Use WeakReference in BindingNotification.

So that it doesn't keep objects alive when cached by
`.Publish().RefCount()` in `ExpressionObserver`. Added a leak test to
test that.

src/Avalonia.Base/AvaloniaObject.cs

@@ -577,7 +577,7 @@ namespace Avalonia
             {
                 if (notification.HasValue)
                 {
-                    notification.Value = TypeUtilities.CastOrDefault(notification.Value, type);
+                    notification.SetValue(TypeUtilities.CastOrDefault(notification.Value, type));
                 }
 
                 return notification;

src/Avalonia.Base/Data/BindingNotification.cs

@@ -44,14 +44,19 @@ namespace Avalonia.Data
         public static readonly BindingNotification UnsetValue =
             new BindingNotification(AvaloniaProperty.UnsetValue);
 
+        // Null cannot be held in WeakReference as it's indistinguishable from an expired value so
+        // use this value in its place.
+        private static readonly object NullValue = new object();
+
+        private WeakReference<object> _value;
+
         /// <summary>
         /// Initializes a new instance of the <see cref="BindingNotification"/> class.
         /// </summary>
         /// <param name="value">The binding value.</param>
         public BindingNotification(object value)
         {
-            Value = value;
-            HasValue = true;
+            _value = new WeakReference<object>(value ?? NullValue);
         }
 
         /// <summary>
@@ -66,7 +71,6 @@ namespace Avalonia.Data
                 throw new ArgumentException($"'errorType' may not be None");
             }
 
-            Value = AvaloniaProperty.UnsetValue;
             Error = error;
             ErrorType = errorType;
         }
@@ -80,20 +84,42 @@ namespace Avalonia.Data
         public BindingNotification(Exception error, BindingErrorType errorType, object fallbackValue)
             : this(error, errorType)
         {
-            Value = fallbackValue;
-            HasValue = true;
+            _value = new WeakReference<object>(fallbackValue ?? NullValue);
         }
 
         /// <summary>
         /// Gets the value that should be passed to the target when <see cref="HasValue"/>
         /// is true.
         /// </summary>
-        public object Value { get; set; }
+        /// <remarks>
+        /// If this property is read when <see cref="HasValue"/> is false then it will return
+        /// <see cref="AvaloniaProperty.UnsetValue"/>.
+        /// </remarks>
+        public object Value
+        {
+            get
+            {
+                if (_value != null)
+                {
+                    object result;
+
+                    if (_value.TryGetTarget(out result))
+                    {
+                        return result == NullValue ? null : result;
+                    }
+                }
+
+                // There's the possibility of a race condition in that HasValue can return true,
+                // and then the value is GC'd before Value is read. We should be ok though as
+                // we return UnsetValue which should be a safe alternative.
+                return AvaloniaProperty.UnsetValue;
+            }
+        }
 
         /// <summary>
         /// Gets a value indicating whether <see cref="Value"/> should be pushed to the target.
         /// </summary>
-        public bool HasValue { get; set; }
+        public bool HasValue => _value != null;
 
         /// <summary>
         /// Gets the error that occurred on the source, if any.
@@ -179,14 +205,7 @@ namespace Avalonia.Data
             Contract.Requires<ArgumentNullException>(e != null);
             Contract.Requires<ArgumentException>(type != BindingErrorType.None);
 
-            if (Error != null)
-            {
-                Error = new AggregateException(Error, e);
-            }
-            else
-            {
-                Error = e;
-            }
+            Error = Error != null ? new AggregateException(Error, e) : e;
 
             if (type == BindingErrorType.Error || ErrorType == BindingErrorType.Error)
             {
@@ -194,10 +213,26 @@ namespace Avalonia.Data
             }
         }
 
+        /// <summary>
+        /// Removes the <see cref="Value"/> and makes <see cref="HasValue"/> return null.
+        /// </summary>
+        public void ClearValue()
+        {
+            _value = null;
+        }
+
+        /// <summary>
+        /// Sets the <see cref="Value"/>.
+        /// </summary>
+        public void SetValue(object value)
+        {
+            _value = new WeakReference<object>(value ?? NullValue);
+        }
+
         private static bool ExceptionEquals(Exception a, Exception b)
         {
             return a?.GetType() == b?.GetType() &&
-                   a.Message == b.Message;
+                   a?.Message == b?.Message;
         }
     }
 }

src/Avalonia.Base/PriorityValue.cs

@@ -259,7 +259,7 @@ namespace Avalonia
 
                 if (notification?.HasValue == true)
                 {
-                    notification.Value = castValue;
+                    notification.SetValue(castValue);
                 }
 
                 if (notification == null || notification.HasValue)

src/Markup/Avalonia.Markup/Data/ExpressionSubject.cs

@@ -255,7 +255,7 @@ namespace Avalonia.Markup.Data
             }
         }
 
-        private BindingNotification Merge(object a, BindingNotification b)
+        private static BindingNotification Merge(object a, BindingNotification b)
         {
             var an = a as BindingNotification;
 
@@ -270,7 +270,7 @@ namespace Avalonia.Markup.Data
             }
         }
 
-        private BindingNotification Merge(BindingNotification a, object b)
+        private static BindingNotification Merge(BindingNotification a, object b)
         {
             var bn = b as BindingNotification;
 
@@ -280,17 +280,22 @@ namespace Avalonia.Markup.Data
             }
             else
             {
-                a.Value = b;
-                a.HasValue = true;
+                a.SetValue(b);
             }
 
             return a;
         }
 
-        private BindingNotification Merge(BindingNotification a, BindingNotification b)
+        private static BindingNotification Merge(BindingNotification a, BindingNotification b)
         {
-            a.Value = b.Value;
-            a.HasValue = b.HasValue;
+            if (b.HasValue)
+            {
+                a.SetValue(b.Value);
+            }
+            else
+            {
+                a.ClearValue();
+            }
 
             if (b.Error != null)
             {

tests/Avalonia.LeakTests/ExpressionObserverTests.cs

@@ -35,6 +35,24 @@ namespace Avalonia.LeakTests
                 Assert.Equal(0, memory.GetObjects(where => where.Type.Is<AvaloniaList<string>>()).ObjectsCount));
         }
 
+        [Fact]
+        public void Should_Not_Keep_Source_Alive_ObservableCollection_With_DataValidation()
+        {
+            Func<ExpressionObserver> run = () =>
+            {
+                var source = new { Foo = new AvaloniaList<string> { "foo", "bar" } };
+                var target = new ExpressionObserver(source, "Foo", true);
+
+                target.Subscribe(_ => { });
+                return target;
+            };
+
+            var result = run();
+
+            dotMemory.Check(memory =>
+                Assert.Equal(0, memory.GetObjects(where => where.Type.Is<AvaloniaList<string>>()).ObjectsCount));
+        }
+
         [Fact]
         public void Should_Not_Keep_Source_Alive_NonIntegerIndexer()
         {

tests/Avalonia.Markup.Xaml.UnitTests/Data/BindingTests.cs

@@ -166,15 +166,16 @@ namespace Avalonia.Markup.Xaml.UnitTests.Data
         [Fact]
         public void DataContext_Binding_Should_Produce_Correct_Results()
         {
+            var viewModel = new { Foo = "bar" };
             var root = new Decorator
             {
-                DataContext = new { Foo = "bar" },
+                DataContext = viewModel,
             };
 
             var child = new Control();
             var values = new List<object>();
 
-            child.GetObservable(Border.DataContextProperty).Subscribe(x => values.Add(x));
+            child.GetObservable(Control.DataContextProperty).Subscribe(x => values.Add(x));
             child.Bind(Control.DataContextProperty, new Binding("Foo"));
 
             // When binding to DataContext and the target isn't found, the binding should produce