Added IsTemplateFocusTarget attached property.

And use it in TreeViewItem's data template to place the focus adorner
around the correct control. Closes #294.

src/Perspex.Controls/Control.cs

@@ -69,8 +69,7 @@ namespace Perspex.Controls
         /// Defines the <see cref="ContextMenu"/> property.
         /// </summary>
         public static readonly StyledProperty<ContextMenu> ContextMenuProperty =
-            PerspexProperty.Register<Control, ContextMenu>(nameof(ContextMenu));        
-
+            PerspexProperty.Register<Control, ContextMenu>(nameof(ContextMenu));
 
         /// <summary>
         /// Event raised when an element wishes to be scrolled into view.
@@ -393,6 +392,15 @@ namespace Perspex.Controls
                 });
         }
 
+        /// <summary>
+        /// Gets the element that recieves the focus adorner.
+        /// </summary>
+        /// <returns>The control that recieves the focus adorner.</returns>
+        protected virtual IControl GetTemplateFocusTarget()
+        {
+            return this;
+        }
+
         /// <summary>
         /// Called when the control is added to a logical tree.
         /// </summary>
@@ -488,8 +496,13 @@ namespace Perspex.Controls
 
                     if (_focusAdorner != null)
                     {
-                        AdornerLayer.SetAdornedElement((Visual)_focusAdorner, this);
-                        adornerLayer.Children.Add(_focusAdorner);
+                        var target = (Visual)GetTemplateFocusTarget();
+
+                        if (target != null)
+                        {
+                            AdornerLayer.SetAdornedElement((Visual)_focusAdorner, target);
+                            adornerLayer.Children.Add(_focusAdorner);
+                        }
                     }
                 }
             }

src/Perspex.Controls/Primitives/TemplatedControl.cs

@@ -74,6 +74,12 @@ namespace Perspex.Controls.Primitives
         public static readonly StyledProperty<IControlTemplate> TemplateProperty =
             PerspexProperty.Register<TemplatedControl, IControlTemplate>("Template");
 
+        /// <summary>
+        /// Defines the IsTemplateFocusTarget attached property.
+        /// </summary>
+        public static readonly AttachedProperty<bool> IsTemplateFocusTargetProperty =
+            PerspexProperty.RegisterAttached<TemplatedControl, Control, bool>("IsTemplateFocusTarget");
+
         /// <summary>
         /// Defines the <see cref="TemplateApplied"/> routed event.
         /// </summary>
@@ -198,6 +204,33 @@ namespace Perspex.Controls.Primitives
             set { SetValue(TemplateProperty, value); }
         }
 
+        /// <summary>
+        /// Gets the value of the IsTemplateFocusTargetProperty attached property on a control.
+        /// </summary>
+        /// <param name="control">The control.</param>
+        /// <returns>The property value.</returns>
+        /// <see cref="SetIsTemplateFocusTarget(Control, bool)"/>
+        public bool GetIsTemplateFocusTarget(Control control)
+        {
+            return control.GetValue(IsTemplateFocusTargetProperty);
+        }
+
+        /// <summary>
+        /// Sets the value of the IsTemplateFocusTargetProperty attached property on a control.
+        /// </summary>
+        /// <param name="control">The control.</param>
+        /// <param name="value">The property value.</param>
+        /// <remarks>
+        /// When a control is navigated to using the keyboard, a focus adorner is shown - usually
+        /// around the control itself. However if the TemplatedControl.IsTemplateFocusTarget 
+        /// attached property is set to true on an element in the control template, then the focus
+        /// adorner will be shown around that control instead.
+        /// </remarks>
+        public void SetIsTemplateFocusTarget(Control control, bool value)
+        {
+            control.SetValue(IsTemplateFocusTargetProperty, value);
+        }
+
         /// <inheritdoc/>
         public sealed override void ApplyTemplate()
         {
@@ -224,6 +257,7 @@ namespace Perspex.Controls.Primitives
             }
         }
 
+        /// <inheritdoc/>
         protected sealed override IndexerDescriptor CreateBindingDescriptor(IndexerDescriptor source)
         {
             var result = base.CreateBindingDescriptor(source);
@@ -240,6 +274,20 @@ namespace Perspex.Controls.Primitives
             return result;
         }
 
+        /// <inheritdoc/>
+        protected override IControl GetTemplateFocusTarget()
+        {
+            foreach (Control child in this.GetTemplateChildren())
+            {
+                if (GetIsTemplateFocusTarget(child))
+                {
+                    return child;
+                }
+            }
+
+            return this;
+        }
+
         /// <summary>
         /// Called when the control's template is applied.
         /// </summary>

src/Perspex.Controls/Templates/TemplateExtensions.cs

@@ -12,7 +12,7 @@ namespace Perspex.Controls.Templates
 {
     public static class TemplateExtensions
     {
-        public static IEnumerable<Control> GetTemplateChildren(this ITemplatedControl control)
+        public static IEnumerable<IControl> GetTemplateChildren(this ITemplatedControl control)
         {
             var visual = control as IVisual;
 
@@ -21,12 +21,12 @@ namespace Perspex.Controls.Templates
                 // TODO: This searches the whole descendent tree - it can stop when it exits the
                 // template.
                 return visual.GetVisualDescendents()
-                    .OfType<Control>()
+                    .OfType<IControl>()
                     .Where(x => x.TemplatedParent == control);
             }
             else
             {
-                return Enumerable.Empty<Control>();
+                return Enumerable.Empty<IControl>();
             }
         }
     }

src/Perspex.Themes.Default/TreeViewItem.paml

@@ -11,18 +11,12 @@
                     Background="{TemplateBinding Background}"
                     BorderBrush="{TemplateBinding BorderBrush}"
                     BorderThickness="{TemplateBinding BorderThickness}"
+                    TemplatedControl.IsTemplateFocusTarget="True"
                     Grid.Column="1">
               <ContentPresenter Name="PART_HeaderPresenter"
                                 Content="{TemplateBinding Header}"
                                 Margin="{TemplateBinding Padding}"/>
             </Border>
-            <Rectangle Name="focus"
-                       IsHitTestVisible="False"
-                       IsVisible="{TemplateBinding IsFocused}"
-                       Stroke="Black"
-                       StrokeThickness="1"
-                       StrokeDashArray="1,2"
-                       Grid.Column="1"/>
           </Grid>
           <ItemsPresenter Name="PART_ItemsPresenter"
                           IsVisible="{TemplateBinding IsExpanded}"