Ensure that the default value can be coerced.

When the coercion function on a property results in the default value being coerced, ensure that state is recorded in the value store.

src/Avalonia.Base/AvaloniaProperty.cs

@@ -499,6 +499,12 @@ namespace Avalonia
         /// <param name="o">The object instance.</param>
         internal abstract void RouteClearValue(AvaloniaObject o);
 
+        /// <summary>
+        /// Routes an untyped CoerceValue call on a property with its default value to a typed call.
+        /// </summary>
+        /// <param name="o">The object instance.</param>
+        internal abstract void RouteCoerceDefaultValue(AvaloniaObject o);
+
         /// <summary>
         /// Routes an untyped GetValue call to a typed call.
         /// </summary>

src/Avalonia.Base/DirectPropertyBase.cs

@@ -117,6 +117,11 @@ namespace Avalonia
             o.ClearValue<TValue>(this);
         }
 
+        internal override void RouteCoerceDefaultValue(AvaloniaObject o)
+        {
+            // Do nothing.
+        }
+
         /// <inheritdoc/>
         internal override object? RouteGetValue(AvaloniaObject o)
         {

src/Avalonia.Base/PropertyStore/EffectiveValue.cs

@@ -36,12 +36,23 @@ namespace Avalonia.PropertyStore
         /// </summary>
         public IValueEntry? BaseValueEntry { get; private set; }
 
+        /// <summary>
+        /// Gets a value indicating whether the property has a coercion function.
+        /// </summary>
+        public bool HasCoercion { get; protected set; }
+
         /// <summary>
         /// Gets a value indicating whether the <see cref="Value"/> was overridden by a call to 
         /// <see cref="AvaloniaObject.SetCurrentValue{T}"/>.
         /// </summary>
         public bool IsOverridenCurrentValue { get; set; }
 
+        /// <summary>
+        /// Gets a value indicating whether the <see cref="Value"/> is the result of the 
+        /// 
+        /// </summary>
+        public bool IsCoercedDefaultValue { get; set; }
+
         /// <summary>
         /// Begins a reevaluation pass on the effective value.
         /// </summary>
@@ -63,10 +74,33 @@ namespace Avalonia.PropertyStore
         /// <summary>
         /// Ends a reevaluation pass on the effective value.
         /// </summary>
+        /// <param name="owner">The associated value store.</param>
+        /// <param name="property">The property being reevaluated.</param>
         /// <remarks>
-        /// This method unsubscribes from any unused value entries.
+        /// Handles coercing the default value if necessary.
         /// </remarks>
-        public void EndReevaluation()
+        public void EndReevaluation(ValueStore owner, AvaloniaProperty property)
+        {
+            if (Priority == BindingPriority.Unset && HasCoercion)
+                CoerceDefaultValueAndRaise(owner, property);
+        }
+
+        /// <summary>
+        /// Gets a value indicating whether the effective value represents the default value of the
+        /// property and can be removed.
+        /// </summary>
+        /// <returns>True if the effective value van be removed; otherwise false.</returns>
+        public bool CanRemove()
+        {
+            return Priority == BindingPriority.Unset &&
+                !IsOverridenCurrentValue &&
+                !IsCoercedDefaultValue;
+        }
+
+        /// <summary>
+        /// Unsubscribes from any unused value entries.
+        /// </summary>
+        public void UnsubscribeIfNecessary()
         {
             if (Priority == BindingPriority.Unset)
             {
@@ -130,6 +164,17 @@ namespace Avalonia.PropertyStore
         /// <param name="property">The property being cleared.</param>
         public abstract void DisposeAndRaiseUnset(ValueStore owner, AvaloniaProperty property);
 
+        /// <summary>
+        /// Coerces the default value, raising <see cref="AvaloniaObject.PropertyChanged"/>
+        /// where necessary.
+        /// </summary>
+        /// <param name="owner">The associated value store.</param>
+        /// <param name="property">The property being coerced.</param>
+        protected abstract void CoerceDefaultValueAndRaise(ValueStore owner, AvaloniaProperty property);
+
+        /// <summary>
+        /// Gets the current effective value as a boxed value.
+        /// </summary>
         protected abstract object? GetBoxedValue();
 
         protected void UpdateValueEntry(IValueEntry? entry, BindingPriority priority)

src/Avalonia.Base/PropertyStore/EffectiveValue`1.cs

@@ -3,7 +3,6 @@ using System.Collections.Generic;
 using System.Diagnostics;
 using System.Diagnostics.CodeAnalysis;
 using Avalonia.Data;
-using static Avalonia.Rendering.Composition.Animations.PropertySetSnapshot;
 
 namespace Avalonia.PropertyStore
 {
@@ -33,19 +32,16 @@ namespace Avalonia.PropertyStore
 
             if (_metadata.CoerceValue is { } coerce)
             {
+                HasCoercion = true;
                 _uncommon = new()
                 {
                     _coerce = coerce,
                     _uncoercedValue = value,
                     _uncoercedBaseValue = value,
                 };
-
-                Value = coerce(owner, value);
-            }
-            else
-            {
-                Value = value;
             }
+
+            Value = value;
         }
 
         /// <summary>
@@ -61,7 +57,7 @@ namespace Avalonia.PropertyStore
             Debug.Assert(priority != BindingPriority.LocalValue);
             UpdateValueEntry(value, priority);
 
-            SetAndRaiseCore(owner,  (StyledProperty<T>)value.Property, GetValue(value), priority, false);
+            SetAndRaiseCore(owner,  (StyledProperty<T>)value.Property, GetValue(value), priority);
 
             if (priority > BindingPriority.LocalValue &&
                 value.GetDataValidationState(out var state, out var error))
@@ -75,7 +71,7 @@ namespace Avalonia.PropertyStore
             StyledProperty<T> property,
             T value)
         {
-            SetAndRaiseCore(owner, property, value, BindingPriority.LocalValue, false);
+            SetAndRaiseCore(owner, property, value, BindingPriority.LocalValue);
         }
 
         public void SetCurrentValueAndRaise(
@@ -83,8 +79,15 @@ namespace Avalonia.PropertyStore
             StyledProperty<T> property,
             T value)
         {
-            IsOverridenCurrentValue = true;
-            SetAndRaiseCore(owner, property, value, Priority, true);
+            SetAndRaiseCore(owner, property, value, Priority, isOverriddenCurrentValue: true);
+        }
+
+        public void SetCoercedDefaultValueAndRaise(
+            ValueStore owner,
+            StyledProperty<T> property,
+            T value)
+        {
+            SetAndRaiseCore(owner, property, value, Priority, isCoercedDefaultValue: true);
         }
 
         public bool TryGetBaseValue([MaybeNullWhen(false)] out T value)
@@ -117,7 +120,7 @@ namespace Avalonia.PropertyStore
             Debug.Assert(Priority != BindingPriority.Animation);
             Debug.Assert(BasePriority != BindingPriority.Unset);
             UpdateValueEntry(null, BindingPriority.Animation);
-            SetAndRaiseCore(owner, (StyledProperty<T>)property, _baseValue!, BasePriority, false);
+            SetAndRaiseCore(owner, (StyledProperty<T>)property, _baseValue!, BasePriority);
         }
 
         public override void CoerceValue(ValueStore owner, AvaloniaProperty property)
@@ -168,6 +171,17 @@ namespace Avalonia.PropertyStore
             }
         }
 
+        protected override void CoerceDefaultValueAndRaise(ValueStore owner, AvaloniaProperty property)
+        {
+            Debug.Assert(_uncommon?._coerce is not null);
+            Debug.Assert(Priority == BindingPriority.Unset);
+
+            var coercedDefaultValue = _uncommon!._coerce!(owner.Owner, _metadata.DefaultValue);
+
+            if (!EqualityComparer<T>.Default.Equals(_metadata.DefaultValue, coercedDefaultValue))
+                SetCoercedDefaultValueAndRaise(owner, (StyledProperty<T>)property, coercedDefaultValue);
+        }
+
         protected override object? GetBoxedValue() => Value;
         
         private static T GetValue(IValueEntry entry)
@@ -183,7 +197,8 @@ namespace Avalonia.PropertyStore
             StyledProperty<T> property,
             T value,
             BindingPriority priority,
-            bool isOverriddenCurrentValue)
+            bool isOverriddenCurrentValue = false,
+            bool isCoercedDefaultValue = false)
         {
             var oldValue = Value;
             var valueChanged = false;
@@ -191,6 +206,7 @@ namespace Avalonia.PropertyStore
             var v = value;
 
             IsOverridenCurrentValue = isOverriddenCurrentValue;
+            IsCoercedDefaultValue = isCoercedDefaultValue;
 
             if (_uncommon?._coerce is { } coerce)
                 v = coerce(owner.Owner, value);

src/Avalonia.Base/PropertyStore/ValueStore.cs

@@ -259,6 +259,27 @@ namespace Avalonia.PropertyStore
         {
             if (_effectiveValues.TryGetValue(property, out var v))
                 v.CoerceValue(this, property);
+            else
+                property.RouteCoerceDefaultValue(Owner);
+        }
+
+        public void CoerceDefaultValue<T>(StyledProperty<T> property)
+        {
+            var metadata = property.GetMetadata(Owner.GetType());
+
+            if (metadata.CoerceValue is null)
+                return;
+
+            var coercedDefaultValue = metadata.CoerceValue(Owner, metadata.DefaultValue);
+
+            if (EqualityComparer<T>.Default.Equals(metadata.DefaultValue, coercedDefaultValue))
+                return;
+
+            // We have a situation where the default value isn't valid according to the coerce
+            // function. In this case, we need to create an EffectiveValue entry.
+            var effectiveValue = CreateEffectiveValue(property);
+            AddEffectiveValue(property, effectiveValue);
+            effectiveValue.SetCoercedDefaultValueAndRaise(this, property, coercedDefaultValue);
         }
 
         public Optional<T> GetBaseValue<T>(StyledProperty<T> property)
@@ -838,20 +859,25 @@ namespace Avalonia.PropertyStore
                         goto restart;
                 }
 
-                if (current?.Priority == BindingPriority.Unset)
+                if (current is not null)
                 {
-                    if (current.BasePriority == BindingPriority.Unset)
-                    {
-                        RemoveEffectiveValue(property);
-                        current.DisposeAndRaiseUnset(this, property);
-                    }
-                    else
+                    current.EndReevaluation(this, property);
+
+                    if (current.CanRemove())
                     {
-                        current.RemoveAnimationAndRaise(this, property);
+                        if (current.BasePriority == BindingPriority.Unset)
+                        {
+                            RemoveEffectiveValue(property);
+                            current.DisposeAndRaiseUnset(this, property);
+                        }
+                        else
+                        {
+                            current.RemoveAnimationAndRaise(this, property);
+                        }
                     }
-                }
 
-                current?.EndReevaluation();
+                    current.UnsubscribeIfNecessary();
+                }
             }
             finally
             {
@@ -923,7 +949,9 @@ namespace Avalonia.PropertyStore
                 {
                     _effectiveValues.GetKeyValue(i, out var key, out var e);
 
-                    if (e.Priority == BindingPriority.Unset && !e.IsOverridenCurrentValue)
+                    e.EndReevaluation(this, key);
+
+                    if (e.CanRemove())
                     {
                         RemoveEffectiveValue(key, i);
                         e.DisposeAndRaiseUnset(this, key);
@@ -932,7 +960,7 @@ namespace Avalonia.PropertyStore
                             break;
                     }
 
-                    e.EndReevaluation();
+                    e.UnsubscribeIfNecessary();
                 }
             }
             finally

src/Avalonia.Base/StyledProperty.cs

@@ -176,6 +176,11 @@ namespace Avalonia
             o.ClearValue<TValue>(this);
         }
 
+        internal override void RouteCoerceDefaultValue(AvaloniaObject o)
+        {
+            o.GetValueStore().CoerceDefaultValue(this);
+        }
+
         /// <inheritdoc/>
         internal override object? RouteGetValue(AvaloniaObject o)
         {

tests/Avalonia.Base.UnitTests/AvaloniaObjectTests_Coercion.cs

@@ -1,7 +1,10 @@
 using System;
 using System.Collections.Generic;
 using System.Reactive.Subjects;
+using Avalonia.Controls;
 using Avalonia.Data;
+using Avalonia.Styling;
+using Avalonia.UnitTests;
 using Xunit;
 
 namespace Avalonia.Base.UnitTests
@@ -182,11 +185,103 @@ namespace Avalonia.Base.UnitTests
             Assert.Equal(-150, target.Foo);
         }
 
+        [Fact]
+        public void Default_Value_Can_Be_Coerced()
+        {
+            var target = new Class1();
+            var raised = 0;
+
+            target.MinFoo = 20;
+
+            target.PropertyChanged += (s, e) =>
+            {
+                Assert.Equal(Class1.FooProperty, e.Property);
+                Assert.Equal(11, e.OldValue);
+                Assert.Equal(20, e.NewValue);
+                Assert.Equal(BindingPriority.Unset, e.Priority);
+                ++raised;
+            };
+
+            target.CoerceValue(Class1.FooProperty);
+
+            Assert.Equal(20, target.Foo);
+            Assert.Equal(1, raised);
+        }
+
+        [Fact]
+        public void ClearValue_Respects_Coerced_Default_Value()
+        {
+            var target = new Class1();
+            var raised = 0;
+
+            target.Foo = 30;
+            target.MinFoo = 20;
+
+            target.PropertyChanged += (s, e) =>
+            {
+                Assert.Equal(Class1.FooProperty, e.Property);
+                Assert.Equal(30, e.OldValue);
+                Assert.Equal(20, e.NewValue);
+                Assert.Equal(BindingPriority.Unset, e.Priority);
+                ++raised;
+            };
+
+            target.ClearValue(Class1.FooProperty);
+
+            Assert.Equal(20, target.Foo);
+            Assert.Equal(1, raised);
+        }
+
+        [Fact]
+        public void Deactivating_Style_Respects_Coerced_Default_Value()
+        {
+            var target = new Control1
+            {
+                MinFoo = 20,
+            };
+
+            var root = new TestRoot
+            {
+                Styles =
+                {
+                    new Style(x => x.OfType<Control1>().Class("foo"))
+                    {
+                        Setters =
+                        {
+                            new Setter(Control1.FooProperty, 50),
+                        },
+                    },
+                },
+                Child = target,
+            };
+
+            var raised = 0;
+
+            target.Classes.Add("foo");
+            root.LayoutManager.ExecuteInitialLayoutPass();
+
+            Assert.Equal(50, target.Foo);
+
+            target.PropertyChanged += (s, e) =>
+            {
+                Assert.Equal(Control1.FooProperty, e.Property);
+                Assert.Equal(50, e.OldValue);
+                Assert.Equal(20, e.NewValue);
+                Assert.Equal(BindingPriority.Unset, e.Priority);
+                ++raised;
+            };
+
+            target.Classes.Remove("foo");
+
+            Assert.Equal(20, target.Foo);
+            Assert.Equal(1, raised);
+        }
+
         private class Class1 : AvaloniaObject
         {
             public static readonly StyledProperty<int> FooProperty =
                 AvaloniaProperty.Register<Class1, int>(
-                    "Qux",
+                    "Foo",
                     defaultValue: 11,
                     coerce: CoerceFoo);
 
@@ -215,13 +310,15 @@ namespace Avalonia.Base.UnitTests
                 set => SetValue(InheritedProperty, value);
             }
 
+            public int MinFoo { get; set; } = 0;
             public int MaxFoo { get; set; } = 100;
 
             public List<AvaloniaPropertyChangedEventArgs> CoreChanges { get; } = new();
 
             public static int CoerceFoo(AvaloniaObject instance, int value)
             {
-                return Math.Min(((Class1)instance).MaxFoo, value);
+                var o = (Class1)instance;
+                return Math.Clamp(value, o.MinFoo, o.MaxFoo);
             }
 
             protected override void OnPropertyChangedCore(AvaloniaPropertyChangedEventArgs change)
@@ -266,5 +363,29 @@ namespace Avalonia.Base.UnitTests
                 return -value;
             }
         }
+
+        private class Control1 : Control 
+        {
+            public static readonly StyledProperty<int> FooProperty =
+                AvaloniaProperty.Register<Control1, int>(
+                    "Foo",
+                    defaultValue: 11,
+                    coerce: CoerceFoo);
+
+            public int Foo
+            {
+                get => GetValue(FooProperty);
+                set => SetValue(FooProperty, value);
+            }
+
+            public int MinFoo { get; set; } = 0;
+            public int MaxFoo { get; set; } = 100;
+
+            public static int CoerceFoo(AvaloniaObject instance, int value)
+            {
+                var o = (Control1)instance;
+                return Math.Clamp(value, o.MinFoo, o.MaxFoo);
+            }
+        }
     }
 }

tests/Avalonia.Base.UnitTests/AvaloniaPropertyTests.cs

@@ -179,6 +179,11 @@ namespace Avalonia.Base.UnitTests
                 throw new NotImplementedException();
             }
 
+            internal override void RouteCoerceDefaultValue(AvaloniaObject o)
+            {
+                throw new NotImplementedException();
+            }
+
             internal override object RouteGetValue(AvaloniaObject o)
             {
                 throw new NotImplementedException();