Bring back the way simpler way to observe DPs in WPF

Author: anaisbetts

ReactiveUI.Platforms/ReactiveUI.Xaml_Net45.csproj

@@ -135,13 +135,13 @@
       <Link>Xaml\Properties\CommonAssemblyInfo.cs</Link>
     </Compile>
     <Compile Include="ComponentModelTypeConverter.cs" />
+    <Compile Include="Xaml\WpfDependencyObjectObservableForProperty.cs" />
     <Compile Include="Xaml\ActivationForViewFetcher.cs" />
     <Compile Include="Xaml\PlatformOperations.cs" />
     <Compile Include="Registrations.cs" />
     <Compile Include="Xaml\AutoDataTemplateBindingHook.cs" />
     <Compile Include="Xaml\BindingTypeConverters.cs" />
     <Compile Include="Xaml\CreatesCommandBinding.cs" />
-    <Compile Include="Xaml\DependencyObjectObservableForProperty.cs" />
     <Compile Include="Xaml\Properties\AssemblyInfo.cs" />
     <Compile Include="Xaml\RoutedViewHost.cs" />
     <Compile Include="Xaml\TransitioningContentControl.cs" />

ReactiveUI.Platforms/Xaml/WpfDependencyObjectObservableForProperty.cs

@@ -0,0 +1,51 @@
+using System;
+using System.Collections.Generic;
+using System.Linq;
+using System.Text;
+using System.Threading.Tasks;
+using System.Windows;
+using System.Reflection;
+using System.ComponentModel;
+using System.Reactive.Linq;
+using System.Reactive.Disposables;
+
+namespace ReactiveUI.Xaml
+{
+    public class DependencyObjectObservableForProperty : ICreatesObservableForProperty
+    {
+        public int GetAffinityForObject(Type type, string propertyName, bool beforeChanged = false)
+        {
+            if (!typeof(DependencyObject).GetTypeInfo().IsAssignableFrom(type.GetTypeInfo())) return 0;
+            return getDependencyProperty(type, propertyName) != null ? 4 : 0;
+        }
+
+        public IObservable<IObservedChange<object, object>> GetNotificationForProperty(object sender, string propertyName, bool beforeChanged = false)
+        {
+            var type = sender.GetType();
+            var dpd = DependencyPropertyDescriptor.FromProperty(getDependencyProperty(type, propertyName), sender.GetType());
+
+            return Observable.Create<IObservedChange<object, object>>(subj => {
+                var handler = new EventHandler((o, e) => {
+                    subj.OnNext(new ObservedChange<object, object>() {
+                        Sender = sender,
+                        PropertyName = propertyName,
+                        Value = null,
+                    });
+                });
+
+                dpd.AddValueChanged(sender, handler);
+                return Disposable.Create(() => dpd.RemoveValueChanged(sender, handler));
+            });
+        }
+
+        DependencyProperty getDependencyProperty(Type type, string propertyName)
+        {
+            var fi = type.GetRuntimeFields().FirstOrDefault(x => x.Name == propertyName + "Property" && x.IsStatic);
+            if (fi != null) {
+                return (DependencyProperty)fi.GetValue(null);
+            }
+
+            return null;
+        }
+    }
+}