Added PerspexProperty plugin to ExpressionObserver.

src/Markup/Perspex.Markup/Data/ExpressionObserver.cs

@@ -21,7 +21,8 @@ namespace Perspex.Markup.Data
         public static readonly IList<IPropertyAccessorPlugin> PropertyAccessors =
             new List<IPropertyAccessorPlugin>
             {
-                new InpcPropertyAccessorPlugin()
+                new PerspexPropertyAccessorPlugin(),
+                new InpcPropertyAccessorPlugin(),
             };
 
         private Func<object> _root;

src/Markup/Perspex.Markup/Data/Plugins/InpcPropertyAccessorPlugin.cs

@@ -54,18 +54,6 @@ namespace Perspex.Markup.Data.Plugins
             }
         }
 
-        private IObservable<object> GetObservable(
-            INotifyPropertyChanged inpc,
-            PropertyInfo property,
-            object o)
-        {
-            return Observable.FromEventPattern<PropertyChangedEventHandler, PropertyChangedEventArgs>(
-                x => inpc.PropertyChanged += x,
-                x => inpc.PropertyChanged -= x)
-                .Where(e => e.EventArgs.PropertyName == property.Name)
-                .Select(_ => property.GetValue(o));
-        }
-
         private class Accessor : IPropertyAccessor
         {
             private object _instance;

src/Markup/Perspex.Markup/Data/Plugins/PerspexPropertyAccessorPlugin.cs

@@ -0,0 +1,101 @@
+// Copyright (c) The Perspex Project. All rights reserved.
+// Licensed under the MIT license. See licence.md file in the project root for full license information.
+
+using System;
+using System.ComponentModel;
+using System.Reactive.Linq;
+using System.Reflection;
+
+namespace Perspex.Markup.Data.Plugins
+{
+    /// <summary>
+    /// Reads a property from a <see cref="PerspexObject"/>.
+    /// </summary>
+    public class PerspexPropertyAccessorPlugin : IPropertyAccessorPlugin
+    {
+        /// <summary>
+        /// Checks whether this plugin can handle accessing the properties of the specified object.
+        /// </summary>
+        /// <param name="instance">The object.</param>
+        /// <returns>True if the plugin can handle the object; otherwise false.</returns>
+        public bool Match(object instance)
+        {
+            Contract.Requires<ArgumentNullException>(instance != null);
+
+            return instance is PerspexObject;
+        }
+
+        /// <summary>
+        /// Starts monitoring the value of a property on an object.
+        /// </summary>
+        /// <param name="instance">The object.</param>
+        /// <param name="propertyName">The property name.</param>
+        /// <param name="changed">A function to call when the property changes.</param>
+        /// <returns>
+        /// An <see cref="IPropertyAccessor"/> interface through which future interactions with the 
+        /// property will be made, or null if the property was not found.
+        /// </returns>
+        public IPropertyAccessor Start(object instance, string propertyName, Action<object> changed)
+        {
+            Contract.Requires<ArgumentNullException>(instance != null);
+            Contract.Requires<ArgumentNullException>(propertyName != null);
+            Contract.Requires<ArgumentNullException>(changed != null);
+
+            var o = (PerspexObject)instance;
+            var p = PerspexPropertyRegistry.Instance.FindRegistered(o, propertyName);
+
+            if (p != null)
+            {
+                return new Accessor(o, p, changed);
+            }
+            else
+            {
+                return null;
+            }
+        }
+
+        private class Accessor : IPropertyAccessor
+        {
+            private PerspexObject _instance;
+            private PerspexProperty _property;
+            private IDisposable _subscription;
+
+            public Accessor(PerspexObject instance, PerspexProperty property, Action<object> changed)
+            {
+                Contract.Requires<ArgumentNullException>(instance != null);
+                Contract.Requires<ArgumentNullException>(property != null);
+
+                _instance = instance;
+                _property = property;
+                _subscription = instance.GetObservable(property).Skip(1).Subscribe(changed);
+            }
+
+            public Type PropertyType
+            {
+                get { return _property.PropertyType; }
+            }
+
+            public object Value
+            {
+                get { return _instance.GetValue(_property); }
+            }
+
+            public void Dispose()
+            {
+                _subscription?.Dispose();
+                _subscription = null;
+            }
+
+            public bool SetValue(object value)
+            {
+                if (!_property.IsReadOnly)
+                {
+                    _instance.SetValue(_property, value);
+                    return true;
+                }
+
+                return false;
+            }
+        }
+    }
+}

src/Markup/Perspex.Markup/Perspex.Markup.csproj

@@ -42,6 +42,7 @@
     <Compile Include="Data\ExpressionNodeBuilder.cs" />
     <Compile Include="Data\ExpressionParseException.cs" />
     <Compile Include="Data\ExpressionSubject.cs" />
+    <Compile Include="Data\Plugins\PerspexPropertyAccessorPlugin.cs" />
     <Compile Include="Data\Plugins\InpcPropertyAccessorPlugin.cs" />
     <Compile Include="Data\Plugins\IPropertyAccessor.cs" />
     <Compile Include="Data\Plugins\IPropertyAccessorPlugin.cs" />

src/Perspex.Base/PerspexProperty.cs

@@ -270,6 +270,11 @@ namespace Perspex
         /// </summary>
         public bool IsDirect { get; }
 
+        /// <summary>
+        /// Gets a value indicating whether this is a readonly property.
+        /// </summary>
+        public bool IsReadOnly => IsDirect && Setter == null;
+
         /// <summary>
         /// Gets an observable that is fired when this property is initialized on a
         /// new <see cref="PerspexObject"/> instance.

tests/Perspex.Markup.UnitTests/Binding/ExpressionObserverTests_PerspexProperty.cs

@@ -0,0 +1,45 @@
+// Copyright (c) The Perspex Project. All rights reserved.
+// Licensed under the MIT license. See licence.md file in the project root for full license information.
+
+using System;
+using System.Collections.Generic;
+using System.Reactive.Linq;
+using Perspex.Markup.Data;
+using Xunit;
+
+namespace Perspex.Markup.UnitTests.Binding
+{
+    public class ExpressionObserverTests_PerspexProperty
+    {
+        [Fact]
+        public async void Should_Get_Simple_Property_Value()
+        {
+            var data = new Class1();
+            var target = new ExpressionObserver(data, "Foo");
+            var result = await target.Take(1);
+
+            Assert.Equal("foo", result);
+        }
+
+        [Fact]
+        public void Should_Track_Simple_Property_Value()
+        {
+            var data = new Class1();
+            var target = new ExpressionObserver(data, "Foo");
+            var result = new List<object>();
+
+            var sub = target.Subscribe(x => result.Add(x));
+            data.SetValue(Class1.FooProperty, "bar");
+
+            Assert.Equal(new[] { "foo", "bar" }, result);
+
+            sub.Dispose();
+        }
+
+        private class Class1 : PerspexObject
+        {
+            public static readonly PerspexProperty<string> FooProperty =
+                PerspexProperty.Register<Class1, string>("Foo", defaultValue: "foo");
+        }
+    }
+}

tests/Perspex.Markup.UnitTests/Perspex.Markup.UnitTests.csproj

@@ -73,6 +73,7 @@
   </ItemGroup>
   <ItemGroup>
     <Compile Include="Binding\ExpressionNodeBuilderTests_Errors.cs" />
+    <Compile Include="Binding\ExpressionObserverTests_PerspexProperty.cs" />
     <Compile Include="Binding\ExpressionSubjectTests.cs" />
     <Compile Include="Binding\ExpressionObserverTests_Observable.cs" />
     <Compile Include="Binding\ExpressionObserverTests_SetValue.cs" />