating whether to automatically scroll to newly selected items.
+        /// </summary>
+        public bool AutoScrollToSelectedItem
+        {
+            get { return GetValue(AutoScrollToSelectedItemProperty); }
+            set { SetValue(AutoScrollToSelectedItemProperty, value); }
+        }
+
         /// <summary>
         /// Gets or sets the selected item.
         /// </summary>
@@ -65,6 +83,11 @@ namespace Perspex.Controls
                 {
                     var container = ItemContainerGenerator.Index.ContainerFromItem(_selectedItem);
                     MarkContainerSelected(container, true);
+
+                    if (AutoScrollToSelectedItem && container != null)
+                    {
+                        container.BringIntoView();
+                    }
                 }
             }
         }
@@ -72,12 +95,14 @@ namespace Perspex.Controls
         /// <inheritdoc/>
         protected override IItemContainerGenerator CreateItemContainerGenerator()
         {
-            return new TreeItemContainerGenerator<TreeViewItem>(
+            var result = new TreeItemContainerGenerator<TreeViewItem>(
                 this,
                 TreeViewItem.HeaderProperty,
                 TreeViewItem.ItemsProperty,
                 TreeViewItem.IsExpandedProperty,
                 new TreeContainerIndex());
+            result.Index.Materialized += ContainerMaterialized;
+            return result;
         }
 
         /// <inheritdoc/>
@@ -190,6 +215,36 @@ namespace Perspex.Controls
             return null;
         }
 
+        /// <summary>
+        /// Called when a new item container is materialized, to set its selected state.
+        /// </summary>
+        /// <param name="sender">The event sender.</param>
+        /// <param name="e">The event args.</param>
+        private void ContainerMaterialized(object sender, ItemContainerEventArgs e)
+        {
+            var selectedItem = SelectedItem;
+
+            if (selectedItem != null)
+            {
+                foreach (var container in e.Containers)
+                {
+                    if (container.Item == selectedItem)
+                    {
+                        ((TreeViewItem)container.ContainerControl).IsSelected = true;
+
+                        if (AutoScrollToSelectedItem)
+                        {
+                            DispatcherTimer.RunOnce(
+                                container.ContainerControl.BringIntoView,
+                                TimeSpan.Zero);
+                        }
+
+                        break;
+                    }
+                }
+            }
+        }
+
         /// <summary>
         /// Sets a container's 'selected' class or <see cref="ISelectable.IsSelected"/>.
         /// </summary>

src/Perspex.Controls/TreeViewItem.cs

@@ -73,16 +73,12 @@ namespace Perspex.Controls
         /// <inheritdoc/>
         protected override IItemContainerGenerator CreateItemContainerGenerator()
         {
-            var result =  new TreeItemContainerGenerator<TreeViewItem>(
+            return new TreeItemContainerGenerator<TreeViewItem>(
                 this,
                 TreeViewItem.HeaderProperty,
                 TreeViewItem.ItemsProperty,
                 TreeViewItem.IsExpandedProperty,
                 _treeView?.ItemContainerGenerator.Index ?? new TreeContainerIndex());
-
-            result.Materialized += ItemMaterialized;
-
-            return result;
         }
 
         /// <inheritdoc/>
@@ -123,21 +119,5 @@ namespace Perspex.Controls
 
             base.OnKeyDown(e);
         }
-
-        private void ItemMaterialized(object sender, ItemContainerEventArgs e)
-        {
-            var selectedItem = _treeView?.SelectedItem;
-
-            if (selectedItem != null)
-            {
-                foreach (var container in e.Containers)
-                {
-                    if (container.Item == selectedItem)
-                    {
-                        ((TreeViewItem)container.ContainerControl).IsSelected = true;
-                    }
-                }
-            }
-        }
     }
 }