Implement batching for AvaloniaObject property values.

Cuts down the amount of notifications raised when controls' stying is attached/detached.

Part of fixing #5027.

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(
@@ -43,6 +46,16 @@ public BindingEntry(
         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;
@@ -52,6 +65,7 @@ public void Dispose()
         {
             _subscription?.Dispose();
             _subscription = null;
+            _isSubscribed = false;
             _sink.Completed(Property, this, _value);
         }
 
@@ -79,13 +93,35 @@ 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)
+        {
+            sink.ValueChanged(new AvaloniaPropertyChangedEventArgs<T>(
+                owner,
+                (AvaloniaProperty<T>)property,
+                oldValue.GetValueOrDefault<T>(),
+                _value,
+                Priority));
+        }
+
         private void UpdateValue(BindingValue<T> value)
         {
             if (value.HasValue && Property.ValidateValue?.Invoke(value.Value) == false)

src/Avalonia.Base/PropertyStore/ConstantValueEntry.cs

@@ -38,5 +38,20 @@ public Optional<T> GetValue(BindingPriority maxPriority = BindingPriority.Animat
 
         public void Dispose() => _sink.Completed(Property, this, _value);
         public void Reparent(IValueSink sink) => _sink = sink;
+        public void Start() { }
+
+        public void RaiseValueChanged(
+            IValueSink sink,
+            IAvaloniaObject owner,
+            AvaloniaProperty property,
+            Optional<object> oldValue)
+        {
+            sink.ValueChanged(new AvaloniaPropertyChangedEventArgs<T>(
+                owner,
+                (AvaloniaProperty<T>)property,
+                oldValue.GetValueOrDefault<T>(),
+                _value,
+                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 +9,14 @@ 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);
     }
 
     /// <summary>

src/Avalonia.Base/PropertyStore/LocalValueEntry.cs

@@ -24,5 +24,20 @@ 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)
+        {
+            sink.ValueChanged(new AvaloniaPropertyChangedEventArgs<T>(
+                owner,
+                (AvaloniaProperty<T>)property,
+                oldValue.GetValueOrDefault<T>(),
+                _value,
+                BindingPriority.LocalValue));
+        }
     }
 }

src/Avalonia.Base/PropertyStore/PriorityValue.cs

@@ -18,14 +18,16 @@ namespace Avalonia.PropertyStore
     /// <see cref="IPriorityValueEntry{T}"/> entries (sorted first by priority and then in the order
     /// they were added) plus a local value.
     /// </remarks>
-    internal class PriorityValue<T> : IValue<T>, IValueSink
+    internal class PriorityValue<T> : IValue<T>, IValueSink, IBatchUpdate
     {
         private readonly IAvaloniaObject _owner;
         private readonly IValueSink _sink;
         private readonly List<IPriorityValueEntry<T>> _entries = new List<IPriorityValueEntry<T>>();
         private readonly Func<IAvaloniaObject, T, T>? _coerceValue;
         private Optional<T> _localValue;
         private Optional<T> _value;
+        private bool _isCalculatingValue;
+        private bool _batchUpdate;
 
         public PriorityValue(
             IAvaloniaObject owner,
@@ -53,6 +55,18 @@ public PriorityValue(
             existing.Reparent(this);
             _entries.Add(existing);
 
+            if (existing is IBindingEntry binding &&
+                existing.Priority == BindingPriority.LocalValue)
+            {
+                // Bit of a special case here: if we have a local value binding that is being
+                // promoted to a priority value we need to make sure the binding is subscribed
+                // even if we've got a batch operation in progress because otherwise we don't know
+                // whether the binding or a subsequent SetValue with local priority will win. A
+                // notification won't be sent during batch update anyway because it will be
+                // caught and stored for later by the ValueStore.
+                binding.Start(ignoreBatchUpdate: true);
+            }
+
             var v = existing.GetValue();
 
             if (v.HasValue)
@@ -78,6 +92,28 @@ public PriorityValue(
         public IReadOnlyList<IPriorityValueEntry<T>> Entries => _entries;
         Optional<object> IValue.GetValue() => _value.ToObject();
 
+        public void BeginBatchUpdate()
+        {
+            _batchUpdate = true;
+
+            foreach (var entry in _entries)
+            {
+                (entry as IBatchUpdate)?.BeginBatchUpdate();
+            }
+        }
+
+        public void EndBatchUpdate()
+        {
+            _batchUpdate = false;
+
+            foreach (var entry in _entries)
+            {
+                (entry as IBatchUpdate)?.EndBatchUpdate();
+            }
+
+            UpdateEffectiveValue(null);
+        }
+
         public void ClearLocalValue()
         {
             UpdateEffectiveValue(new AvaloniaPropertyChangedEventArgs<T>(
@@ -137,7 +173,22 @@ public BindingEntry<T> AddBinding(IObservable<BindingValue<T>> source, BindingPr
             return binding;
         }
 
-        public void CoerceValue() => UpdateEffectiveValue(null);
+        public void UpdateEffectiveValue() => UpdateEffectiveValue(null);
+        public void Start() => UpdateEffectiveValue(null);
+
+        public void RaiseValueChanged(
+            IValueSink sink,
+            IAvaloniaObject owner,
+            AvaloniaProperty property,
+            Optional<object> oldValue)
+        {
+            sink.ValueChanged(new AvaloniaPropertyChangedEventArgs<T>(
+                owner,
+                (AvaloniaProperty<T>)property,
+                oldValue.GetValueOrDefault<T>(),
+                _value,
+                Priority));
+        }
 
         void IValueSink.ValueChanged<TValue>(AvaloniaPropertyChangedEventArgs<TValue> change)
         {
@@ -146,7 +197,7 @@ void IValueSink.ValueChanged<TValue>(AvaloniaPropertyChangedEventArgs<TValue> ch
                 _localValue = default;
             }
 
-            if (change is AvaloniaPropertyChangedEventArgs<T> c)
+            if (!_isCalculatingValue && change is AvaloniaPropertyChangedEventArgs<T> c)
             {
                 UpdateEffectiveValue(c);
             }
@@ -188,41 +239,47 @@ private int FindInsertPoint(BindingPriority priority)
 
         public (Optional<T>, BindingPriority) CalculateValue(BindingPriority maxPriority)
         {
-            var reachedLocalValues = false;
+            _isCalculatingValue = true;
 
-            for (var i = _entries.Count - 1; i >= 0; --i)
+            try
             {
-                var entry = _entries[i];
-
-                if (entry.Priority < maxPriority)
+                for (var i = _entries.Count - 1; i >= 0; --i)
                 {
-                    continue;
+                    var entry = _entries[i];
+
+                    if (entry.Priority < maxPriority)
+                    {
+                        continue;
+                    }
+
+                    entry.Start();
+
+                    if (entry.Priority >= BindingPriority.LocalValue &&
+                        maxPriority <= BindingPriority.LocalValue &&
+                        _localValue.HasValue)
+                    {
+                        return (_localValue, BindingPriority.LocalValue);
+                    }
+
+                    var entryValue = entry.GetValue();
+
+                    if (entryValue.HasValue)
+                    {
+                        return (entryValue, entry.Priority);
+                    }
                 }
 
-                if (!reachedLocalValues &&
-                    entry.Priority >= BindingPriority.LocalValue &&
-                    maxPriority <= BindingPriority.LocalValue &&
-                    _localValue.HasValue)
+                if (maxPriority <= BindingPriority.LocalValue && _localValue.HasValue)
                 {
                     return (_localValue, BindingPriority.LocalValue);
                 }
 
-                var entryValue = entry.GetValue();
-
-                if (entryValue.HasValue)
-                {
-                    return (entryValue, entry.Priority);
-                }
+                return (default, BindingPriority.Unset);
             }
-
-            if (!reachedLocalValues &&
-                maxPriority <= BindingPriority.LocalValue &&
-                _localValue.HasValue)
+            finally
             {
-                return (_localValue, BindingPriority.LocalValue);
+                _isCalculatingValue = false;
             }
-
-            return (default, BindingPriority.Unset);
         }
 
         private void UpdateEffectiveValue(AvaloniaPropertyChangedEventArgs<T>? change)