Added a AddClassHandler<TTarget> extension method.

For PerspexProperties, similar to RoutedEvent.AddClassHandler<TTarget>.

Perspex.Base/Perspex.Base.csproj

@@ -39,6 +39,7 @@
     <Compile Include="Binding.cs" />
     <Compile Include="Diagnostics\PerspexObjectExtensions.cs" />
     <Compile Include="GlobalSuppressions.cs" />
+    <Compile Include="PerspexObjectExtensions.cs" />
     <Compile Include="PerspexProperty`1.cs" />
     <Compile Include="PriorityBindingEntry.cs" />
     <Compile Include="Collections\IPerspexList.cs" />

Perspex.Base/PerspexObjectExtensions.cs

@@ -0,0 +1,53 @@
+// -----------------------------------------------------------------------
+// <copyright file="PerspexObjectExtensions.cs" company="Steven Kirk">
+// Copyright 2015 MIT Licence. See licence.md for more information.
+// </copyright>
+// -----------------------------------------------------------------------
+
+namespace Perspex
+{
+    using System;
+    using System.Reactive.Linq;
+
+    /// <summary>
+    /// Provides extension methods for <see cref="PerspexObject"/> and related classes.
+    /// </summary>
+    public static class PerspexObjectExtensions
+    {
+        /// <summary>
+        /// Subscribes to a property changed notifications for changes that originate from a
+        /// <typeparamref name="TTarget"/>.
+        /// </summary>
+        /// <typeparam name="TTarget">The type of the property change sender.</typeparam>
+        /// <param name="observable">The property changed observable.</param>
+        /// <param name="handler">Given a TTarget, returns the handler.</param>
+        /// <returns>A disposable that can be used to terminate the subscription.</returns>
+        public static IDisposable AddClassHandler<TTarget>(
+            this IObservable<PerspexPropertyChangedEventArgs> observable,
+            Func<TTarget, Action<PerspexPropertyChangedEventArgs>> handler)
+            where TTarget : class
+        {
+            return observable.Subscribe(e => SubscribeAdapter(e, handler));
+        }
+
+        /// <summary>
+        /// Observer method for <see cref="AddClassHandler{TTarget}(IObservable{PerspexPropertyChangedEventArgs},
+        /// Func{TTarget, Action{PerspexPropertyChangedEventArgs}})"/>.
+        /// </summary>
+        /// <typeparam name="TTarget">The sender type to accept.</typeparam>
+        /// <param name="e">The event args.</param>
+        /// <param name="handler">Given a TTarget, returns the handler.</param>
+        private static void SubscribeAdapter<TTarget>(
+            PerspexPropertyChangedEventArgs e,
+            Func<TTarget, Action<PerspexPropertyChangedEventArgs>> handler)
+            where TTarget : class
+        {
+            var target = e.Sender as TTarget;
+
+            if (target != null)
+            {
+                handler(target)(e);
+            }
+        }
+    }
+}

Perspex.Controls/MenuItem.cs

@@ -84,7 +84,7 @@ namespace Perspex.Controls
             FocusableProperty.OverrideDefaultValue<MenuItem>(true);
             ClickEvent.AddClassHandler<MenuItem>(x => x.OnClick);
             SubmenuOpenedEvent.AddClassHandler<MenuItem>(x => x.OnSubmenuOpened);
-            IsSubMenuOpenProperty.Changed.Subscribe(SubMenuOpenChanged);
+            IsSubMenuOpenProperty.Changed.AddClassHandler<MenuItem>(x => x.SubMenuOpenChanged);
         }
 
         /// <summary>
@@ -383,23 +383,19 @@ namespace Perspex.Controls
         /// Called when the <see cref="IsSubMenuOpen"/> property changes.
         /// </summary>
         /// <param name="e">The property change event.</param>
-        private static void SubMenuOpenChanged(PerspexPropertyChangedEventArgs e)
+        private void SubMenuOpenChanged(PerspexPropertyChangedEventArgs e)
         {
-            var sender = e.Sender as MenuItem;
             var value = (bool)e.NewValue;
 
-            if (sender != null)
+            if (value)
             {
-                if (value)
-                {
-                    sender.RaiseEvent(new RoutedEventArgs(SubmenuOpenedEvent));
-                    sender.IsSelected = true;
-                }
-                else
-                {
-                    sender.CloseSubmenus();
-                    sender.SelectedIndex = -1;
-                }
+                this.RaiseEvent(new RoutedEventArgs(SubmenuOpenedEvent));
+                this.IsSelected = true;
+            }
+            else
+            {
+                this.CloseSubmenus();
+                this.SelectedIndex = -1;
             }
         }