Add Control.SizeChanged Event

src/Avalonia.Base/Interactivity/RoutedEventArgs.cs

@@ -2,29 +2,59 @@
 
 namespace Avalonia.Interactivity
 {
+    /// <summary>
+    /// Provides state information and data specific to a routed event.
+    /// </summary>
     public class RoutedEventArgs : EventArgs
     {
+        /// <summary>
+        /// Initializes a new instance of the <see cref="RoutedEventArgs"/> class.
+        /// </summary>
         public RoutedEventArgs()
         {
         }
 
+        /// <summary>
+        /// Initializes a new instance of the <see cref="RoutedEventArgs"/> class.
+        /// </summary>
+        /// <param name="routedEvent">The routed event associated with these event args.</param>
         public RoutedEventArgs(RoutedEvent? routedEvent)
         {
             RoutedEvent = routedEvent;
         }
 
+        /// <summary>
+        /// Initializes a new instance of the <see cref="RoutedEventArgs"/> class.
+        /// </summary>
+        /// <param name="routedEvent">The routed event associated with these event args.</param>
+        /// <param name="source">The source object that raised the routed event.</param>
         public RoutedEventArgs(RoutedEvent? routedEvent, IInteractive? source)
         {
             RoutedEvent = routedEvent;
             Source = source;
         }
 
+        /// <summary>
+        /// Gets or sets a value indicating whether the routed event has already been handled.
+        /// </summary>
+        /// <remarks>
+        /// Once handled, a routed event should be ignored.
+        /// </remarks>
         public bool Handled { get; set; }
 
+        /// <summary>
+        /// Gets or sets the routed event associated with these event args.
+        /// </summary>
         public RoutedEvent? RoutedEvent { get; set; }
 
+        /// <summary>
+        /// Gets or sets the routing strategy (direct, bubbling, or tunneling) of the routed event.
+        /// </summary>
         public RoutingStrategies Route { get; set; }
 
+        /// <summary>
+        /// Gets or sets the source object that raised the routed event.
+        /// </summary>
         public IInteractive? Source { get; set; }
     }
 }

src/Avalonia.Controls/Control.cs

@@ -84,6 +84,13 @@ public class Control : InputElement, IControl, INamed, IVisualBrushInitialize, I
                 nameof(Unloaded),
                 RoutingStrategies.Direct);
 
+        /// <summary>
+        /// Defines the <see cref="SizeChanged"/> event.
+        /// </summary>
+        public static readonly RoutedEvent<SizeChangedEventArgs> SizeChangedEvent =
+            RoutedEvent.Register<Control, SizeChangedEventArgs>(
+                nameof(SizeChanged), RoutingStrategies.Direct);
+
         /// <summary>
         /// Defines the <see cref="FlowDirection"/> property.
         /// </summary>
@@ -211,6 +218,15 @@ public event EventHandler<RoutedEventArgs>? Unloaded
             remove => RemoveHandler(UnloadedEvent, value);
         }
 
+        /// <summary>
+        /// Occurs when the bounds (actual size) of the control have changed.
+        /// </summary>
+        public event EventHandler<SizeChangedEventArgs>? SizeChanged
+        {
+            add => AddHandler(SizeChangedEvent, value);
+            remove => RemoveHandler(SizeChangedEvent, value);
+        }
+
         public new IControl? Parent => (IControl?)base.Parent;
 
         /// <summary>
@@ -530,14 +546,35 @@ protected override void OnKeyUp(KeyEventArgs e)
             }
         }
 
+        /// <inheritdoc/>
         protected override void OnPropertyChanged(AvaloniaPropertyChangedEventArgs change)
         {
             base.OnPropertyChanged(change);
 
-            if (change.Property == FlowDirectionProperty)
+            if (change.Property == BoundsProperty)
+            {
+                var oldValue = change.GetOldValue<Rect>();
+                var newValue = change.GetNewValue<Rect>();
+
+                // Bounds is a Rect with an X/Y Position as well as Height/Width.
+                // This means it is possible for the Rect to change position but not size.
+                // Therefore, we want to explicity check only the size and raise an event
+                // only when that size has changed.
+                if (newValue.Size != oldValue.Size)
+                {
+                    var sizeChangedEventArgs = new SizeChangedEventArgs(
+                        SizeChangedEvent,
+                        source: this,
+                        previousSize: new Size(oldValue.Width, oldValue.Height),
+                        newSize: new Size(newValue.Width, newValue.Height));
+
+                    RaiseEvent(sizeChangedEventArgs);
+                }
+            }
+            else if (change.Property == FlowDirectionProperty)
             {
                 InvalidateMirrorTransform();
-                
+
                 foreach (var visual in VisualChildren)
                 {
                     if (visual is Control child)

src/Avalonia.Controls/SizeChangedEventArgs.cs

@@ -0,0 +1,83 @@
+using Avalonia.Interactivity;
+using Avalonia.Layout;
+using Avalonia.Utilities;
+
+namespace Avalonia.Controls
+{
+    /// <summary>
+    /// Provides data specific to a SizeChanged event.
+    /// </summary>
+    public class SizeChangedEventArgs : RoutedEventArgs
+    {
+        /// <summary>
+        /// Initializes a new instance of the <see cref="SizeChangedEventArgs"/> class.
+        /// </summary>
+        /// <param name="routedEvent">The routed event associated with these event args.</param>
+        public SizeChangedEventArgs(RoutedEvent? routedEvent)
+            : base (routedEvent)
+        {
+        }
+
+        /// <summary>
+        /// Initializes a new instance of the <see cref="SizeChangedEventArgs"/> class.
+        /// </summary>
+        /// <param name="routedEvent">The routed event associated with these event args.</param>
+        /// <param name="source">The source object that raised the routed event.</param>
+        public SizeChangedEventArgs(RoutedEvent? routedEvent, IInteractive? source)
+            : base(routedEvent, source)
+        {
+        }
+
+        /// <summary>
+        /// Initializes a new instance of the <see cref="SizeChangedEventArgs"/> class.
+        /// </summary>
+        /// <param name="routedEvent">The routed event associated with these event args.</param>
+        /// <param name="source">The source object that raised the routed event.</param>
+        /// <param name="previousSize">The previous size (or bounds) of the object.</param>
+        /// <param name="newSize">The new size (or bounds) of the object.</param>
+        public SizeChangedEventArgs(
+            RoutedEvent? routedEvent,
+            IInteractive? source,
+            Size previousSize,
+            Size newSize)
+            : base(routedEvent, source)
+        {
+            PreviousSize = previousSize;
+            NewSize = newSize;
+        }
+
+        /// <summary>
+        /// Gets a value indicating whether the height of the new size is considered
+        /// different than the previous size height.
+        /// </summary>
+        /// <remarks>
+        /// This will take into account layout epsilon and will not be true if both
+        /// heights are considered equivalent for layout purposes. Remember there can
+        /// be small variations in the calculations between layout cycles due to
+        /// rounding and precision even when the size has not otherwise changed.
+        /// </remarks>
+        public bool HeightChanged => !MathUtilities.AreClose(NewSize.Height, PreviousSize.Height, LayoutHelper.LayoutEpsilon);
+
+        /// <summary>
+        /// Gets the new size (or bounds) of the object.
+        /// </summary>
+        public Size NewSize { get; init; }
+
+        /// <summary>
+        /// Gets the previous size (or bounds) of the object.
+        /// </summary>
+        public Size PreviousSize { get; init; }
+
+        /// <summary>
+        /// Gets a value indicating whether the width of the new size is considered
+        /// different than the previous size width.
+        /// </summary>
+        /// <remarks>
+        /// This will take into account layout epsilon and will not be true if both
+        /// widths are considered equivalent for layout purposes. Remember there can
+        /// be small variations in the calculations between layout cycles due to
+        /// rounding and precision even when the size has not otherwise changed.
+        /// </remarks>
+        public bool WidthChanged => !MathUtilities.AreClose(NewSize.Width, PreviousSize.Width, LayoutHelper.LayoutEpsilon);
+    }
+}