Fix OneTime bindings.

OneTime bindings were failing in BindingTest because the initial binding
error was being counted as the single value to transfer. Don't do this
with OneTime bindings - only transfer valid values.

src/Avalonia.Base/Data/BindingNotification.cs

@@ -166,6 +166,21 @@ namespace Avalonia.Data
             return !(a == b);
         }
 
+        /// <summary>
+        /// Gets a value from an object that may be a <see cref="BindingNotification"/>.
+        /// </summary>
+        /// <param name="o">The object.</param>
+        /// <returns>The value.</returns>
+        /// <remarks>
+        /// If <paramref name="o"/> is a <see cref="BindingNotification"/> then returns the binding
+        /// notification's <see cref="Value"/>. If not, returns the object unchanged.
+        /// </remarks>
+        public static object ExtractValue(object o)
+        {
+            var notification = o as BindingNotification;
+            return notification != null ? notification.Value : o;
+        }
+
         /// <summary>
         /// Compares an object to an instance of <see cref="BindingNotification"/> for equality.
         /// </summary>

src/Avalonia.Base/Data/BindingOperations.cs

@@ -54,7 +54,10 @@ namespace Avalonia.Data
 
                     if (source != null)
                     {
-                        return source.Take(1).Subscribe(x => target.SetValue(property, x, binding.Priority));
+                        return source
+                            .Where(x => BindingNotification.ExtractValue(x) != AvaloniaProperty.UnsetValue)
+                            .Take(1)
+                            .Subscribe(x => target.SetValue(property, x, binding.Priority));
                     }
                     else
                     {

tests/Avalonia.Base.UnitTests/AvaloniaObjectTests_Binding.cs

@@ -54,6 +54,36 @@ namespace Avalonia.Base.UnitTests
             Assert.False(target.IsSet(Class1.QuxProperty));
         }
 
+        [Fact]
+        public void OneTime_Binding_Ignores_UnsetValue()
+        {
+            var target = new Class1();
+            var source = new Subject<object>();
+
+            target.Bind(Class1.QuxProperty, new TestOneTimeBinding(source));
+
+            source.OnNext(AvaloniaProperty.UnsetValue);
+            Assert.Equal(5.6, target.GetValue(Class1.QuxProperty));
+
+            source.OnNext(6.7);
+            Assert.Equal(6.7, target.GetValue(Class1.QuxProperty));
+        }
+
+        [Fact]
+        public void OneTime_Binding_Ignores_Binding_Errors()
+        {
+            var target = new Class1();
+            var source = new Subject<object>();
+
+            target.Bind(Class1.QuxProperty, new TestOneTimeBinding(source));
+
+            source.OnNext(new BindingNotification(new Exception(), BindingErrorType.Error));
+            Assert.Equal(5.6, target.GetValue(Class1.QuxProperty));
+
+            source.OnNext(6.7);
+            Assert.Equal(6.7, target.GetValue(Class1.QuxProperty));
+        }
+
         [Fact]
         public void Bind_Throws_Exception_For_Unregistered_Property()
         {
@@ -352,5 +382,24 @@ namespace Avalonia.Base.UnitTests
             public static readonly StyledProperty<string> BarProperty =
                 AvaloniaProperty.Register<Class2, string>("Bar", "bardefault");
         }
+
+        private class TestOneTimeBinding : IBinding
+        {
+            private IObservable<object> _source;
+
+            public TestOneTimeBinding(IObservable<object> source)
+            {
+                _source = source;
+            }
+
+            public InstancedBinding Initiate(
+                IAvaloniaObject target,
+                AvaloniaProperty targetProperty,
+                object anchor = null,
+                bool enableDataValidation = false)
+            {
+                return new InstancedBinding(_source, BindingMode.OneTime);
+            }
+        }
     }
 }