PR Feedback.

src/Avalonia.Base/AvaloniaObject.cs

@@ -51,10 +51,19 @@ namespace Avalonia
         /// </summary>
         private EventHandler<AvaloniaPropertyChangedEventArgs> _propertyChanged;
 
+        private DeferredSetter<AvaloniaProperty, object> _directDeferredSetter;
+
         /// <summary>
         /// Delayed setter helper for direct properties. Used to fix #855.
         /// </summary>
-        private readonly DeferredSetter<AvaloniaProperty, object> directDelayedSetter = new DeferredSetter<AvaloniaProperty, object>();
+        private DeferredSetter<AvaloniaProperty, object> DirectDelayedSetter
+        {
+            get
+            {
+                return _directDeferredSetter ??
+                    (_directDeferredSetter = new DeferredSetter<AvaloniaProperty, object>());
+            }
+        }
 
 
         /// <summary>
@@ -559,7 +568,7 @@ namespace Avalonia
         protected bool SetAndRaise<T>(AvaloniaProperty<T> property, ref T field, T value)
         {
             VerifyAccess();
-            if (!directDelayedSetter.IsNotifying(property))
+            if (!DirectDelayedSetter.IsNotifying(property))
             {
                 var valueChanged = false;
                 if (!object.Equals(field, value))
@@ -569,16 +578,16 @@ namespace Avalonia
                     valueChanged = true;
                 }
 
-                while (directDelayedSetter.HasPendingSet(property))
+                while (DirectDelayedSetter.HasPendingSet(property))
                 {
-                    SetAndRaiseCore(property, ref field, (T)directDelayedSetter.GetFirstPendingSet(property));
+                    SetAndRaiseCore(property, ref field, (T)DirectDelayedSetter.GetFirstPendingSet(property));
                     valueChanged = true;
                 }
                 return valueChanged;
             }
             else if(!object.Equals(field, value))
             {
-                directDelayedSetter.AddPendingSet(property, value);
+                DirectDelayedSetter.AddPendingSet(property, value);
             }
             return false;
         }
@@ -588,7 +597,7 @@ namespace Avalonia
             var old = field;
             field = value;
 
-            using (directDelayedSetter.MarkNotifying(property))
+            using (DirectDelayedSetter.MarkNotifying(property))
             {
                 RaisePropertyChanged(property, old, value, BindingPriority.LocalValue);
             }
@@ -598,14 +607,15 @@ namespace Avalonia
             AvaloniaProperty<T> property,
             Action<T, Action<Action>> setterCallback,
             T value,
-            Predicate<T> pendingSetCondition = null)
+            Predicate<T> pendingSetCondition)
         {
             Contract.Requires<ArgumentNullException>(setterCallback != null);
-            directDelayedSetter.SetAndNotify(
+            Contract.Requires<ArgumentNullException>(pendingSetCondition != null);
+            DirectDelayedSetter.SetAndNotify(
                 property,
                 (val, notify) => setterCallback((T)val, notify),
                 value,
-                pendingSetCondition != null ? o => pendingSetCondition((T)o) : (Predicate<object>)null);
+                o => pendingSetCondition((T)o));
         }
 
         /// <summary>

src/Avalonia.Base/PriorityValue.cs

@@ -28,9 +28,10 @@ namespace Avalonia
     {
         private readonly Type _valueType;
         private readonly SingleOrDictionary<int, PriorityLevel> _levels = new SingleOrDictionary<int, PriorityLevel>();
-        private object _value;
+
         private readonly Func<object, object> _validate;
         private static readonly DeferredSetter<PriorityValue, (object value, int priority)> delayedSetter = new DeferredSetter<PriorityValue, (object, int)>();
+        private (object value, int priority) _value;
 
         /// <summary>
         /// Initializes a new instance of the <see cref="PriorityValue"/> class.
@@ -47,9 +48,7 @@ namespace Avalonia
         {
             Owner = owner;
             Property = property;
-            _valueType = valueType;
+            _value = (AvaloniaProperty.UnsetValue, int.MaxValue);
-            _value = AvaloniaProperty.UnsetValue;
-            ValuePriority = int.MaxValue;
             _validate = validate;
         }
 
@@ -66,16 +65,12 @@ namespace Avalonia
         /// <summary>
         /// Gets the current value.
         /// </summary>
-        public object Value => _value;
+        public object Value => _value.value;
 
         /// <summary>
         /// Gets the priority of the binding that is currently active.
         /// </summary>
-        public int ValuePriority
+        public int ValuePriority => _value.priority;
-        {
-            get;
-            private set;
-        }
 
         /// <summary>
         /// Adds a new binding.
@@ -238,7 +233,7 @@ namespace Avalonia
             delayedSetter.SetAndNotify(this,
                 UpdateCore,
                 (value, priority),
-                val => !object.Equals(val.value, _value));
+                val => !object.Equals(val.value, val.value));
         }
 
         private void UpdateCore((object value, int priority) update, Action<Action> notify)
@@ -254,15 +249,14 @@ namespace Avalonia
 
             if (TypeUtilities.TryConvertImplicit(_valueType, val, out castValue))
             {
-                var old = _value;
+                var old = this._value.value;
 
                 if (_validate != null && castValue != AvaloniaProperty.UnsetValue)
                 {
                     castValue = _validate(castValue);
                 }
 
-                ValuePriority = update.priority;
+                this._value = (castValue, update.priority);
-                _value = castValue;
 
                 if (notification?.HasValue == true)
                 {
@@ -271,7 +265,7 @@ namespace Avalonia
 
                 if (notification == null || notification.HasValue)
                 {
-                    notify(() => Owner?.Changed(this, old, _value));
+                    notify(() => Owner?.Changed(this, old, Value));
                 }
 
                 if (notification != null)

src/Avalonia.Base/Utilities/DeferredSetter.cs

@@ -14,11 +14,11 @@ namespace Avalonia.Utilities
     class DeferredSetter<TProperty, TValue>
         where TProperty: class
     {
-        private struct NotifyDisposable : IDisposable
+        internal struct NotifyDisposable : IDisposable
         {
             private readonly SettingStatus status;
 
-            public NotifyDisposable(SettingStatus status)
+            internal NotifyDisposable(SettingStatus status)
             {
                 this.status = status;
                 status.Notifying = true;
@@ -33,7 +33,7 @@ namespace Avalonia.Utilities
         /// <summary>
         /// Information on current setting/notification status of a property.
         /// </summary>
-        private class SettingStatus
+        internal class SettingStatus
         {
             public bool Notifying { get; set; }
 
@@ -55,7 +55,7 @@ namespace Avalonia.Utilities
         /// </summary>
         /// <param name="property">The property to mark as notifying.</param>
         /// <returns>Returns a disposable that when disposed, marks the property as done notifying.</returns>
-        internal IDisposable MarkNotifying(TProperty property)
+        internal NotifyDisposable MarkNotifying(TProperty property)
         {
             Contract.Requires<InvalidOperationException>(!IsNotifying(property));
             
@@ -119,6 +119,7 @@ namespace Avalonia.Utilities
             Predicate<TValue> pendingSetCondition)
         {
             Contract.Requires<ArgumentNullException>(setterCallback != null);
+            Contract.Requires<ArgumentNullException>(pendingSetCondition != null);
             if (!IsNotifying(property))
             {
                 setterCallback(value, notification =>
@@ -139,7 +140,7 @@ namespace Avalonia.Utilities
                     });
                 }
             }
-            else if(pendingSetCondition?.Invoke(value) ?? true)
+            else if(pendingSetCondition(value))
             {
                 AddPendingSet(property, value);
             }

src/Avalonia.Controls/Primitives/SelectingItemsControl.cs

@@ -151,7 +151,7 @@ namespace Avalonia.Controls.Primitives
             {
                 if (_updateCount == 0)
                 {
-                    SetAndRaise(SelectedIndexProperty, (val, notifierWrapper) =>
+                    SetAndRaise(SelectedIndexProperty, (val, notifyWrapper) =>
                     {
                         var old = SelectedIndex;
                         var effective = (val >= 0 && val < Items?.Cast<object>().Count()) ? val : -1;
@@ -159,10 +159,15 @@ namespace Avalonia.Controls.Primitives
                         if (old != effective)
                         {
                             _selectedIndex = effective;
-                            notifierWrapper(() => RaisePropertyChanged(SelectedIndexProperty, old, effective, BindingPriority.LocalValue));
+                            notifyWrapper(() =>
+                                RaisePropertyChanged(
+                                    SelectedIndexProperty,
+                                    old,
+                                    effective,
+                                    BindingPriority.LocalValue));
                             SelectedItem = ElementAt(Items, effective);
                         }
-                    }, value);
+                    }, value, val => val != SelectedIndex);
                 }
                 else
                 {
@@ -196,7 +201,12 @@ namespace Avalonia.Controls.Primitives
                         {
                             _selectedItem = effective;
 
-                            notifyWrapper(() => RaisePropertyChanged(SelectedItemProperty, old, effective, BindingPriority.LocalValue));
+                            notifyWrapper(() =>
+                                RaisePropertyChanged(
+                                    SelectedItemProperty,
+                                    old,
+                                    effective,
+                                    BindingPriority.LocalValue));
 
                             SelectedIndex = index;
 
@@ -215,7 +225,7 @@ namespace Avalonia.Controls.Primitives
                                 SelectedItems.Clear();
                             }
                         }
-                    }, value);
+                    }, value, val => val != SelectedItem);
                 }
                 else
                 {

tests/Avalonia.Controls.UnitTests/ListBoxTests_Single.cs

@@ -223,7 +223,7 @@ namespace Avalonia.Controls.UnitTests
 
             Assert.Equal(0, viewModel.SetterInvokedCount);
 
-            //here in real life stack overflow exception is thrown issue #855
+            // In Issue #855, a Stackoverflow occured here.
             target.SelectedItem = viewModel.Items[2];
 
             Assert.Equal(viewModel.Items[1], target.SelectedItem);