Allow setting values via ExpressionObserver.

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

@@ -19,6 +19,26 @@ namespace Perspex.Markup.Binding
             Nodes = ExpressionNodeBuilder.Build(expression);
         }
 
+        public bool SetValue(object value)
+        {
+            var last = Nodes.Last() as PropertyAccessorNode;
+
+            if (last != null)
+            {
+                try
+                {
+                    IncrementCount();
+                    return last.SetValue(value);
+                }
+                finally
+                {
+                    DecrementCount();
+                }
+            }
+
+            return false;
+        }
+
         public object Root { get; }
 
         public IList<ExpressionNode> Nodes { get; }

src/Markup/Perspex.Markup/Binding/PropertyAccessorNode.cs

@@ -16,6 +16,17 @@ namespace Perspex.Markup.Binding
             PropertyName = propertyName;
         }
 
+        public bool SetValue(object value)
+        {
+            if (_propertyInfo != null)
+            {
+                _propertyInfo.SetValue(Target, value);
+                return true;
+            }
+
+            return false;
+        }
+
         public string PropertyName { get; }
 
         protected override void SubscribeAndUpdate(object target)

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

@@ -142,6 +142,53 @@ namespace Perspex.Markup.UnitTests.Binding
             Assert.Equal(0, old.SubscriptionCount);
         }
 
+        [Fact]
+        public void SetValue_Should_Set_Simple_Property_Value()
+        {
+            var data = new Class1 { Foo = "foo" };
+            var target = new ExpressionObserver(data, "Foo");
+
+            Assert.True(target.SetValue("bar"));
+            Assert.Equal("bar", data.Foo);
+        }
+
+        [Fact]
+        public void SetValue_Should_Set_Property_At_The_End_Of_Chain()
+        {
+            var data = new Class1 { Next = new Class2 { Bar = "bar" } };
+            var target = new ExpressionObserver(data, "Next.Bar");
+
+            Assert.True(target.SetValue("baz"));
+            Assert.Equal("baz", ((Class2)data.Next).Bar);
+        }
+
+        [Fact]
+        public void SetValue_Should_Return_False_For_Missing_Property()
+        {
+            var data = new Class1 { Next = new WithoutBar()};
+            var target = new ExpressionObserver(data, "Next.Bar");
+
+            Assert.False(target.SetValue("baz"));
+        }
+
+        [Fact]
+        public void SetValue_Should_Return_False_For_Missing_Object()
+        {
+            var data = new Class1();
+            var target = new ExpressionObserver(data, "Next.Bar");
+
+            Assert.False(target.SetValue("baz"));
+        }
+
+        [Fact]
+        public void SetValue_Should_Throw_For_Wrong_Type()
+        {
+            var data = new Class1 { Foo = "foo" };
+            var target = new ExpressionObserver(data, "Foo");
+
+            Assert.Throws<ArgumentException>(() => target.SetValue(1.2));
+        }
+
         private interface INext
         {
             int SubscriptionCount { get; }

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

@@ -86,6 +86,9 @@
       <Name>Perspex.Markup</Name>
     </ProjectReference>
   </ItemGroup>
+  <ItemGroup>
+    <Service Include="{82A7F48D-3B50-4B1E-B82E-3ADA8210C358}" />
+  </ItemGroup>
   <Import Project="$(MSBuildToolsPath)\Microsoft.CSharp.targets" />
   <Target Name="EnsureNuGetPackageBuildImports" BeforeTargets="PrepareForBuild">
     <PropertyGroup>