Added ZoomChanged event

Fixes #38

samples/AvaloniaDemo/MainWindow.axaml.cs

@@ -1,4 +1,5 @@
-using Avalonia;
+using System.Diagnostics;
+using Avalonia;
 using Avalonia.Controls;
 using Avalonia.Controls.PanAndZoom;
 using Avalonia.Input;
@@ -19,6 +20,8 @@ namespace AvaloniaDemo
             if (_zoomBorder != null)
             {
                 _zoomBorder.KeyDown += ZoomBorder_KeyDown;
+                
+                _zoomBorder.ZoomChanged += ZoomBorder_ZoomChanged;
             }
         }
 
@@ -46,5 +49,10 @@ namespace AvaloniaDemo
                     break;
             }
         }
+
+        private void ZoomBorder_ZoomChanged(object sender, ZoomChangedEventArgs e)
+        {
+            Debug.WriteLine($"[ZoomChanged] {e.ZoomX} {e.ZoomY} {e.OffsetX} {e.OffsetY}");
+        }
     }
 }

src/Avalonia.Controls.PanAndZoom/ZoomBorder.cs

@@ -7,6 +7,39 @@ using static System.Math;
 
 namespace Avalonia.Controls.PanAndZoom
 {
+    /// <summary>
+    /// Zoom changed event arguments.
+    /// </summary>
+    public class ZoomChangedEventArgs : EventArgs
+    {
+        /// <summary>
+        /// Gets the zoom ratio for x axis.
+        /// </summary>
+        public double ZoomX { get; set; }
+        
+        /// <summary>
+        /// Gets the zoom ratio for y axis.
+        /// </summary>
+        public double ZoomY { get; set; }
+        
+        /// <summary>
+        /// Gets the pan offset for x axis.
+        /// </summary>
+        public double OffsetX { get; set; }
+        
+        /// <summary>
+        /// Gets the pan offset for y axis.
+        /// </summary>
+        public double OffsetY { get; set; }
+    }
+
+    /// <summary>
+    /// Zoom changed event handler.
+    /// </summary>
+    /// <param name="sender">The sender object.</param>
+    /// <param name="e">Zoom changed event arguments.</param>
+    public delegate void ZoomChangedEventHandler(object sender, ZoomChangedEventArgs e);
+
     /// <summary>
     /// Pan and zoom control for Avalonia.
     /// </summary>
@@ -191,6 +224,11 @@ namespace Avalonia.Controls.PanAndZoom
         private double _offsetY = 0.0;
         private bool _captured = false;
 
+        /// <summary>
+        /// Zoom changed event.
+        /// </summary>
+        public event ZoomChangedEventHandler ZoomChanged;
+        
         /// <summary>
         /// Gets or sets invalidate action for border child element.
         /// </summary>
@@ -394,6 +432,18 @@ namespace Avalonia.Controls.PanAndZoom
             this.GetObservable(ChildProperty).Subscribe(ChildChanged);
         }
 
+        private void RaiseZoomChanged()
+        {
+            var args = new ZoomChangedEventArgs()
+            {
+                ZoomX = _zoomX,
+                ZoomY =  _zoomY,
+                OffsetX = _offsetX,
+                OffsetY =  _offsetY
+            };
+            OnZoomChanged(args);
+        }
+
         private void Defaults()
         {
             _isPanning = false;
@@ -583,6 +633,15 @@ namespace Avalonia.Controls.PanAndZoom
             _matrix = new Matrix(zoomX, 0.0, 0.0, zoomY, offsetX, offsetY);
         }
 
+        /// <summary>
+        /// Raises <see cref="ZoomChanged"/> event.
+        /// </summary>
+        /// <param name="e">Zoom changed event arguments.</param>
+        protected virtual void OnZoomChanged(ZoomChangedEventArgs e)
+        {
+            ZoomChanged?.Invoke(this, e);
+        }
+
         /// <summary>
         /// Invalidate child element.
         /// </summary>
@@ -612,6 +671,7 @@ namespace Avalonia.Controls.PanAndZoom
             _element.RenderTransformOrigin = new RelativePoint(new Point(0, 0), RelativeUnit.Relative);
             _element.RenderTransform = new MatrixTransform(_matrix);
             _element.InvalidateVisual();
+            RaiseZoomChanged();
         }
 
         /// <summary>