Use SingleOrQueue instead of Queue in DeferredSetter. Make sure tests pass.

src/Avalonia.Base/AvaloniaObject.cs

@@ -45,21 +45,8 @@ namespace Avalonia
         /// </summary>
         private EventHandler<AvaloniaPropertyChangedEventArgs> _propertyChanged;
 
-        private DeferredSetter<AvaloniaProperty, object> _directDeferredSetter;
         private ValueStore _values;
-
+        private ValueStore Values => _values ?? (_values = new ValueStore(this));
-        /// <summary>
-        /// Delayed setter helper for direct properties. Used to fix #855.
-        /// </summary>
-        private DeferredSetter<AvaloniaProperty, object> DirectPropertyDeferredSetter
-        {
-            get
-            {
-                return _directDeferredSetter ??
-                    (_directDeferredSetter = new DeferredSetter<AvaloniaProperty, object>());
-            }
-        }
-
 
         /// <summary>
         /// Initializes a new instance of the <see cref="AvaloniaObject"/> class.
@@ -223,9 +210,9 @@ namespace Avalonia
             {
                 return ((IDirectPropertyAccessor)GetRegistered(property)).GetValue(this);
             }
-            else if (_values != null)
+            else if (Values != null)
             {
-                var result = _values.GetValue(property);
+                var result = Values.GetValue(property);
 
                 if (result == AvaloniaProperty.UnsetValue)
                 {
@@ -263,7 +250,7 @@ namespace Avalonia
             Contract.Requires<ArgumentNullException>(property != null);
             VerifyAccess();
 
-            return _values?.IsAnimating(property) ?? false;
+            return Values?.IsAnimating(property) ?? false;
         }
 
         /// <summary>
@@ -280,7 +267,7 @@ namespace Avalonia
             Contract.Requires<ArgumentNullException>(property != null);
             VerifyAccess();
 
-            return _values?.IsSet(property) ?? false;
+            return Values?.IsSet(property) ?? false;
         }
 
         /// <summary>
@@ -376,12 +363,7 @@ namespace Avalonia
                     description,
                     priority);
 
-                if (_values == null)
+                return Values.AddBinding(property, source, priority);
-                {
-                    _values = new ValueStore(this);
-                }
-
-                return _values.AddBinding(property, source, priority);
             }
         }
 
@@ -412,7 +394,7 @@ namespace Avalonia
         public void Revalidate(AvaloniaProperty property)
         {
             VerifyAccess();
-            _values?.Revalidate(property);
+            Values?.Revalidate(property);
         }
         
         internal void PriorityValueChanged(AvaloniaProperty property, int priority, object oldValue, object newValue)
@@ -454,7 +436,7 @@ namespace Avalonia
         /// Gets all priority values set on the object.
         /// </summary>
         /// <returns>A collection of property/value tuples.</returns>
-        internal IDictionary<AvaloniaProperty, PriorityValue> GetSetValues() => _values?.GetSetValues();
+        internal IDictionary<AvaloniaProperty, PriorityValue> GetSetValues() => Values?.GetSetValues();
 
         /// <summary>
         /// Forces revalidation of properties when a property value changes.
@@ -564,15 +546,15 @@ namespace Avalonia
             T value)
         {
             Contract.Requires<ArgumentNullException>(setterCallback != null);
-            return _values.Setter.SetAndNotify(
+            return Values.Setter.SetAndNotify(
                 property,
                 ref field,
-                ((object value, int) update, ref T backing, Action<Action> notify) =>
+                (object update, ref T backing, Action<Action> notify) =>
                 {
-                    setterCallback((T)update.value, ref backing, notify);
+                    setterCallback((T)update, ref backing, notify);
                     return true;
                 },
-                (value, 0));
+                value);
         }
 
         /// <summary>
@@ -735,13 +717,8 @@ namespace Avalonia
                     originalValue?.GetType().FullName ?? "(null)"));
             }
 
-            if (_values == null)
-            {
-                _values = new ValueStore(this);
-            }
-
             LogPropertySet(property, value, priority);
-            _values.AddValue(property, value, (int)priority);
+            Values.AddValue(property, value, (int)priority);
         }
 
         /// <summary>

src/Avalonia.Base/IPriorityValueOwner.cs

@@ -33,6 +33,6 @@ namespace Avalonia
         /// </summary>
         void VerifyAccess();
 
-        DeferredSetter<AvaloniaProperty, (object value, int priority)> Setter { get; }
+        DeferredSetter<AvaloniaProperty, object> Setter { get; }
     }
 }

src/Avalonia.Base/PriorityValue.cs

@@ -248,6 +248,12 @@ namespace Avalonia
                 (value, priority));
         }
 
+        private bool UpdateCore(
+            object update,
+            ref (object value, int priority) backing,
+            Action<Action> notify)
+            => UpdateCore(((object, int))update, ref backing, notify);
+
         private bool UpdateCore(
             (object value, int priority) update,
             ref (object value, int priority) backing,
@@ -255,13 +261,14 @@ namespace Avalonia
         {
             var val = update.value;
             var notification = val as BindingNotification;
+            object castValue;
 
             if (notification != null)
             {
                 val = (notification.HasValue) ? notification.Value : null;
             }
 
-            if (TypeUtilities.TryConvertImplicit(_valueType, val, out object castValue))
+            if (TypeUtilities.TryConvertImplicit(_valueType, val, out castValue))
             {
                 var old = backing.value;
 

src/Avalonia.Base/Utilities/DeferredSetter.cs

@@ -8,6 +8,7 @@ namespace Avalonia.Utilities
 {
     /// <summary>
     /// A utility class to enable deferring assignment until after property-changed notifications are sent.
+    /// Used to fix #855.
     /// </summary>
     /// <typeparam name="TProperty">The type of the object that represents the property.</typeparam>
     /// <typeparam name="TSetRecord">The type of value with which to track the delayed assignment.</typeparam>
@@ -37,13 +38,13 @@ namespace Avalonia.Utilities
         {
             public bool Notifying { get; set; }
 
-            private Queue<TSetRecord> pendingValues;
+            private SingleOrQueue<TSetRecord> pendingValues;
             
-            public Queue<TSetRecord> PendingValues
+            public SingleOrQueue<TSetRecord> PendingValues
             {
                 get
                 {
-                    return pendingValues ?? (pendingValues = new Queue<TSetRecord>());
+                    return pendingValues ?? (pendingValues = new SingleOrQueue<TSetRecord>());
                 }
             }
         }
@@ -89,7 +90,7 @@ namespace Avalonia.Utilities
         /// <returns>If the property has any pending assignments.</returns>
         private bool HasPendingSet(TProperty property)
         {
-            return setRecords.TryGetValue(property, out var status) && status.PendingValues.Count != 0;
+            return setRecords.TryGetValue(property, out var status) && !status.PendingValues.Empty;
         }
 
         /// <summary>

src/Avalonia.Base/Utilities/SingleOrQueue.cs

@@ -0,0 +1,54 @@
+using System;
+using System.Collections.Generic;
+using System.Text;
+
+namespace Avalonia.Utilities
+{
+    public class SingleOrQueue<T>
+    {
+        private T _head;
+        private Queue<T> _tail;
+
+        private Queue<T> Tail => _tail ?? (_tail = new Queue<T>());
+
+        private bool HasTail => _tail != null;
+
+        public bool Empty { get; private set; } = true;
+
+        public void Enqueue(T value)
+        {
+            if (Empty)
+            {
+                _head = value;
+            }
+            else
+            {
+                Tail.Enqueue(value);
+            }
+
+            Empty = false;
+        }
+
+        public T Dequeue()
+        {
+            if (Empty)
+            {
+                throw new InvalidOperationException("Cannot dequeue from an empty queue!");
+            }
+
+            var result = _head;
+
+            if (HasTail && Tail.Count != 0)
+            {
+                _head = Tail.Dequeue();
+            }
+            else
+            {
+                _head = default;
+                Empty = true;
+            }
+
+            return result;
+        }
+    }
+}

src/Avalonia.Base/ValueStore.cs

@@ -116,7 +116,7 @@ namespace Avalonia
 
         public bool IsAnimating(AvaloniaProperty property)
         {
-            return _values.TryGetValue(property, out var value) ? (value as PriorityValue)?.IsAnimating ?? false : false;
+            return _values.TryGetValue(property, out var value) && value is PriorityValue priority && priority.IsAnimating;
         }
 
         public bool IsSet(AvaloniaProperty property)
@@ -168,14 +168,14 @@ namespace Avalonia
             return value;
         }
 
-        private DeferredSetter<AvaloniaProperty, (object value, int priority)> _defferedSetter;
+        private DeferredSetter<AvaloniaProperty, object> _defferedSetter;
 
-        public DeferredSetter<AvaloniaProperty, (object value, int priority)> Setter
+        public DeferredSetter<AvaloniaProperty, object> Setter
         {
             get
             {
                 return _defferedSetter ??
-                    (_defferedSetter = new DeferredSetter<AvaloniaProperty, (object value, int priority)>());
+                    (_defferedSetter = new DeferredSetter<AvaloniaProperty, object>());
             }
         }
     }

tests/Avalonia.Base.UnitTests/PriorityValueTests.cs

@@ -1,11 +1,12 @@
 // Copyright (c) The Avalonia Project. All rights reserved.
 // Licensed under the MIT license. See licence.md file in the project root for full license information.
 
+using Avalonia.Utilities;
+using Moq;
 using System;
 using System.Linq;
 using System.Reactive.Linq;
 using System.Reactive.Subjects;
-using Moq;
 using Xunit;
 
 namespace Avalonia.Base.UnitTests
@@ -21,7 +22,7 @@ namespace Avalonia.Base.UnitTests
         [Fact]
         public void Initial_Value_Should_Be_UnsetValue()
         {
-            var target = new PriorityValue(null, TestProperty, typeof(string));
+            var target = new PriorityValue(GetMockOwner().Object, TestProperty, typeof(string));
 
             Assert.Same(AvaloniaProperty.UnsetValue, target.Value);
         }
@@ -29,7 +30,7 @@ namespace Avalonia.Base.UnitTests
         [Fact]
         public void First_Binding_Sets_Value()
         {
-            var target = new PriorityValue(null, TestProperty, typeof(string));
+            var target = new PriorityValue(GetMockOwner().Object, TestProperty, typeof(string));
 
             target.Add(Single("foo"), 0);
 
@@ -39,7 +40,7 @@ namespace Avalonia.Base.UnitTests
         [Fact]
         public void Changing_Binding_Should_Set_Value()
         {
-            var target = new PriorityValue(null, TestProperty, typeof(string));
+            var target = new PriorityValue(GetMockOwner().Object, TestProperty, typeof(string));
             var subject = new BehaviorSubject<string>("foo");
 
             target.Add(subject, 0);
@@ -51,7 +52,7 @@ namespace Avalonia.Base.UnitTests
         [Fact]
         public void Setting_Direct_Value_Should_Override_Binding()
         {
-            var target = new PriorityValue(null, TestProperty, typeof(string));
+            var target = new PriorityValue(GetMockOwner().Object, TestProperty, typeof(string));
 
             target.Add(Single("foo"), 0);
             target.SetValue("bar", 0);
@@ -62,7 +63,7 @@ namespace Avalonia.Base.UnitTests
         [Fact]
         public void Binding_Firing_Should_Override_Direct_Value()
         {
-            var target = new PriorityValue(null, TestProperty, typeof(string));
+            var target = new PriorityValue(GetMockOwner().Object, TestProperty, typeof(string));
             var source = new BehaviorSubject<object>("initial");
 
             target.Add(source, 0);
@@ -76,7 +77,7 @@ namespace Avalonia.Base.UnitTests
         [Fact]
         public void Earlier_Binding_Firing_Should_Not_Override_Later()
         {
-            var target = new PriorityValue(null, TestProperty, typeof(string));
+            var target = new PriorityValue(GetMockOwner().Object, TestProperty, typeof(string));
             var nonActive = new BehaviorSubject<object>("na");
             var source = new BehaviorSubject<object>("initial");
 
@@ -92,7 +93,7 @@ namespace Avalonia.Base.UnitTests
         [Fact]
         public void Binding_Completing_Should_Revert_To_Direct_Value()
         {
-            var target = new PriorityValue(null, TestProperty, typeof(string));
+            var target = new PriorityValue(GetMockOwner().Object, TestProperty, typeof(string));
             var source = new BehaviorSubject<object>("initial");
 
             target.Add(source, 0);
@@ -108,7 +109,7 @@ namespace Avalonia.Base.UnitTests
         [Fact]
         public void Binding_With_Lower_Priority_Has_Precedence()
         {
-            var target = new PriorityValue(null, TestProperty, typeof(string));
+            var target = new PriorityValue(GetMockOwner().Object, TestProperty, typeof(string));
 
             target.Add(Single("foo"), 1);
             target.Add(Single("bar"), 0);
@@ -120,7 +121,7 @@ namespace Avalonia.Base.UnitTests
         [Fact]
         public void Later_Binding_With_Same_Priority_Should_Take_Precedence()
         {
-            var target = new PriorityValue(null, TestProperty, typeof(string));
+            var target = new PriorityValue(GetMockOwner().Object, TestProperty, typeof(string));
 
             target.Add(Single("foo"), 1);
             target.Add(Single("bar"), 0);
@@ -133,7 +134,7 @@ namespace Avalonia.Base.UnitTests
         [Fact]
         public void Changing_Binding_With_Lower_Priority_Should_Set_Not_Value()
         {
-            var target = new PriorityValue(null, TestProperty, typeof(string));
+            var target = new PriorityValue(GetMockOwner().Object, TestProperty, typeof(string));
             var subject = new BehaviorSubject<string>("bar");
 
             target.Add(Single("foo"), 0);
@@ -146,7 +147,7 @@ namespace Avalonia.Base.UnitTests
         [Fact]
         public void UnsetValue_Should_Fall_Back_To_Next_Binding()
         {
-            var target = new PriorityValue(null, TestProperty, typeof(string));
+            var target = new PriorityValue(GetMockOwner().Object, TestProperty, typeof(string));
             var subject = new BehaviorSubject<object>("bar");
 
             target.Add(subject, 0);
@@ -162,7 +163,7 @@ namespace Avalonia.Base.UnitTests
         [Fact]
         public void Adding_Value_Should_Call_OnNext()
         {
-            var owner = new Mock<IPriorityValueOwner>();
+            var owner = GetMockOwner();
             var target = new PriorityValue(owner.Object, TestProperty, typeof(string));
 
             target.Add(Single("foo"), 0);
@@ -173,7 +174,7 @@ namespace Avalonia.Base.UnitTests
         [Fact]
         public void Changing_Value_Should_Call_OnNext()
         {
-            var owner = new Mock<IPriorityValueOwner>();
+            var owner = GetMockOwner();
             var target = new PriorityValue(owner.Object, TestProperty, typeof(string));
             var subject = new BehaviorSubject<object>("foo");
 
@@ -186,7 +187,7 @@ namespace Avalonia.Base.UnitTests
         [Fact]
         public void Disposing_A_Binding_Should_Revert_To_Next_Value()
         {
-            var target = new PriorityValue(null, TestProperty, typeof(string));
+            var target = new PriorityValue(GetMockOwner().Object, TestProperty, typeof(string));
 
             target.Add(Single("foo"), 0);
             var disposable = target.Add(Single("bar"), 0);
@@ -199,7 +200,7 @@ namespace Avalonia.Base.UnitTests
         [Fact]
         public void Disposing_A_Binding_Should_Remove_BindingEntry()
         {
-            var target = new PriorityValue(null, TestProperty, typeof(string));
+            var target = new PriorityValue(GetMockOwner().Object, TestProperty, typeof(string));
 
             target.Add(Single("foo"), 0);
             var disposable = target.Add(Single("bar"), 0);
@@ -212,7 +213,7 @@ namespace Avalonia.Base.UnitTests
         [Fact]
         public void Completing_A_Binding_Should_Revert_To_Previous_Binding()
         {
-            var target = new PriorityValue(null, TestProperty, typeof(string));
+            var target = new PriorityValue(GetMockOwner().Object, TestProperty, typeof(string));
             var source = new BehaviorSubject<object>("bar");
 
             target.Add(Single("foo"), 0);
@@ -226,7 +227,7 @@ namespace Avalonia.Base.UnitTests
         [Fact]
         public void Completing_A_Binding_Should_Revert_To_Lower_Priority()
         {
-            var target = new PriorityValue(null, TestProperty, typeof(string));
+            var target = new PriorityValue(GetMockOwner().Object, TestProperty, typeof(string));
             var source = new BehaviorSubject<object>("bar");
 
             target.Add(Single("foo"), 1);
@@ -240,7 +241,7 @@ namespace Avalonia.Base.UnitTests
         [Fact]
         public void Completing_A_Binding_Should_Remove_BindingEntry()
         {
-            var target = new PriorityValue(null, TestProperty, typeof(string));
+            var target = new PriorityValue(GetMockOwner().Object, TestProperty, typeof(string));
             var subject = new BehaviorSubject<object>("bar");
 
             target.Add(Single("foo"), 0);
@@ -254,7 +255,7 @@ namespace Avalonia.Base.UnitTests
         [Fact]
         public void Direct_Value_Should_Be_Coerced()
         {
-            var target = new PriorityValue(null, TestProperty, typeof(int), x => Math.Min((int)x, 10));
+            var target = new PriorityValue(GetMockOwner().Object, TestProperty, typeof(int), x => Math.Min((int)x, 10));
 
             target.SetValue(5, 0);
             Assert.Equal(5, target.Value);
@@ -265,7 +266,7 @@ namespace Avalonia.Base.UnitTests
         [Fact]
         public void Bound_Value_Should_Be_Coerced()
         {
-            var target = new PriorityValue(null, TestProperty, typeof(int), x => Math.Min((int)x, 10));
+            var target = new PriorityValue(GetMockOwner().Object, TestProperty, typeof(int), x => Math.Min((int)x, 10));
             var source = new Subject<object>();
 
             target.Add(source, 0);
@@ -279,7 +280,7 @@ namespace Avalonia.Base.UnitTests
         public void Revalidate_Should_ReCoerce_Value()
         {
             var max = 10;
-            var target = new PriorityValue(null, TestProperty, typeof(int), x => Math.Min((int)x, max));
+            var target = new PriorityValue(GetMockOwner().Object, TestProperty, typeof(int), x => Math.Min((int)x, max));
             var source = new Subject<object>();
 
             target.Add(source, 0);
@@ -302,5 +303,12 @@ namespace Avalonia.Base.UnitTests
         {
             return Observable.Never<T>().StartWith(value);
         }
+
+        private static Mock<IPriorityValueOwner> GetMockOwner()
+        {
+            var owner = new Mock<IPriorityValueOwner>();
+            owner.SetupGet(o => o.Setter).Returns(new DeferredSetter<AvaloniaProperty, object>());
+            return owner;
+        }
     }
 }

tests/Avalonia.Base.UnitTests/Utilities/SingleOrQueueTests.cs

@@ -0,0 +1,50 @@
+using Avalonia.Utilities;
+using System;
+using System.Collections.Generic;
+using System.Text;
+using Xunit;
+
+namespace Avalonia.Base.UnitTests.Utilities
+{
+    public class SingleOrQueueTests
+    {
+        [Fact]
+        public void New_SingleOrQueue_Is_Empty()
+        {
+            Assert.True(new SingleOrQueue<object>().Empty);
+        }
+
+        [Fact]
+        public void Dequeue_Throws_When_Empty()
+        {
+            var queue = new SingleOrQueue<object>();
+
+            Assert.Throws<InvalidOperationException>(() => queue.Dequeue());
+        }
+
+        [Fact]
+        public void Enqueue_Adds_Element()
+        {
+            var queue = new SingleOrQueue<int>();
+
+            queue.Enqueue(1);
+
+            Assert.False(queue.Empty);
+
+            Assert.Equal(1, queue.Dequeue());
+        }
+
+        [Fact]
+        public void Multiple_Elements_Dequeued_In_Correct_Order()
+        {
+            var queue = new SingleOrQueue<int>();
+
+            queue.Enqueue(1);
+            queue.Enqueue(2);
+            queue.Enqueue(3);
+            Assert.Equal(1, queue.Dequeue());
+            Assert.Equal(2, queue.Dequeue());
+            Assert.Equal(3, queue.Dequeue());
+        }
+    }
+}