Switch Expander.IsExpanded to a StyledProperty

src/Avalonia.Controls/Expander.cs

@@ -59,12 +59,11 @@ namespace Avalonia.Controls
         /// <summary>
         /// Defines the <see cref="IsExpanded"/> property.
         /// </summary>
-        public static readonly DirectProperty<Expander, bool> IsExpandedProperty =
-            AvaloniaProperty.RegisterDirect<Expander, bool>(
+        public static readonly StyledProperty<bool> IsExpandedProperty =
+            AvaloniaProperty.Register<Expander, bool>(
                 nameof(IsExpanded),
-                o => o.IsExpanded,
-                (o, v) => o.IsExpanded = v,
-                defaultBindingMode: Data.BindingMode.TwoWay);
+                defaultBindingMode: BindingMode.TwoWay,
+                coerce: CoerceIsExpanded);
 
         /// <summary>
         /// Defines the <see cref="Collapsed"/> event.
@@ -99,7 +98,6 @@ namespace Avalonia.Controls
                 RoutingStrategies.Bubble);
 
         private bool _ignorePropertyChanged = false;
-        private bool _isExpanded;
         private CancellationTokenSource? _lastTransitionCts;
 
         /// <summary>
@@ -134,50 +132,8 @@ namespace Avalonia.Controls
         /// </summary>
         public bool IsExpanded
         {
-            get => _isExpanded;
-            set
-            {
-                // It is important here that IsExpanded is a direct property so events can be invoked
-                // BEFORE the property system gets notified of updated values. This is because events
-                // may be canceled by external code.
-                if (_isExpanded != value)
-                {
-                    RoutedEventArgs eventArgs;
-
-                    if (value)
-                    {
-                        eventArgs = new RoutedEventArgs(ExpandingEvent, this);
-                        OnExpanding(eventArgs);
-                    }
-                    else
-                    {
-                        eventArgs = new RoutedEventArgs(CollapsingEvent, this);
-                        OnCollapsing(eventArgs);
-                    }
-
-                    if (eventArgs.Handled)
-                    {
-                        // If the event was externally handled (canceled) we must still notify the value has changed.
-                        // This property changed notification will update any external code observing this property that itself may have set the new value.
-                        // We are essentially reverted any external state change along with ignoring the IsExpanded property set.
-                        // Remember IsExpanded is usually controlled by a ToggleButton in the control theme.
-                        _ignorePropertyChanged = true;
-
-                        RaisePropertyChanged(
-                            IsExpandedProperty,
-                            oldValue: value,
-                            newValue: _isExpanded,
-                            BindingPriority.LocalValue,
-                            isEffectiveValue: true);
-
-                        _ignorePropertyChanged = false;
-                    }
-                    else
-                    {
-                        SetAndRaise(IsExpandedProperty, ref _isExpanded, value);
-                    }
-                }
-            }
+            get => GetValue(IsExpandedProperty);
+            set => SetValue(IsExpandedProperty, value);
         }
 
         /// <summary>
@@ -332,5 +288,63 @@ namespace Avalonia.Controls
 
             PseudoClasses.Set(":expanded", IsExpanded);
         }
+
+        /// <summary>
+        /// Called when the <see cref="IsExpanded"/> property has to be coerced.
+        /// </summary>
+        /// <param name="value">The value to coerce.</param>
+        protected virtual bool OnCoerceIsExpanded(bool value)
+        {
+            RoutedEventArgs eventArgs;
+
+            if (value)
+            {
+                eventArgs = new RoutedEventArgs(ExpandingEvent, this);
+                OnExpanding(eventArgs);
+            }
+            else
+            {
+                eventArgs = new RoutedEventArgs(CollapsingEvent, this);
+                OnCollapsing(eventArgs);
+            }
+
+            if (eventArgs.Handled)
+            {
+                // If the event was externally handled (canceled) we must still notify the value has changed.
+                // This property changed notification will update any external code observing this property that itself may have set the new value.
+                // We are essentially reverted any external state change along with ignoring the IsExpanded property set.
+                // Remember IsExpanded is usually controlled by a ToggleButton in the control theme and is also used for animations.
+                _ignorePropertyChanged = true;
+
+                RaisePropertyChanged(
+                    IsExpandedProperty,
+                    oldValue: value,
+                    newValue: !value,
+                    BindingPriority.LocalValue,
+                    isEffectiveValue: true);
+
+                _ignorePropertyChanged = false;
+
+                return !value;
+            }
+
+            return value;
+        }
+
+        /// <summary>
+        /// Coerces/validates the <see cref="IsExpanded"/> property value.
+        /// </summary>
+        /// <param name="instance">The <see cref="Expander"/> instance.</param>
+        /// <param name="value">The value to coerce.</param>
+        /// <returns>The coerced/validated value.</returns>
+        private static bool CoerceIsExpanded(AvaloniaObject instance, bool value)
+        {
+            if (instance is Expander expander)
+            {
+                return expander.OnCoerceIsExpanded(value);
+            }
+
+            return value;
+        }
     }
 }