Ensure active direct bindings don't get collected.

src/Perspex.Base/PerspexObject.cs

@@ -25,6 +25,17 @@ namespace Perspex
     /// </remarks>
     public class PerspexObject : IPerspexObject, IPerspexObjectDebug, INotifyPropertyChanged
     {
+        /// <summary>
+        /// Maintains a list of direct property binding subscriptions so that the binding source
+        /// doesn't get collected.
+        /// </summary>
+        /// <remarks>
+        /// If/when we provide a ClearBindings() method, then this collection will be need to be
+        /// moved to an instance field and indexed by property, but until that point a static
+        /// collection will suffice.
+        /// </remarks>
+        private static List<IDisposable> s_directBindings = new List<IDisposable>();
+
         /// <summary>
         /// The parent object that inherited values are inherited from.
         /// </summary>
@@ -401,9 +412,20 @@ namespace Perspex
                     property,
                     GetDescription(source));
 
-                return source
+                IDisposable subscription = null;
+
+                subscription = source
                     .Select(x => TypeUtilities.CastOrDefault(x, property.PropertyType))
+                    .Do(_ => { }, () => s_directBindings.Remove(subscription))
                     .Subscribe(x => SetValue(property, x));
+
+                s_directBindings.Add(subscription);
+
+                return Disposable.Create(() =>
+                {
+                    subscription.Dispose();
+                    s_directBindings.Remove(subscription);
+                });
             }
             else
             {

tests/Perspex.Base.UnitTests/PerspexObjectTests_Direct.cs

@@ -333,6 +333,50 @@ namespace Perspex.Base.UnitTests
             Assert.Equal("second", target.Foo);
         }
 
+        [Fact]
+        public void Binding_To_Direct_Property_Does_Not_Get_Collected()
+        {
+            var target = new Class2();
+
+            Func<WeakReference> setupBinding = () =>
+            {
+                var source = new Subject<string>();
+                var sub = target.Bind((PerspexProperty)Class1.FooProperty, source);
+                return new WeakReference(source);
+            };
+
+            var weakSource = setupBinding();
+
+            GC.Collect();
+
+            Assert.True(weakSource.IsAlive);
+        }
+
+        [Fact]
+        public void Binding_To_Direct_Property_Gets_Collected_When_Completed()
+        {
+            var target = new Class2();
+
+            Func<WeakReference> setupBinding = () =>
+            {
+                var source = new Subject<string>();
+                var sub = target.Bind((PerspexProperty)Class1.FooProperty, source);
+                return new WeakReference(source);
+            };
+        
+            var weakSource = setupBinding();
+
+            Action completeSource = () =>
+            {
+                ((ISubject<string>)weakSource.Target).OnCompleted();
+            };
+
+            completeSource();
+            GC.Collect();
+
+            Assert.False(weakSource.IsAlive);
+        }
+
         [Fact]
         public void Property_Notifies_Initialized()
         {