Added a Scroll event to ScrollBar

The event is fired when the ScrollBar's value is changed through
interaction with the ScrollBar's Track

src/Avalonia.Controls/Primitives/ScrollBar.cs

@@ -6,9 +6,21 @@ using System.Reactive;
 using System.Reactive.Linq;
 using Avalonia.Data;
 using Avalonia.Interactivity;
+using Avalonia.Input;
 
 namespace Avalonia.Controls.Primitives
 {
+    public class ScrollEventArgs : EventArgs
+    {
+        public ScrollEventArgs(ScrollEventType eventType, double newValue)
+        {
+            ScrollEventType = eventType;
+            NewValue = newValue;
+        }
+        public double NewValue { get; private set; }
+        public ScrollEventType ScrollEventType { get; private set; }
+    }
+
     /// <summary>
     /// A scrollbar control.
     /// </summary>
@@ -44,6 +56,9 @@ namespace Avalonia.Controls.Primitives
         {
             PseudoClass(OrientationProperty, o => o == Orientation.Vertical, ":vertical");
             PseudoClass(OrientationProperty, o => o == Orientation.Horizontal, ":horizontal");
+
+            Thumb.DragDeltaEvent.AddClassHandler<ScrollBar>(o => o.OnThumbDragDelta, RoutingStrategies.Bubble);
+            Thumb.DragCompletedEvent.AddClassHandler<ScrollBar>(o => o.OnThumbDragComplete, RoutingStrategies.Bubble);
         }
 
         /// <summary>
@@ -88,6 +103,8 @@ namespace Avalonia.Controls.Primitives
             set { SetValue(OrientationProperty, value); }
         }
 
+        public event EventHandler<ScrollEventArgs> Scroll;
+
         /// <summary>
         /// Calculates whether the scrollbar should be visible.
         /// </summary>
@@ -140,6 +157,8 @@ namespace Avalonia.Controls.Primitives
             _pageUpButton = e.NameScope.Find<Button>("PART_PageUpButton");
             _pageDownButton = e.NameScope.Find<Button>("PART_PageDownButton");
 
+
+
             if (_lineUpButton != null)
             {
                 _lineUpButton.Click += LineUpClick;
@@ -184,21 +203,39 @@ namespace Avalonia.Controls.Primitives
         private void SmallDecrement()
         {
             Value = Math.Max(Value - SmallChange * ViewportSize, Minimum);
+            OnScroll(ScrollEventType.SmallDecrement);
         }
 
         private void SmallIncrement()
         {
             Value = Math.Min(Value + SmallChange * ViewportSize, Maximum);
+            OnScroll(ScrollEventType.SmallIncrement);
         }
 
         private void LargeDecrement()
         {
             Value = Math.Max(Value - LargeChange * ViewportSize, Minimum);
+            OnScroll(ScrollEventType.LargeDecrement);
         }
 
         private void LargeIncrement()
         {
             Value = Math.Min(Value + LargeChange * ViewportSize, Maximum);
+            OnScroll(ScrollEventType.LargeIncrement);
+        }
+
+        private void OnThumbDragDelta(VectorEventArgs e)
+        {
+            OnScroll(ScrollEventType.ThumbTrack);
+        }
+        private void OnThumbDragComplete(VectorEventArgs e)
+        {
+            OnScroll(ScrollEventType.EndScroll);
+        }
+
+        protected void OnScroll(ScrollEventType scrollEventType)
+        {
+            Scroll?.Invoke(this, new ScrollEventArgs(scrollEventType, Value));
         }
     }
 }

src/Avalonia.Controls/Primitives/ScrollEventType.cs

@@ -0,0 +1,53 @@
+// Copyright (c) The Avalonia Project. All rights reserved.
+// Licensed under the MIT license. See licence.md file in the project root for full license information.
+
+namespace Avalonia.Controls.Primitives
+{
+    /// <summary>    
+    /// Specifies the type of Avalonia.Controls.Primitives.ScrollBar.Scroll event
+    /// that occurred.
+    /// </summary>
+    public enum ScrollEventType
+    {
+        /// <summary>    
+        /// Specifies that the Avalonia.Controls.Primitives.Thumb moved a specified
+        /// distance, as determined by the value of Avalonia.Controls.Primitives.RangeBase.SmallChange.
+        /// The Avalonia.Controls.Primitives.Thumb moved to the left for a horizontal
+        /// Avalonia.Controls.Primitives.ScrollBar or upward for a vertical Avalonia.Controls.Primitives.ScrollBar.
+        /// </summary>
+        SmallDecrement = 0,
+        /// <summary>    
+        /// Specifies that the Avalonia.Controls.Primitives.Thumb moved a specified
+        /// distance, as determined by the value of Avalonia.Controls.Primitives.RangeBase.SmallChange.
+        /// The Avalonia.Controls.Primitives.Thumb moved to the right for a horizontal
+        /// Avalonia.Controls.Primitives.ScrollBar or downward for a vertical Avalonia.Controls.Primitives.ScrollBar.
+        /// </summary>
+        SmallIncrement = 1,
+        /// <summary>    
+        /// Specifies that the Avalonia.Controls.Primitives.Thumb moved a specified
+        /// distance, as determined by the value of Avalonia.Controls.Primitives.RangeBase.LargeChange.
+        /// The Avalonia.Controls.Primitives.Thumb moved to the left for a horizontal
+        /// Avalonia.Controls.Primitives.ScrollBar or upward for a vertical Avalonia.Controls.Primitives.ScrollBar.
+        /// </summary>
+        LargeDecrement = 2,
+        /// <summary>    
+        /// Specifies that the Avalonia.Controls.Primitives.Thumb moved a specified
+        /// distance, as determined by the value of Avalonia.Controls.Primitives.RangeBase.LargeChange.
+        /// The Avalonia.Controls.Primitives.Thumb moved to the right for a horizontal
+        /// Avalonia.Controls.Primitives.ScrollBar or downward for a vertical Avalonia.Controls.Primitives.ScrollBar.
+        /// </summary>
+        LargeIncrement = 3,
+        /// <summary>    
+        /// The Avalonia.Controls.Primitives.Thumb was dragged and caused a Avalonia.UIElement.MouseMove
+        /// event. A Avalonia.Controls.Primitives.ScrollBar.Scroll event of this Avalonia.Controls.Primitives.ScrollEventType
+        /// may occur more than one time when the Avalonia.Controls.Primitives.Thumb
+        /// is dragged in the Avalonia.Controls.Primitives.ScrollBar.
+        /// </summary>
+        ThumbTrack = 4,
+        /// <summary>    
+        /// Specifies that the Avalonia.Controls.Primitives.Thumb was dragged to a
+        /// new position and is now no longer being dragged by the user.
+        /// </summary>
+        EndScroll = 5
+    }
+}