Fix TreeView selection.

TreeView cannot inherit from SelectingItemsControl as it makes no sense
to have a SelectedIndex property.

Perspex.Controls/TreeView.cs

@@ -6,20 +6,44 @@
 
 namespace Perspex.Controls
 {
-    using System.Collections;
-    using System.Collections.Generic;
+    using System;
     using Perspex.Controls.Generators;
-    using Perspex.Controls.Primitives;
     using Perspex.Input;
+    using Perspex.VisualTree;
+    using System.Collections;
+    using System.Collections.Generic;
+    using System.Linq;
 
-    public class TreeView : SelectingItemsControl
+    public class TreeView : ItemsControl
     {
+        public static readonly PerspexProperty<object> SelectedItemProperty =
+            PerspexProperty.Register<TreeView, object>("SelectedItem", coerce: CoerceSelectedItem);
+
+        static TreeView()
+        {
+            SelectedItemProperty.Changed.Subscribe(x =>
+            {
+                var control = x.Sender as TreeView;
+
+                if (control != null)
+                {
+                    control.SelectedItemChanged(x.NewValue);
+                }
+            });
+        }
+
+        public object SelectedItem
+        {
+            get { return this.GetValue(SelectedItemProperty); }
+            set { this.SetValue(SelectedItemProperty, value); }
+        }
+
         protected override ItemContainerGenerator CreateItemContainerGenerator()
         {
             return new TreeItemContainerGenerator<TreeViewItem>(this);
         }
 
-        protected override void MoveSelection(FocusNavigationDirection direction)
+        protected virtual void MoveSelection(FocusNavigationDirection direction)
         {
             // TODO: Up and down movement is a *HACK* and probably pretty slow. Probably needs
             // rewriting at some point.
@@ -70,6 +94,85 @@ namespace Perspex.Controls
             }
         }
 
+        protected override void OnKeyDown(KeyEventArgs e)
+        {
+            base.OnKeyDown(e);
+
+            if (!e.Handled)
+            {
+                switch (e.Key)
+                {
+                    case Key.Up:
+                        this.MoveSelection(FocusNavigationDirection.Up);
+                        break;
+                    case Key.Down:
+                        this.MoveSelection(FocusNavigationDirection.Down);
+                        break;
+                    case Key.Left:
+                        this.MoveSelection(FocusNavigationDirection.Left);
+                        break;
+                    case Key.Right:
+                        this.MoveSelection(FocusNavigationDirection.Right);
+                        break;
+                    default:
+                        return;
+                }
+
+                var selected = this.SelectedItem;
+
+                if (selected != null)
+                {
+                    var container = this.ItemContainerGenerator.GetContainerForItem(selected);
+
+                    if (container != null)
+                    {
+                        container.BringIntoView();
+                        FocusManager.Instance.Focus(container, true);
+                    }
+                }
+
+                e.Handled = true;
+            }
+        }
+
+        protected override void OnPointerPressed(PointerPressEventArgs e)
+        {
+            IVisual source = (IVisual)e.Source;
+            var selectable = source.GetVisualAncestors()
+                .OfType<ISelectable>()
+                .OfType<Control>()
+                .FirstOrDefault();
+
+            if (selectable != null)
+            {
+                var item = this.ItemContainerGenerator.GetItemForContainer(selectable);
+
+                if (item != null)
+                {
+                    this.SelectedItem = item;
+                    selectable.BringIntoView();
+                    FocusManager.Instance.Focus(selectable);
+                }
+            }
+
+            e.Handled = true;
+        }
+
+        private static object CoerceSelectedItem(PerspexObject o, object value)
+        {
+            //var control = o as SelectingItemsControl;
+
+            //if (control != null)
+            //{
+            //    if (value != null && (control.Items == null || control.Items.IndexOf(value) == -1))
+            //    {
+            //        return null;
+            //    }
+            //}
+
+            return value;
+        }
+
         private List<object> Flatten()
         {
             var result = new List<object>();
@@ -93,5 +196,25 @@ namespace Perspex.Controls
                 }
             }
         }
+
+        private void SelectedItemChanged(object selected)
+        {
+            var containers = this.ItemContainerGenerator.GetAll()
+                .Select(x => x.Item2)
+                .OfType<ISelectable>();
+            var selectedContainer = (selected != null) ?
+                this.ItemContainerGenerator.GetContainerForItem(selected) :
+                null;
+
+            if (this.Presenter != null && this.Presenter.Panel != null)
+            {
+                KeyboardNavigation.SetTabOnceActiveElement(this.Presenter.Panel, selectedContainer);
+            }
+
+            foreach (var item in containers)
+            {
+                item.IsSelected = item == selectedContainer;
+            }
+        }
     }
 }