Remove AvaloniaProperty.Initialized.

It was only needed for pseudoclasses. Move pseudoclass registration to an engine that doesn't require `AvaloniaProperty.Initialized`, implemented in `PseudoclassEngine`.

src/Avalonia.Base/AvaloniaObject.cs

@@ -34,7 +34,6 @@ namespace Avalonia
         public AvaloniaObject()
         {
             VerifyAccess();
-            AvaloniaPropertyRegistry.Instance.NotifyInitialized(this);
         }
 
         /// <summary>

src/Avalonia.Base/AvaloniaProperty.cs

@@ -22,7 +22,6 @@ namespace Avalonia
         public static readonly object UnsetValue = new UnsetValueType();
 
         private static int s_nextId;
-        private readonly Subject<AvaloniaPropertyChangedEventArgs> _initialized;
         private readonly Subject<AvaloniaPropertyChangedEventArgs> _changed;
         private readonly PropertyMetadata _defaultMetadata;
         private readonly Dictionary<Type, PropertyMetadata> _metadata;
@@ -55,7 +54,6 @@ namespace Avalonia
                 throw new ArgumentException("'name' may not contain periods.");
             }
 
-            _initialized = new Subject<AvaloniaPropertyChangedEventArgs>();
             _changed = new Subject<AvaloniaPropertyChangedEventArgs>();
             _metadata = new Dictionary<Type, PropertyMetadata>();
 
@@ -83,7 +81,6 @@ namespace Avalonia
             Contract.Requires<ArgumentNullException>(source != null);
             Contract.Requires<ArgumentNullException>(ownerType != null);
 
-            _initialized = source._initialized;
             _changed = source._changed;
             _metadata = new Dictionary<Type, PropertyMetadata>();
 
@@ -138,22 +135,6 @@ namespace Avalonia
         /// </summary>
         public virtual bool IsReadOnly => false;
 
-        /// <summary>
-        /// Gets an observable that is fired when this property is initialized on a
-        /// new <see cref="AvaloniaObject"/> instance.
-        /// </summary>
-        /// <remarks>
-        /// This observable is fired each time a new <see cref="AvaloniaObject"/> is constructed
-        /// for all properties registered on the object's type. The default value of the property
-        /// for the object is passed in the args' NewValue (OldValue will always be
-        /// <see cref="UnsetValue"/>.
-        /// </remarks>
-        /// <value>
-        /// An observable that is fired when this property is initialized on a new
-        /// <see cref="AvaloniaObject"/> instance.
-        /// </value>
-        public IObservable<AvaloniaPropertyChangedEventArgs> Initialized => _initialized;
-
         /// <summary>
         /// Gets an observable that is fired when this property changes on any
         /// <see cref="AvaloniaObject"/> instance.
@@ -482,26 +463,6 @@ namespace Avalonia
             return Name;
         }
 
-        /// <summary>
-        /// True if <see cref="Initialized"/> has any observers.
-        /// </summary>
-        internal bool HasNotifyInitializedObservers => _initialized.HasObservers;
-
-        /// <summary>
-        /// Notifies the <see cref="Initialized"/> observable.
-        /// </summary>
-        /// <param name="o">The object being initialized.</param>
-        internal abstract void NotifyInitialized(IAvaloniaObject o);
-
-        /// <summary>
-        /// Notifies the <see cref="Initialized"/> observable.
-        /// </summary>
-        /// <param name="e">The observable arguments.</param>
-        internal void NotifyInitialized(AvaloniaPropertyChangedEventArgs e)
-        {
-            _initialized.OnNext(e);
-        }
-
         /// <summary>
         /// Notifies the <see cref="Changed"/> observable.
         /// </summary>

src/Avalonia.Base/AvaloniaPropertyRegistry.cs

@@ -407,51 +407,6 @@ namespace Avalonia
             _inheritedCache.Clear();
         }
 
-        internal void NotifyInitialized(AvaloniaObject o)
-        {
-            Contract.Requires<ArgumentNullException>(o != null);
-
-            var type = o.GetType();
-
-            if (!_initializedCache.TryGetValue(type, out var initializationData))
-            {
-                var visited = new HashSet<AvaloniaProperty>();
-
-                initializationData = new List<PropertyInitializationData>();
-
-                foreach (AvaloniaProperty property in GetRegistered(type))
-                {
-                    if (property.IsDirect)
-                    {
-                        initializationData.Add(new PropertyInitializationData(property, (IDirectPropertyAccessor)property));
-                    }
-                    else
-                    {
-                        initializationData.Add(new PropertyInitializationData(property, (IStyledPropertyAccessor)property, type));
-                    }
-
-                    visited.Add(property);
-                }
-
-                foreach (AvaloniaProperty property in GetRegisteredAttached(type))
-                {
-                    if (!visited.Contains(property))
-                    {
-                        initializationData.Add(new PropertyInitializationData(property, (IStyledPropertyAccessor)property, type));
-
-                        visited.Add(property);
-                    }
-                }
-
-                _initializedCache.Add(type, initializationData);
-            }
-
-            foreach (PropertyInitializationData data in initializationData)
-            {
-                data.Property.NotifyInitialized(o);
-            }
-        }
-
         private readonly struct PropertyInitializationData
         {
             public AvaloniaProperty Property { get; }

src/Avalonia.Base/DirectPropertyBase.cs

@@ -101,21 +101,6 @@ namespace Avalonia
             return (DirectPropertyMetadata<TValue>)base.GetMetadata(type);
         }
 
-        /// <inheritdoc/>
-        internal override void NotifyInitialized(IAvaloniaObject o)
-        {
-            if (HasNotifyInitializedObservers)
-            {
-                var e = new AvaloniaPropertyChangedEventArgs<TValue>(
-                    o,
-                    this,
-                    default,
-                    InvokeGetter(o),
-                    BindingPriority.Unset);
-                NotifyInitialized(e);
-            }
-        }
-
         /// <inheritdoc/>
         internal override void RouteClearValue(IAvaloniaObject o)
         {

src/Avalonia.Base/StyledPropertyBase.cs

@@ -181,21 +181,6 @@ namespace Avalonia
         /// <inheritdoc/>
         object IStyledPropertyAccessor.GetDefaultValue(Type type) => GetDefaultBoxedValue(type);
 
-        /// <inheritdoc/>
-        internal override void NotifyInitialized(IAvaloniaObject o)
-        {
-            if (HasNotifyInitializedObservers)
-            {
-                var e = new AvaloniaPropertyChangedEventArgs<TValue>(
-                    o,
-                    this,
-                    default,
-                    o.GetValue(this),
-                    BindingPriority.Unset);
-                NotifyInitialized(e);
-            }
-        }
-
         /// <inheritdoc/>
         internal override void RouteClearValue(IAvaloniaObject o)
         {

src/Avalonia.Styling/PseudoclassEngine.cs

@@ -0,0 +1,120 @@
+using System;
+using System.Collections.Generic;
+using Avalonia.Controls;
+
+namespace Avalonia
+{
+    internal static class PseudoclassEngine
+    {
+        private static Dictionary<Type, List<IEntry>> _entries = new Dictionary<Type, List<IEntry>>();
+        private static Dictionary<AvaloniaProperty, List<IEntry>> _properties = new Dictionary<AvaloniaProperty, List<IEntry>>();
+        private static Dictionary<Type, List<IEntry>> _cache = new Dictionary<Type, List<IEntry>>();
+
+        public static void Created(StyledElement element)
+        {
+            var t = element.GetType();
+
+            if (!_cache.TryGetValue(t, out var list))
+            {
+                list = new List<IEntry>();
+
+                foreach (var i in _entries)
+                {
+                    if (i.Key.IsAssignableFrom(t))
+                    {
+                        list.AddRange(i.Value);
+                    }
+                }
+
+                _cache.Add(t, list);
+            }
+
+            foreach (var i in list)
+            {
+                i.Update(element);
+            }
+        }
+
+        public static void Register<T>(
+            Type type,
+            AvaloniaProperty<T> property,
+            Func<T, bool> selector,
+            string className)
+        {
+            if (!_entries.TryGetValue(type, out var list))
+            {
+                list = new List<IEntry>();
+                _entries.Add(type, list);
+            }
+
+            var entry = new Entry<T>(type, property, selector, className);
+            list.Add(entry);
+            
+            if (!_properties.TryGetValue(property, out list))
+            {
+                list = new List<IEntry>();
+                _properties.Add(property, list);
+                property.Changed.Subscribe(ValueChanged);
+            }
+
+            list.Add(entry);
+        }
+
+        private static void ValueChanged(AvaloniaPropertyChangedEventArgs e)
+        {
+            if (e.Sender is StyledElement element &&
+                _properties.TryGetValue(e.Property, out var list))
+            {
+                var t = e.Sender.GetType();
+
+                foreach (var i in list)
+                {
+                    if (i.Type.IsAssignableFrom(t))
+                    {
+                        i.Update(element);
+                    }
+                }
+            }
+        }
+
+        private interface IEntry
+        {
+            Type Type { get; }
+            void Update(StyledElement element);
+        }
+
+        private class Entry<T> : IEntry
+        {
+            public Entry(
+                Type type,
+                AvaloniaProperty<T> property,
+                Func<T, bool> selector,
+                string className)
+            {
+                Type = type;
+                Property = property;
+                Selector = selector;
+                ClassName = className;
+            }
+
+            public Type Type { get; }
+            public AvaloniaProperty<T> Property { get; }
+            public Func<T, bool> Selector { get; }
+            public string ClassName { get; }
+
+            public void Update(StyledElement element)
+            {
+                var value = element.GetValue(Property);
+
+                if (Selector(value))
+                {
+                    ((IPseudoClasses)element.Classes).Add(ClassName);
+                }
+                else
+                {
+                    ((IPseudoClasses)element.Classes).Remove(ClassName);
+                }
+            }
+        }
+    }
+}

src/Avalonia.Styling/StyledElement.cs

@@ -82,6 +82,7 @@ namespace Avalonia
         public StyledElement()
         {
             _isAttachedToLogicalTree = this is IStyleRoot;
+            PseudoclassEngine.Created(this);
         }
 
         /// <summary>
@@ -557,19 +558,11 @@ namespace Avalonia
                 throw new ArgumentException("Cannot supply an empty className.");
             }
 
-            property.Changed.Merge(property.Initialized)
-                .Where(e => e.Sender is TOwner)
-                .Subscribe(e =>
-                {
-                    if (selector((TProperty)e.NewValue))
-                    {
-                        ((StyledElement)e.Sender).PseudoClasses.Add(className);
-                    }
-                    else
-                    {
-                        ((StyledElement)e.Sender).PseudoClasses.Remove(className);
-                    }
-                });
+            PseudoclassEngine.Register(
+                typeof(TOwner),
+                property,
+                selector,
+                className);
         }
 
         protected virtual void LogicalChildrenCollectionChanged(object sender, NotifyCollectionChangedEventArgs e)

tests/Avalonia.Base.UnitTests/AvaloniaObjectTests_Attached.cs

@@ -16,19 +16,6 @@ namespace Avalonia.Base.UnitTests
             Assert.Equal("foodefault", target.GetValue(Class2.FooProperty));
         }
 
-        [Fact]
-        public void AvaloniaProperty_Initialized_Is_Called_For_Attached_Property()
-        {
-            bool raised = false;
-
-            using (Class1.FooProperty.Initialized.Subscribe(x => raised = true))
-            {
-                new Class3();
-            }
-
-            Assert.True(raised);
-        }
-
         private class Base : AvaloniaObject
         {
         }
@@ -46,9 +33,5 @@ namespace Avalonia.Base.UnitTests
             public static readonly AttachedProperty<string> FooProperty =
                 Class1.FooProperty.AddOwner<Class2>();
         }
-
-        private class Class3 : Base
-        {
-        }
     }
 }

tests/Avalonia.Base.UnitTests/AvaloniaObjectTests_Direct.cs

@@ -424,22 +424,6 @@ namespace Avalonia.Base.UnitTests
             Assert.Equal("second", target.Foo);
         }
 
-        [Fact]
-        public void Property_Notifies_Initialized()
-        {
-            bool raised = false;
-
-            Class1.FooProperty.Initialized.Subscribe(e =>
-                raised = e.Property == Class1.FooProperty &&
-                         e.OldValue == AvaloniaProperty.UnsetValue &&
-                         (string)e.NewValue == "initial" &&
-                         e.Priority == BindingPriority.Unset);
-
-            var target = new Class1();
-
-            Assert.True(raised);
-        }
-
         [Fact]
         public void Binding_Error_Reverts_To_Default_Value()
         {

tests/Avalonia.Base.UnitTests/AvaloniaPropertyTests.cs

@@ -78,24 +78,6 @@ namespace Avalonia.Base.UnitTests
             Assert.Equal(BindingMode.TwoWay, result.DefaultBindingMode);
         }
 
-        [Fact]
-        public void Initialized_Observable_Fired()
-        {
-            bool invoked = false;
-
-            Class1.FooProperty.Initialized.Subscribe(x =>
-            {
-                Assert.Equal(AvaloniaProperty.UnsetValue, x.OldValue);
-                Assert.Equal("default", x.NewValue);
-                Assert.Equal(BindingPriority.Unset, x.Priority);
-                invoked = true;
-            });
-
-            var target = new Class1();
-
-            Assert.True(invoked);
-        }
-
         [Fact]
         public void Changed_Observable_Fired()
         {
@@ -141,11 +123,6 @@ namespace Avalonia.Base.UnitTests
                 OverrideMetadata(typeof(T), metadata);
             }
 
-            internal override void NotifyInitialized(IAvaloniaObject o)
-            {
-                throw new NotImplementedException();
-            }
-
             internal override IDisposable RouteBind(
                 IAvaloniaObject o,
                 IObservable<BindingValue<object>> source,

tests/Avalonia.Base.UnitTests/DirectPropertyTests.cs

@@ -1,33 +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 System;
-using System.Reactive.Subjects;
-using Avalonia.Data;
 using Xunit;
 
 namespace Avalonia.Base.UnitTests
 {
     public class DirectPropertyTests
     {
-        [Fact]
-        public void Initialized_Observable_Fired()
-        {
-            bool invoked = false;
-
-            Class1.FooProperty.Initialized.Subscribe(x =>
-            {
-                Assert.Equal(AvaloniaProperty.UnsetValue, x.OldValue);
-                Assert.Equal("foo", x.NewValue);
-                Assert.Equal(BindingPriority.Unset, x.Priority);
-                invoked = true;
-            });
-
-            var target = new Class1();
-
-            Assert.True(invoked);
-        }
-
         [Fact]
         public void IsDirect_Property_Returns_True()
         {
@@ -69,7 +48,6 @@ namespace Avalonia.Base.UnitTests
             var p2 = p1.AddOwner<Class2>(o => null, (o, v) => { });
 
             Assert.Same(p1.Changed, p2.Changed);
-            Assert.Same(p1.Initialized, p2.Initialized);
         }
 
         private class Class1 : AvaloniaObject