Refactored style activators.

- Always evaluate the active state from current information, don't rely on subscriptions to fire as the current state may not be up-to-date
- Don't notify the `IStyleActivatorSink` of a change immediately on subscription

src/Avalonia.Base/Styling/Activators/AndActivator.cs

@@ -1,6 +1,4 @@
-#nullable enable
+using System.Collections.Generic;
-
-using System.Collections.Generic;
 
 namespace Avalonia.Styling.Activators
 {
@@ -11,49 +9,35 @@ namespace Avalonia.Styling.Activators
     internal class AndActivator : StyleActivatorBase, IStyleActivatorSink
     {
         private List<IStyleActivator>? _sources;
-        private ulong _flags;
-        private ulong _mask;
 
         public int Count => _sources?.Count ?? 0;
 
-        public override bool IsActive
-        {
-            get
-            {
-                if (_sources is null)
-                    return false;
-
-                foreach (var source in _sources)
-                {
-                    if (!source.IsActive)
-                        return false;
-                }
-
-                return true;
-            }
-        }
-
         public void Add(IStyleActivator activator)
         {
+            if (IsSubscribed)
+                throw new AvaloniaInternalException("AndActivator is already subscribed.");
             _sources ??= new List<IStyleActivator>();
             _sources.Add(activator);
         }
 
-        void IStyleActivatorSink.OnNext(bool value, int tag)
+        void IStyleActivatorSink.OnNext(bool value, int tag) => ReevaluateIsActive();
+
+        protected override bool EvaluateIsActive()
         {
-            if (value)
+            if (_sources is null || _sources.Count == 0)
-            {
+                return true;
-                _flags |= 1ul << tag;
+
-            }
+            var count = _sources.Count;
-            else
+            var mask = (1ul << count) - 1;
-            {
+            var flags = 0UL;
-                _flags &= ~(1ul << tag);
-            }
 
-            if (_mask != 0)
+            for (var i = 0; i < count; ++i)
             {
-                PublishNext(_flags == _mask);
+                if (_sources[i].IsActive)
+                    flags |= 1ul << i;
             }
+
+            return flags == mask;
         }
 
         protected override void Initialize()
@@ -66,9 +50,6 @@ namespace Avalonia.Styling.Activators
                 {
                     source.Subscribe(this, i++);
                 }
-
-                _mask = (1ul << Count) - 1;
-                PublishNext(_flags == _mask);
             }
         }
 
@@ -81,8 +62,6 @@ namespace Avalonia.Styling.Activators
                     source.Unsubscribe(this);
                 }
             }
-
-            _mask = 0;
         }
     }
 }

src/Avalonia.Base/Styling/Activators/IStyleActivator.cs

@@ -21,13 +21,27 @@ namespace Avalonia.Styling.Activators
         /// <summary>
         /// Gets a value indicating whether the style is activated.
         /// </summary>
+        /// <remarks>
+        /// This property should read directly from its inputs and not rely on any subscriptions 
+        /// to fire in order to be up-to-date. If a change in active state occurs when reading
+        /// this property then any subscribed <see cref="IStyleActivatorSink"/> should not be
+        /// notified of the change.
+        /// </remarks>
         bool IsActive { get; }
 
+        /// <summary>
+        /// Gets a value indicating whether the style is subscribed.
+        /// </summary>
+        bool IsSubscribed { get; }
+
         /// <summary>
         /// Subscribes to the activator.
         /// </summary>
         /// <param name="sink">The listener.</param>
         /// <param name="tag">An optional tag.</param>
+        /// <remarks>
+        /// This method should not call <see cref="IStyleActivatorSink.OnNext(bool, int)"/>.
+        /// </remarks>
         void Subscribe(IStyleActivatorSink sink, int tag = 0);
 
         /// <summary>

src/Avalonia.Base/Styling/Activators/NotActivator.cs

@@ -1,6 +1,4 @@
-#nullable enable
+namespace Avalonia.Styling.Activators
-
-namespace Avalonia.Styling.Activators
 {
     /// <summary>
     /// An <see cref="IStyleActivator"/> which inverts the state of an input activator.
@@ -9,8 +7,8 @@ namespace Avalonia.Styling.Activators
     {
         private readonly IStyleActivator _source;
         public NotActivator(IStyleActivator source) => _source = source;
-        public override bool IsActive => !_source.IsActive;
+        void IStyleActivatorSink.OnNext(bool value, int tag) => ReevaluateIsActive();
-        void IStyleActivatorSink.OnNext(bool value, int tag) => PublishNext(!value);
+        protected override bool EvaluateIsActive() => !_source.IsActive;
         protected override void Initialize() => _source.Subscribe(this, 0);
         protected override void Deinitialize() => _source.Unsubscribe(this);
     }

src/Avalonia.Base/Styling/Activators/NthChildActivator.cs

@@ -26,30 +26,25 @@ namespace Avalonia.Styling.Activators
             _reversed = reversed;
         }
 
-        public override bool IsActive => NthChildSelector.Evaluate(_control, _provider, _step, _offset, _reversed).IsMatch;
+        protected override bool EvaluateIsActive()
-
-        protected override void Initialize()
         {
-            PublishNext(IsActive);
+            return NthChildSelector.Evaluate(_control, _provider, _step, _offset, _reversed).IsMatch;
-            _provider.ChildIndexChanged += ChildIndexChanged;
         }
 
-        protected override void Deinitialize()
+        protected override void Initialize() => _provider.ChildIndexChanged += ChildIndexChanged;
-        {
+        protected override void Deinitialize() => _provider.ChildIndexChanged -= ChildIndexChanged;
-            _provider.ChildIndexChanged -= ChildIndexChanged;
-        }
 
         private void ChildIndexChanged(object? sender, ChildIndexChangedEventArgs e)
         {
             // Run matching again if:
             // 1. Selector is reversed, so other item insertion/deletion might affect total count without changing subscribed item index.
-            // 2. e.Child is null, when all children indeces were changed.
+            // 2. e.Child is null, when all children indices were changed.
             // 3. Subscribed child index was changed.
             if (_reversed
-                || e.Child is null                
+                || e.Child is null
                 || e.Child == _control)
             {
-                PublishNext(IsActive);
+                ReevaluateIsActive();
             }
         }
     }

src/Avalonia.Base/Styling/Activators/OrActivator.cs

@@ -1,6 +1,4 @@
-#nullable enable
+using System.Collections.Generic;
-
-using System.Collections.Generic;
 
 namespace Avalonia.Styling.Activators
 {
@@ -11,49 +9,29 @@ namespace Avalonia.Styling.Activators
     internal class OrActivator : StyleActivatorBase, IStyleActivatorSink
     {
         private List<IStyleActivator>? _sources;
-        private ulong _flags;
-        private bool _initializing;
 
         public int Count => _sources?.Count ?? 0;
 
-        public override bool IsActive
-        {
-            get
-            {
-                if (_sources is null)
-                    return false;
-
-                foreach (var source in _sources)
-                {
-                    if (source.IsActive)
-                        return true;
-                }
-
-                return false;
-            }
-        }
-
         public void Add(IStyleActivator activator)
         {
             _sources ??= new List<IStyleActivator>();
             _sources.Add(activator);
         }
 
-        void IStyleActivatorSink.OnNext(bool value, int tag)
+        void IStyleActivatorSink.OnNext(bool value, int tag) => ReevaluateIsActive();
+
+        protected override bool EvaluateIsActive()
         {
-            if (value)
+            if (_sources is null || _sources.Count == 0)
-            {
+                return true;
-                _flags |= 1ul << tag;
-            }
-            else
-            {
-                _flags &= ~(1ul << tag);
-            }
 
-            if (!_initializing)
+            foreach (var source in _sources)
             {
-                PublishNext(_flags != 0);
+                if (source.IsActive)
+                    return true;
             }
+
+            return false;
         }
 
         protected override void Initialize()
@@ -62,15 +40,10 @@ namespace Avalonia.Styling.Activators
             {
                 var i = 0;
 
-                _initializing = true;
-
                 foreach (var source in _sources)
                 {
                     source.Subscribe(this, i++);
                 }
-
-                _initializing = false;
-                PublishNext(_flags != 0);
             }
         }
 

src/Avalonia.Base/Styling/Activators/PropertyEqualsActivator.cs

@@ -1,7 +1,5 @@
 using System;
 
-#nullable enable
-
 namespace Avalonia.Styling.Activators
 {
     /// <summary>
@@ -24,13 +22,10 @@ namespace Avalonia.Styling.Activators
             _value = value;
         }
 
-        public override bool IsActive
+        protected override bool EvaluateIsActive()
         {
-            get
+            var value = _control.GetValue(_property);
-            {
+            return PropertyEqualsSelector.Compare(_property.PropertyType, value, _value);
-                var value = _control.GetValue(_property);
-                return PropertyEqualsSelector.Compare(_property.PropertyType, value, _value);
-            }
         }
 
         protected override void Initialize()
@@ -42,6 +37,6 @@ namespace Avalonia.Styling.Activators
 
         void IObserver<object?>.OnCompleted() { }
         void IObserver<object?>.OnError(Exception error) { }
-        void IObserver<object?>.OnNext(object? value) => PublishNext(IsActive);
+        void IObserver<object?>.OnNext(object? value) => ReevaluateIsActive();
     }
 }

src/Avalonia.Base/Styling/Activators/StyleActivatorBase.cs

@@ -7,22 +7,23 @@ namespace Avalonia.Styling.Activators
     {
         private IStyleActivatorSink? _sink;
         private int _tag;
-        private bool? _value;
+        private bool _value;
 
-        public abstract bool IsActive { get; }
+        public bool IsActive => _value = EvaluateIsActive();
+
+        public bool IsSubscribed => _sink is not null;
 
         public void Subscribe(IStyleActivatorSink sink, int tag = 0)
         {
             if (_sink is null)
             {
+                Initialize();
                 _sink = sink;
                 _tag = tag;
-                _value = null;
-                Initialize();
             }
             else
             {
-                throw new AvaloniaInternalException("Cannot subscribe to a StyleActivator more than once.");
+                throw new AvaloniaInternalException("StyleActivator is already subscribed.");
             }
         }
 
@@ -37,22 +38,51 @@ namespace Avalonia.Styling.Activators
             Deinitialize();
         }
 
-        public void PublishNext(bool value)
+        public void Dispose()
         {
-            if (_value != value)
+            _sink = null;
+            Deinitialize();
+        }
+
+        /// <summary>
+        /// Evaluates the <see cref="IsActive"/> value.
+        /// </summary>
+        /// <remarks>
+        /// This method should read directly from its inputs and not rely on any subscriptions to
+        /// fire in order to be up-to-date.
+        /// </remarks>
+        protected abstract bool EvaluateIsActive();
+
+        /// <summary>
+        /// Called from a derived class when the <see cref="IsActive"/> state should be re-evaluated
+        /// and the subscriber notified of any change.
+        /// </summary>
+        /// <returns>
+        /// The evaluated active state;
+        /// </returns>
+        protected bool ReevaluateIsActive()
+        {
+            var value = EvaluateIsActive();
+
+            if (value != _value)
             {
                 _value = value;
                 _sink?.OnNext(value, _tag);
             }
-        }
 
-        public void Dispose()
+            return value;
-        {
-            _sink = null;
-            Deinitialize();
         }
 
+        /// <summary>
+        /// Called in response to a <see cref="Subscribe(IStyleActivatorSink, int)"/> to allow the
+        /// derived class to set up any necessary subscriptions.
+        /// </summary>
         protected abstract void Initialize();
+
+        /// <summary>
+        /// Called in response to an <see cref="Unsubscribe(IStyleActivatorSink)"/> or
+        /// <see cref="Dispose"/> to allow the derived class to dispose any active subscriptions.
+        /// </summary>
         protected abstract void Deinitialize();
     }
 }

src/Avalonia.Base/Styling/Activators/StyleClassActivator.cs

@@ -1,10 +1,6 @@
 using System.Collections.Generic;
-using System.Collections.Specialized;
-using Avalonia.Collections;
 using Avalonia.Controls;
 
-#nullable enable
-
 namespace Avalonia.Styling.Activators
 {
     /// <summary>
@@ -22,8 +18,6 @@ namespace Avalonia.Styling.Activators
             _match = match;
         }
 
-        public override bool IsActive => AreClassesMatching(_classes, _match);
-
         public static bool AreClassesMatching(IReadOnlyList<string> classes, IList<string> toMatch)
         {
             int remainingMatches = toMatch.Count;
@@ -54,20 +48,9 @@ namespace Avalonia.Styling.Activators
             return remainingMatches == 0;
         }
 
-        void IClassesChangedListener.Changed()
+        void IClassesChangedListener.Changed() => ReevaluateIsActive();
-        {
+        protected override bool EvaluateIsActive() => AreClassesMatching(_classes, _match);
-            PublishNext(IsActive);
+        protected override void Initialize() => _classes.AddListener(this);
-        }
+        protected override void Deinitialize() => _classes.RemoveListener(this);
-
-        protected override void Initialize()
-        {
-            PublishNext(IsActive);
-            _classes.AddListener(this);
-        }
-
-        protected override void Deinitialize()
-        {
-            _classes.RemoveListener(this);
-        }
     }
 }

src/Avalonia.Base/Styling/StyleInstance.cs

@@ -20,8 +20,6 @@ namespace Avalonia.Styling
     {
         private readonly IStyleActivator? _activator;
         private List<ISetterInstance>? _setters;
-        private bool _isActivatorInitializing;
-        private bool _isActivatorSubscribed;
 
         public StyleInstance(IStyle style, IStyleActivator? activator)
         {
@@ -36,14 +34,8 @@ namespace Avalonia.Styling
         {
             get
             {
-                if (_activator is object && !_isActivatorSubscribed)
+                if (_activator?.IsSubscribed == false)
-                {
-                    _isActivatorInitializing = true;
                     _activator.Subscribe(this);
-                    _isActivatorInitializing = false;
-                    _isActivatorSubscribed = true;
-                }
-
                 return _activator?.IsActive ?? true;
             }
         }
@@ -65,10 +57,6 @@ namespace Avalonia.Styling
             _activator?.Dispose();
         }
 
-        void IStyleActivatorSink.OnNext(bool value, int tag)
+        void IStyleActivatorSink.OnNext(bool value, int tag) => Owner?.OnFrameActivationChanged(this);
-        {
-            if (!_isActivatorInitializing)
-                Owner?.OnFrameActivationChanged(this);
-        }
     }
 }

tests/Avalonia.Base.UnitTests/Styling/SelectorTests_Nesting.cs

@@ -292,7 +292,13 @@ namespace Avalonia.Base.UnitTests.Styling
 
         private class ActivatorSink : IStyleActivatorSink
         {
-            public ActivatorSink(IStyleActivator source) => source.Subscribe(this);
+            public ActivatorSink(IStyleActivator source)
+            {
+                source.Subscribe(this);
+                Active = source.IsActive;
+            }
+
+
             public bool Active { get; private set; }
             public void OnNext(bool value, int tag) => Active = value;
         }

tests/Avalonia.Base.UnitTests/Styling/StyleActivatorExtensions.cs

@@ -33,8 +33,14 @@ namespace Avalonia.Base.UnitTests.Styling
             private readonly IStyleActivator _source;
             
             public ObservableAdapter(IStyleActivator source) => _source = source;
+
             protected override void Initialize() => _source.Subscribe(this);
             protected override void Deinitialize() => _source.Unsubscribe(this);
+            
+            protected override void Subscribed(IObserver<bool> observer, bool first)
+            {
+                observer.OnNext(_source.IsActive);
+            }
 
             void IStyleActivatorSink.OnNext(bool value, int tag)
             {