From ca7584ce3fef1ba5f8a7e8b7bb916dc4617a1f21 Mon Sep 17 00:00:00 2001
From: Steven Kirk <grokys@gmail.com>
Date: Tue, 17 Oct 2017 23:45:00 +0200
Subject: [PATCH 1/2] Added failing test for #1218

---
 .../Avalonia.Styling.UnitTests/SetterTests.cs | 37 +++++++++++++++++++
 1 file changed, 37 insertions(+)

diff --git a/tests/Avalonia.Styling.UnitTests/SetterTests.cs b/tests/Avalonia.Styling.UnitTests/SetterTests.cs
index 84536fa47b..9c43097a21 100644
--- a/tests/Avalonia.Styling.UnitTests/SetterTests.cs
+++ b/tests/Avalonia.Styling.UnitTests/SetterTests.cs
@@ -8,6 +8,9 @@ using Avalonia.Data;
 using Xunit;
 using System;
 using Avalonia.Controls.Templates;
+using Avalonia.Markup.Xaml.Data;
+using Avalonia.Markup;
+using System.Globalization;
 
 namespace Avalonia.Styling.UnitTests
 {
@@ -61,5 +64,39 @@ namespace Avalonia.Styling.UnitTests
 
             Assert.NotNull(NameScope.GetNameScope((Control)control.Child));
         }
+
+        [Fact]
+        public void Does_Not_Call_Converter_ConvertBack_On_OneWay_Binding()
+        {
+            var control = new Decorator { Name = "foo" };
+            var style = Mock.Of<IStyle>();
+            var binding = new Binding("Name", BindingMode.OneWay)
+            {
+                Converter = new TestConverter(),
+                RelativeSource = new RelativeSource(RelativeSourceMode.Self),
+            };
+            var setter = new Setter(Decorator.TagProperty, binding);
+            var activator = new BehaviorSubject<bool>(true);
+
+            setter.Apply(style, control, activator);
+            Assert.Equal("foobar", control.Tag);
+
+            // Issue #1218 caused TestConverter.ConvertBack to throw here.
+            activator.OnNext(false);
+            Assert.Null(control.Tag);
+        }
+
+        private class TestConverter : IValueConverter
+        {
+            public object Convert(object value, Type targetType, object parameter, CultureInfo culture)
+            {
+                return value.ToString() + "bar";
+            }
+
+            public object ConvertBack(object value, Type targetType, object parameter, CultureInfo culture)
+            {
+                throw new NotImplementedException();
+            }
+        }
     }
 }

From dede306531fc38eef26807cd0eb028f9dbd66470 Mon Sep 17 00:00:00 2001
From: Steven Kirk <grokys@gmail.com>
Date: Tue, 17 Oct 2017 23:45:35 +0200
Subject: [PATCH 2/2] Fixed #1218

---
 src/Avalonia.Styling/Styling/Setter.cs | 12 ++++++------
 1 file changed, 6 insertions(+), 6 deletions(-)

diff --git a/src/Avalonia.Styling/Styling/Setter.cs b/src/Avalonia.Styling/Styling/Setter.cs
index d065b231ce..eb5218cd6f 100644
--- a/src/Avalonia.Styling/Styling/Setter.cs
+++ b/src/Avalonia.Styling/Styling/Setter.cs
@@ -141,20 +141,20 @@ namespace Avalonia.Styling
             {
                 var description = style?.ToString();
 
-                if (sourceInstance.Subject != null)
+                if (sourceInstance.Mode == BindingMode.TwoWay || sourceInstance.Mode == BindingMode.OneWayToSource)
                 {
                     var activated = new ActivatedSubject(activator, sourceInstance.Subject, description);
                     cloned = new InstancedBinding(activated, sourceInstance.Mode, BindingPriority.StyleTrigger);
                 }
-                else if (sourceInstance.Observable != null)
+                else if (sourceInstance.Mode == BindingMode.OneTime)
                 {
-                    var activated = new ActivatedObservable(activator, sourceInstance.Observable, description);
-                    cloned = new InstancedBinding(activated, sourceInstance.Mode, BindingPriority.StyleTrigger);
+                    var activated = new ActivatedValue(activator, sourceInstance.Value, description);
+                    cloned = new InstancedBinding(activated, BindingMode.OneWay, BindingPriority.StyleTrigger);
                 }
                 else
                 {
-                    var activated = new ActivatedValue(activator, sourceInstance.Value, description);
-                    cloned = new InstancedBinding(activated, BindingMode.OneWay, BindingPriority.StyleTrigger);
+                    var activated = new ActivatedObservable(activator, sourceInstance.Observable ?? sourceInstance.Subject, description);
+                    cloned = new InstancedBinding(activated, sourceInstance.Mode, BindingPriority.StyleTrigger);
                 }
             }
             else