Added TreeViewItem.IsExpanded.

Perspex/Controls/StackPanel.cs

@@ -107,7 +107,7 @@ namespace Perspex.Controls
                 arrangedWidth = 0;
             }
 
-            foreach (Control child in this.Children)
+            foreach (Control child in this.Children.Where(x => x.DesiredSize.HasValue))
             {
                 double childWidth = child.DesiredSize.Value.Width;
                 double childHeight = child.DesiredSize.Value.Height;

Perspex/Controls/TreeViewItem.cs

@@ -6,12 +6,15 @@
 
 namespace Perspex.Controls
 {
-    using System;
-    using System.Collections;
-    using System.Linq;
-    using System.Reactive.Linq;
-
     public class TreeViewItem : HeaderedItemsControl
     {
+        public static readonly PerspexProperty<bool> IsExpandedProperty =
+            PerspexProperty.Register<TreeViewItem, bool>("IsExpanded");
+
+        public bool IsExpanded
+        {
+            get { return this.GetValue(IsExpandedProperty); }
+            set { this.SetValue(IsExpandedProperty, value); }
+        }
     }
 }

Perspex/Themes/Default/TreeViewItemStyle.cs

@@ -61,6 +61,7 @@ namespace Perspex.Themes.Default
                         Margin = new Thickness(24, 0, 0, 0),
                         [~ItemsPresenter.ItemsProperty] = control[~TreeViewItem.ItemsProperty],
                         [~ItemsPresenter.ItemsPanelProperty] = control[~TreeViewItem.ItemsPanelProperty],
+                        [~ItemsPresenter.IsVisibleProperty] = control[~TreeViewItem.IsExpandedProperty],
                     }
                 }
             };