Add SizeChanged event

src/Avalonia.Controls/Control.cs

@@ -84,6 +84,13 @@ namespace Avalonia.Controls
                 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.Bubble);
+
         /// <summary>
         /// Defines the <see cref="FlowDirection"/> property.
         /// </summary>
@@ -211,6 +218,15 @@ namespace Avalonia.Controls
             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 +546,28 @@ namespace Avalonia.Controls
             }
         }
 
+        /// <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>();
+
+                var sizeChangedEventArgs = new SizeChangedEventArgs(
+                    SizeChangedEvent,
+                    source: this,
+                    newSize: new Size(newValue.Width, newValue.Height),
+                    previousSize: new Size(oldValue.Width, oldValue.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 +1,60 @@
+using System;
+using System.Collections.Generic;
+using System.Linq;
+using System.Text;
+using System.Threading.Tasks;
+using Avalonia.Interactivity;
+
+namespace Avalonia.Controls
+{
+    /// <summary>
+    /// Provides data specific to a SizeChanged event.
+    /// </summary>
+    public class SizeChangedEventArgs : RoutedEventArgs
+    {
+        public SizeChangedEventArgs(RoutedEvent? routedEvent)
+            : base (routedEvent)
+        {
+        }
+
+        public SizeChangedEventArgs(RoutedEvent? routedEvent, IInteractive? source)
+            : base(routedEvent, source)
+        {
+        }
+
+        public SizeChangedEventArgs(
+            RoutedEvent? routedEvent,
+            IInteractive? source,
+            Size newSize,
+            Size previousSize)
+            : base(routedEvent, source)
+        {
+            NewSize = newSize;
+            PreviousSize = previousSize;
+            HeightChanged = newSize.Height != previousSize.Height;
+            WidthChanged = newSize.Width != previousSize.Width;
+        }
+
+        /// <summary>
+        /// Gets a value indicating whether the height of the new size is different
+        /// than the previous size height.
+        /// </summary>
+        public bool HeightChanged { get; init; }
+
+        /// <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 different
+        /// than the previous size width.
+        /// </summary>
+        public bool WidthChanged { get; init; }
+    }
+}