Batching for AvaloniaObject property values.

src/Avalonia.Base/AvaloniaObject.cs

@@ -23,7 +23,7 @@ public class AvaloniaObject : IAvaloniaObject, IAvaloniaObjectDebug, INotifyProp
         private EventHandler<AvaloniaPropertyChangedEventArgs> _propertyChanged;
         private List<IAvaloniaObject> _inheritanceChildren;
         private ValueStore _values;
-        private ValueStore Values => _values ?? (_values = new ValueStore(this));
+        private bool _batchUpdate;
 
         /// <summary>
         /// Initializes a new instance of the <see cref="AvaloniaObject"/> class.
@@ -117,6 +117,22 @@ public IBinding this[IndexerDescriptor binding]
             set { this.Bind(binding.Property, value); }
         }
 
+        private ValueStore Values
+        {
+            get
+            {
+                if (_values is null)
+                {
+                    _values = new ValueStore(this);
+
+                    if (_batchUpdate)
+                        _values.BeginBatchUpdate();
+                }
+
+                return _values;
+            }
+        }
+
         public bool CheckAccess() => Dispatcher.UIThread.CheckAccess();
 
         public void VerifyAccess() => Dispatcher.UIThread.VerifyAccess();
@@ -434,6 +450,28 @@ public void CoerceValue<T>(StyledPropertyBase<T> property)
             _values?.CoerceValue(property);
         }
 
+        public void BeginBatchUpdate()
+        {
+            if (_batchUpdate)
+            {
+                throw new InvalidOperationException("Batch update already in progress.");
+            }
+
+            _batchUpdate = true;
+            _values?.BeginBatchUpdate();
+        }
+
+        public void EndBatchUpdate()
+        {
+            if (!_batchUpdate)
+            {
+                throw new InvalidOperationException("No batch update in progress.");
+            }
+
+            _batchUpdate = false;
+            _values?.EndBatchUpdate();
+        }
+
         /// <inheritdoc/>
         void IAvaloniaObject.AddInheritanceChild(IAvaloniaObject child)
         {

src/Avalonia.Base/PropertyStore/BindingEntry.cs

@@ -9,8 +9,9 @@ namespace Avalonia.PropertyStore
     /// <summary>
     /// Represents an untyped interface to <see cref="BindingEntry{T}"/>.
     /// </summary>
-    internal interface IBindingEntry : IPriorityValueEntry, IDisposable
+    internal interface IBindingEntry : IBatchUpdate, IPriorityValueEntry, IDisposable
     {
+        void Start(bool ignoreBatchUpdate);
     }
 
     /// <summary>
@@ -22,6 +23,8 @@ internal class BindingEntry<T> : IBindingEntry, IPriorityValueEntry<T>, IObserve
         private readonly IAvaloniaObject _owner;
         private IValueSink _sink;
         private IDisposable? _subscription;
+        private bool _isSubscribed;
+        private bool _batchUpdate;
         private Optional<T> _value;
 
         public BindingEntry(
@@ -39,10 +42,20 @@ public BindingEntry(
         }
 
         public StyledPropertyBase<T> Property { get; }
-        public BindingPriority Priority { get; }
+        public BindingPriority Priority { get; private set; }
         public IObservable<BindingValue<T>> Source { get; }
         Optional<object> IValue.GetValue() => _value.ToObject();
 
+        public void BeginBatchUpdate() => _batchUpdate = true;
+
+        public void EndBatchUpdate()
+        {
+            _batchUpdate = false;
+
+            if (_sink is ValueStore)
+                Start();
+        }
+
         public Optional<T> GetValue(BindingPriority maxPriority)
         {
             return Priority >= maxPriority ? _value : Optional<T>.Empty;
@@ -52,10 +65,17 @@ public void Dispose()
         {
             _subscription?.Dispose();
             _subscription = null;
-            _sink.Completed(Property, this, _value);
+            _isSubscribed = false;
+            OnCompleted();
         }
 
-        public void OnCompleted() => _sink.Completed(Property, this, _value);
+        public void OnCompleted()
+        {
+            var oldValue = _value;
+            _value = default;
+            Priority = BindingPriority.Unset;
+            _sink.Completed(Property, this, oldValue);
+        }
 
         public void OnError(Exception error)
         {
@@ -79,13 +99,36 @@ public void OnNext(BindingValue<T> value)
             }
         }
 
-        public void Start()
+        public void Start() => Start(false);
+
+        public void Start(bool ignoreBatchUpdate)
         {
-            _subscription = Source.Subscribe(this);
+            // We can't use _subscription to check whether we're subscribed because it won't be set
+            // until Subscribe has finished, which will be too late to prevent reentrancy.
+            if (!_isSubscribed && (!_batchUpdate || ignoreBatchUpdate))
+            {
+                _isSubscribed = true;
+                _subscription = Source.Subscribe(this);
+            }
         }
 
         public void Reparent(IValueSink sink) => _sink = sink;
-
+
+        public void RaiseValueChanged(
+            IValueSink sink,
+            IAvaloniaObject owner,
+            AvaloniaProperty property,
+            Optional<object> oldValue,
+            Optional<object> newValue)
+        {
+            sink.ValueChanged(new AvaloniaPropertyChangedEventArgs<T>(
+                owner,
+                (AvaloniaProperty<T>)property,
+                oldValue.GetValueOrDefault<T>(),
+                newValue.GetValueOrDefault<T>(),
+                Priority));
+        }
+
         private void UpdateValue(BindingValue<T> value)
         {
             if (value.HasValue && Property.ValidateValue?.Invoke(value.Value) == false)

src/Avalonia.Base/PropertyStore/ConstantValueEntry.cs

@@ -1,4 +1,5 @@
 using System;
+using System.Diagnostics.CodeAnalysis;
 using Avalonia.Data;
 
 #nullable enable
@@ -17,7 +18,7 @@ internal class ConstantValueEntry<T> : IPriorityValueEntry<T>, IDisposable
 
         public ConstantValueEntry(
             StyledPropertyBase<T> property,
-            T value,
+            [AllowNull] T value,
             BindingPriority priority,
             IValueSink sink)
         {
@@ -28,15 +29,38 @@ public ConstantValueEntry(
         }
 
         public StyledPropertyBase<T> Property { get; }
-        public BindingPriority Priority { get; }
+        public BindingPriority Priority { get; private set; }
         Optional<object> IValue.GetValue() => _value.ToObject();
 
         public Optional<T> GetValue(BindingPriority maxPriority = BindingPriority.Animation)
         {
             return Priority >= maxPriority ? _value : Optional<T>.Empty;
         }
 
-        public void Dispose() => _sink.Completed(Property, this, _value);
+        public void Dispose()
+        {
+            var oldValue = _value;
+            _value = default;
+            Priority = BindingPriority.Unset;
+            _sink.Completed(Property, this, oldValue);
+        }
+
         public void Reparent(IValueSink sink) => _sink = sink;
+        public void Start() { }
+
+        public void RaiseValueChanged(
+            IValueSink sink,
+            IAvaloniaObject owner,
+            AvaloniaProperty property,
+            Optional<object> oldValue,
+            Optional<object> newValue)
+        {
+            sink.ValueChanged(new AvaloniaPropertyChangedEventArgs<T>(
+                owner,
+                (AvaloniaProperty<T>)property,
+                oldValue.GetValueOrDefault<T>(),
+                newValue.GetValueOrDefault<T>(),
+                Priority));
+        }
     }
 }

src/Avalonia.Base/PropertyStore/IBatchUpdate.cs

@@ -0,0 +1,8 @@
+namespace Avalonia.PropertyStore
+{
+    internal interface IBatchUpdate
+    {
+        void BeginBatchUpdate();
+        void EndBatchUpdate();
+    }
+}

src/Avalonia.Base/PropertyStore/IValue.cs

@@ -9,8 +9,15 @@ namespace Avalonia.PropertyStore
     /// </summary>
     internal interface IValue
     {
-        Optional<object> GetValue();
         BindingPriority Priority { get; }
+        Optional<object> GetValue();
+        void Start();
+        void RaiseValueChanged(
+            IValueSink sink,
+            IAvaloniaObject owner,
+            AvaloniaProperty property,
+            Optional<object> oldValue,
+            Optional<object> newValue);
     }
 
     /// <summary>

src/Avalonia.Base/PropertyStore/LocalValueEntry.cs

@@ -24,5 +24,21 @@ public Optional<T> GetValue(BindingPriority maxPriority)
         }
 
         public void SetValue(T value) => _value = value;
+        public void Start() { }
+
+        public void RaiseValueChanged(
+            IValueSink sink,
+            IAvaloniaObject owner,
+            AvaloniaProperty property,
+            Optional<object> oldValue,
+            Optional<object> newValue)
+        {
+            sink.ValueChanged(new AvaloniaPropertyChangedEventArgs<T>(
+                owner,
+                (AvaloniaProperty<T>)property,
+                oldValue.GetValueOrDefault<T>(),
+                newValue.GetValueOrDefault<T>(),
+                BindingPriority.LocalValue));
+        }
     }
 }