Added ExpressionSubject.

src/Markup/Perspex.Markup/Binding/ExpressionObserver.cs

@@ -2,15 +2,13 @@
 // 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.Linq;
 using System.Reactive;
 using System.Reactive.Disposables;
 
 namespace Perspex.Markup.Binding
 {
     /// <summary>
-    /// Observes the value of an expression on a root object.
+    /// Observes and sets the value of an expression on an object.
     /// </summary>
     public class ExpressionObserver : ObservableBase<ExpressionValue>
     {

src/Markup/Perspex.Markup/Binding/ExpressionSubject.cs

@@ -0,0 +1,48 @@
+// 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.Reactive.Linq;
+using System.Reactive.Subjects;
+
+namespace Perspex.Markup.Binding
+{
+    /// <summary>
+    /// Turns an <see cref="ExpressionObserver"/> into a subject that can be bound two-ways.
+    /// </summary>
+    public class ExpressionSubject : ISubject<object>
+    {
+        private ExpressionObserver _inner;
+
+        /// <summary>
+        /// Initializes a new instance of the <see cref="ExpressionObserver"/> class.
+        /// </summary>
+        /// <param name="inner">The <see cref="ExpressionObserver"/>.</param>
+        public ExpressionSubject(ExpressionObserver inner)
+        {
+            _inner = inner;
+        }
+
+        /// <inheritdoc/>
+        public void OnCompleted()
+        {
+        }
+
+        /// <inheritdoc/>
+        public void OnError(Exception error)
+        {
+        }
+
+        /// <inheritdoc/>
+        public void OnNext(object value)
+        {
+            _inner.SetValue(value);
+        }
+
+        /// <inheritdoc/>
+        public IDisposable Subscribe(IObserver<object> observer)
+        {
+            return _inner.Select(x => x.Value).Distinct().Subscribe(observer);
+        }
+    }
+}

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

@@ -36,6 +36,7 @@
   <ItemGroup>
     <Compile Include="Binding\ExpressionNodeBuilder.cs" />
     <Compile Include="Binding\ExpressionParseException.cs" />
+    <Compile Include="Binding\ExpressionSubject.cs" />
     <Compile Include="Binding\ExpressionValue.cs" />
     <Compile Include="Binding\LogicalNotNode.cs" />
     <Compile Include="Binding\IndexerNode.cs" />