Add :mouseover pseudoclass.

Perspex.Windows/Window.cs

@@ -134,7 +134,12 @@ namespace Perspex.Windows
 
         private void MouseMove(Visual visual, Point p)
         {
-            visual.IsMouseOver = visual.Bounds.Contains(p);
+            Control control = visual as Control;
+
+            if (control != null)
+            {
+                control.IsMouseOver = visual.Bounds.Contains(p);
+            }
 
             foreach (Visual child in visual.VisualChildren)
             {

Perspex/Controls/Control.cs

@@ -44,6 +44,9 @@ namespace Perspex.Controls
         public static readonly PerspexProperty<double> BorderThicknessProperty =
             PerspexProperty.Register<Control, double>("BorderThickness");
 
+        public static readonly PerspexProperty<bool> IsMouseOverProperty =
+            PerspexProperty.Register<Visual, bool>("IsMouseOver");
+
         public static readonly PerspexProperty<HorizontalAlignment> HorizontalAlignmentProperty =
             PerspexProperty.Register<Control, HorizontalAlignment>("HorizontalAlignment");
 
@@ -58,10 +61,21 @@ namespace Perspex.Controls
 
         public Control()
         {
-            this.Classes = new ObservableCollection<string>();
+            this.Classes = new PerspexList<string>();
-            this.Styles = new ObservableCollection<Style>();
             this.GetObservableWithHistory(ParentPropertyRW).Subscribe(this.ParentChanged);
 
+            this.GetObservable(IsMouseOverProperty).Subscribe(x =>
+            {
+                if (x)
+                {
+                    this.Classes.Add(":mouseover");
+                }
+                else
+                {
+                    this.Classes.Remove(":mouseover");
+                }
+            });
+
             // Hacky hack hack!
             this.GetObservable(BackgroundProperty).Skip(1).Subscribe(_ => this.InvalidateMeasure());
         }
@@ -84,7 +98,7 @@ namespace Perspex.Controls
             set { this.SetValue(BorderThicknessProperty, value); }
         }
 
-        public ObservableCollection<string> Classes
+        public PerspexList<string> Classes
         {
             get;
             private set;
@@ -102,6 +116,12 @@ namespace Perspex.Controls
             set;
         }
 
+        public bool IsMouseOver
+        {
+            get { return this.GetValue(IsMouseOverProperty); }
+            set { this.SetValue(IsMouseOverProperty, value); }
+        }
+
         public HorizontalAlignment HorizontalAlignment
         {
             get { return this.GetValue(HorizontalAlignmentProperty); }

Perspex/Perspex.csproj

@@ -87,6 +87,7 @@
     <Compile Include="Media\Matrix.cs" />
     <Compile Include="Media\Pen.cs" />
     <Compile Include="Media\SolidColorBrush.cs" />
+    <Compile Include="PerspexList.cs" />
     <Compile Include="PerspexObject.cs" />
     <Compile Include="PerspexProperty.cs" />
     <Compile Include="PerspexPropertyChangedEventArgs.cs" />

Perspex/PerspexList.cs

@@ -0,0 +1,30 @@
+namespace Perspex
+{
+    using System;
+    using System.Collections;
+    using System.Collections.Generic;
+    using System.Collections.ObjectModel;
+    using System.Collections.Specialized;
+    using System.Linq;
+    using System.Reactive.Linq;
+    using System.Reactive.Subjects;
+    using System.Text;
+    using System.Threading.Tasks;
+
+    public class PerspexList<T> : ObservableCollection<T>
+    {
+        public PerspexList()
+        {
+            this.Changed = Observable.FromEvent<NotifyCollectionChangedEventHandler, NotifyCollectionChangedEventArgs>(
+                handler => (sender, e) => handler(e),
+                handler => this.CollectionChanged += handler,
+                handler => this.CollectionChanged -= handler);
+        }
+
+        public IObservable<NotifyCollectionChangedEventArgs> Changed
+        {
+            get;
+            private set;
+        }
+    }
+}

Perspex/Selectors.cs

@@ -8,6 +8,7 @@ namespace Perspex
 {
     using System;
     using System.Collections.Generic;
+    using System.Collections.Specialized;
     using System.Diagnostics.Contracts;
     using System.Linq;
     using System.Reactive.Linq;
@@ -34,6 +35,29 @@ namespace Perspex
             }
         }
 
+        public static Match Class(this Match selector, string name)
+        {
+            Contract.Requires<ArgumentNullException>(name != null);
+
+            if (selector != null)
+            {
+                IObservable<bool> match = Observable
+                    .Return(selector.Control.Classes.Contains(name))
+                    .Concat(selector.Control.Classes.Changed.Select(e => selector.Control.Classes.Contains(name)));
+
+                return new Match
+                {
+                    Control = selector.Control,
+                    Observable = match,
+                    Previous = selector,
+                };
+            }
+            else
+            {
+                return null;
+            }
+        }
+
         public static Match PropertyEquals<T>(this Match selector, PerspexProperty<T> property, T value)
         {
             Contract.Requires<ArgumentNullException>(property != null);

Perspex/Visual.cs

@@ -17,9 +17,6 @@ namespace Perspex
         public static readonly ReadOnlyPerspexProperty<Rect> BoundsProperty =
             new ReadOnlyPerspexProperty<Rect>(BoundsPropertyRW);
 
-        public static readonly PerspexProperty<bool> IsMouseOverProperty =
-            PerspexProperty.Register<Visual, bool>("IsMouseOver");
-
         private static readonly PerspexProperty<Rect> BoundsPropertyRW =
             PerspexProperty.Register<Visual, Rect>("Bounds");
 
@@ -31,12 +28,6 @@ namespace Perspex
             protected set { this.SetValue(BoundsPropertyRW, value); }
         }
 
-        public bool IsMouseOver
-        {
-            get { return this.GetValue(IsMouseOverProperty); }
-            set { this.SetValue(IsMouseOverProperty, value); }
-        }
-
         public virtual IEnumerable<Visual> VisualChildren
         {
             get { return Enumerable.Empty<Visual>(); }

TestApplication/Program.cs

@@ -22,11 +22,11 @@ namespace TestApplication
 
             Window window = new Window
             {
-                Styles = new ObservableCollection<Style>
+                Styles = new[]
                 {
                     new Style
                     {
-                        Selector = x => x.Select<Button>().PropertyEquals(Visual.IsMouseOverProperty, true),
+                        Selector = x => x.Select<Button>().Class(":mouseover"),
                         Setters = new[]
                         {
                             new Setter