Fix large/small scroll in ScrollViewer.

For non-logical scrolling:

- Use 16 for small scroll size (value taken from WPF)
- Use viewport size for large scroll

For logical scrolling, use the `ScrollSize`/`PageScrollSize` defined on `ILogicalScrollable`. Note that this required a small breaking change to `ILogicalScrollable`.

Fixed #3245

src/Avalonia.Controls/Presenters/ItemVirtualizer.cs

@@ -6,6 +6,7 @@ using Avalonia.Controls.Primitives;
 using Avalonia.Controls.Utils;
 using Avalonia.Input;
 using Avalonia.Layout;
+using Avalonia.VisualTree;
 
 namespace Avalonia.Controls.Presenters
 {
@@ -99,9 +100,14 @@ namespace Avalonia.Controls.Presenters
         {
             get
             {
-                return Vertical ?
-                    new Size(Owner.Panel.DesiredSize.Width, ExtentValue) :
-                    new Size(ExtentValue, Owner.Panel.DesiredSize.Height);
+                if (IsLogicalScrollEnabled)
+                {
+                    return Vertical ?
+                        new Size(Owner.Panel.DesiredSize.Width, ExtentValue) :
+                        new Size(ExtentValue, Owner.Panel.DesiredSize.Height);
+                }
+
+                return default;
             }
         }
 
@@ -112,9 +118,14 @@ namespace Avalonia.Controls.Presenters
         {
             get
             {
-                return Vertical ? 
-                    new Size(Owner.Panel.Bounds.Width, ViewportValue) :
-                    new Size(ViewportValue, Owner.Panel.Bounds.Height);
+                if (IsLogicalScrollEnabled)
+                {
+                    return Vertical ?
+                        new Size(Owner.Panel.Bounds.Width, ViewportValue) :
+                        new Size(ViewportValue, Owner.Panel.Bounds.Height);
+                }
+
+                return default;
             }
         }
 
@@ -125,11 +136,21 @@ namespace Avalonia.Controls.Presenters
         {
             get
             {
-                return Vertical ? new Vector(_crossAxisOffset, OffsetValue) : new Vector(OffsetValue, _crossAxisOffset);
+                if (IsLogicalScrollEnabled)
+                {
+                    return Vertical ? new Vector(_crossAxisOffset, OffsetValue) : new Vector(OffsetValue, _crossAxisOffset);
+                }
+
+                return default;
             }
 
             set
             {
+                if (!IsLogicalScrollEnabled)
+                {
+                    throw new NotSupportedException("Logical scrolling disabled.");
+                }
+
                 var oldCrossAxisOffset = _crossAxisOffset;
 
                 if (Vertical)
@@ -164,10 +185,10 @@ namespace Avalonia.Controls.Presenters
             }
 
             var virtualizingPanel = owner.Panel as IVirtualizingPanel;
-            var scrollable = (ILogicalScrollable)owner;
+            var scrollContentPresenter = owner.Parent as IScrollable;
             ItemVirtualizer result = null;
 
-            if (virtualizingPanel != null && scrollable.InvalidateScroll != null)
+            if (virtualizingPanel != null && scrollContentPresenter is object)
             {
                 switch (owner.VirtualizationMode)
                 {
@@ -277,6 +298,6 @@ namespace Avalonia.Controls.Presenters
         /// <summary>
         /// Invalidates the current scroll.
         /// </summary>
-        protected void InvalidateScroll() => ((ILogicalScrollable)Owner).InvalidateScroll?.Invoke();
+        protected void InvalidateScroll() => ((ILogicalScrollable)Owner).RaiseScrollInvalidated(EventArgs.Empty);
     }
 }

src/Avalonia.Controls/Presenters/ItemsPresenter.cs

@@ -21,6 +21,7 @@ namespace Avalonia.Controls.Presenters
 
         private bool _canHorizontallyScroll;
         private bool _canVerticallyScroll;
+        EventHandler _scrollInvalidated;
 
         /// <summary>
         /// Initializes static members of the <see cref="ItemsPresenter"/> class.
@@ -95,13 +96,17 @@ namespace Avalonia.Controls.Presenters
         Size IScrollable.Viewport => Virtualizer?.Viewport ?? Bounds.Size;
 
         /// <inheritdoc/>
-        Action ILogicalScrollable.InvalidateScroll { get; set; }
+        event EventHandler ILogicalScrollable.ScrollInvalidated
+        {
+            add => _scrollInvalidated += value;
+            remove => _scrollInvalidated -= value;
+        }
 
         /// <inheritdoc/>
-        Size ILogicalScrollable.ScrollSize => new Size(1, 1);
+        Size ILogicalScrollable.ScrollSize => new Size(16, 1);
 
         /// <inheritdoc/>
-        Size ILogicalScrollable.PageScrollSize => new Size(0, 1);
+        Size ILogicalScrollable.PageScrollSize => Virtualizer?.Viewport ?? new Size(16, 16);
 
         internal ItemVirtualizer Virtualizer { get; private set; }
 
@@ -117,6 +122,12 @@ namespace Avalonia.Controls.Presenters
             return Virtualizer?.GetControlInDirection(direction, from);
         }
 
+        /// <inheritdoc/>
+        void ILogicalScrollable.RaiseScrollInvalidated(EventArgs e)
+        {
+            _scrollInvalidated?.Invoke(this, e);
+        }
+
         public override void ScrollIntoView(object item)
         {
             Virtualizer?.ScrollIntoView(item);
@@ -138,7 +149,7 @@ namespace Avalonia.Controls.Presenters
         {
             Virtualizer?.Dispose();
             Virtualizer = ItemVirtualizer.Create(this);
-            ((ILogicalScrollable)this).InvalidateScroll?.Invoke();
+            _scrollInvalidated?.Invoke(this, EventArgs.Empty);
 
             KeyboardNavigation.SetTabNavigation(
                 (InputElement)Panel,
@@ -162,7 +173,7 @@ namespace Avalonia.Controls.Presenters
         {
             Virtualizer?.Dispose();
             Virtualizer = ItemVirtualizer.Create(this);
-            ((ILogicalScrollable)this).InvalidateScroll?.Invoke();
+            _scrollInvalidated?.Invoke(this, EventArgs.Empty);
         }
     }
 }

src/Avalonia.Controls/Presenters/ScrollContentPresenter.cs

@@ -3,8 +3,10 @@ using System.Collections.Generic;
 using System.Linq;
 using System.Reactive.Disposables;
 using System.Reactive.Linq;
+using System.Runtime.InteropServices.ComTypes;
 using Avalonia.Controls.Primitives;
 using Avalonia.Input;
+using Avalonia.LogicalTree;
 using Avalonia.VisualTree;
 
 namespace Avalonia.Controls.Presenters
@@ -349,7 +351,7 @@ namespace Avalonia.Controls.Presenters
 
             if (scrollable != null)
             {
-                scrollable.InvalidateScroll = () => UpdateFromScrollable(scrollable);
+                scrollable.ScrollInvalidated += ScrollInvalidated;
 
                 if (scrollable.IsLogicalScrollEnabled)
                 {
@@ -360,12 +362,17 @@ namespace Avalonia.Controls.Presenters
                             .Subscribe(x => scrollable.CanVerticallyScroll = x),
                         this.GetObservable(OffsetProperty)
                             .Skip(1).Subscribe(x => scrollable.Offset = x),
-                        Disposable.Create(() => scrollable.InvalidateScroll = null));
+                        Disposable.Create(() => scrollable.ScrollInvalidated -= ScrollInvalidated));
                     UpdateFromScrollable(scrollable);
                 }
             }
         }
 
+        private void ScrollInvalidated(object sender, EventArgs e)
+        {
+            UpdateFromScrollable((ILogicalScrollable)sender);
+        }
+
         private void UpdateFromScrollable(ILogicalScrollable scrollable)
         {
             var logicalScroll = _logicalScrollSubscription != null;

src/Avalonia.Controls/Primitives/ILogicalScrollable.cs

@@ -31,22 +31,6 @@ namespace Avalonia.Controls.Primitives
         /// </summary>
         bool IsLogicalScrollEnabled { get; }
 
-        /// <summary>
-        /// Gets or sets the scroll invalidation method.
-        /// </summary>
-        /// <remarks>
-        /// <para>
-        /// This method notifies the attached <see cref="ScrollViewer"/> of a change in 
-        /// the <see cref="IScrollable.Extent"/>, <see cref="IScrollable.Offset"/> or 
-        /// <see cref="IScrollable.Viewport"/> properties.
-        /// </para>
-        /// <para>
-        /// This property is set by the parent <see cref="ScrollViewer"/> when the 
-        /// <see cref="ILogicalScrollable"/> is placed inside it.
-        /// </para>
-        /// </remarks>
-        Action InvalidateScroll { get; set; }
-
         /// <summary>
         /// Gets the size to scroll by, in logical units.
         /// </summary>
@@ -57,6 +41,15 @@ namespace Avalonia.Controls.Primitives
         /// </summary>
         Size PageScrollSize { get; }
 
+        /// <summary>
+        /// Raised when the scroll is invalidated.
+        /// </summary>
+        /// <remarks>
+        /// This event notifies an attached <see cref="ScrollViewer"/> of a change in 
+        /// one of the scroll properties.
+        /// </remarks>
+        event EventHandler ScrollInvalidated;
+
         /// <summary>
         /// Attempts to bring a portion of the target visual into view by scrolling the content.
         /// </summary>
@@ -72,5 +65,11 @@ namespace Avalonia.Controls.Primitives
         /// <param name="from">The control from which movement begins.</param>
         /// <returns>The control.</returns>
         IControl GetControlInDirection(NavigationDirection direction, IControl from);
+
+        /// <summary>
+        /// Raises the <see cref="ScrollInvalidated"/> event.
+        /// </summary>
+        /// <param name="e">The event args.</param>
+        void RaiseScrollInvalidated(EventArgs e);
     }
 }

src/Avalonia.Controls/ScrollViewer.cs

@@ -10,6 +10,8 @@ namespace Avalonia.Controls
     /// </summary>
     public class ScrollViewer : ContentControl, IScrollable, IScrollAnchorProvider
     {
+        private static readonly Size s_defaultSmallChange = new Size(16, 16);
+
         /// <summary>
         /// Defines the <see cref="CanHorizontallyScroll"/> property.
         /// </summary>
@@ -59,6 +61,22 @@ namespace Avalonia.Controls
                 o => o.Viewport,
                 (o, v) => o.Viewport = v);
 
+        /// <summary>
+        /// Defines the <see cref="LargeChange"/> property.
+        /// </summary>
+        public static readonly DirectProperty<ScrollViewer, Size> LargeChangeProperty =
+            AvaloniaProperty.RegisterDirect<ScrollViewer, Size>(
+                nameof(LargeChange),
+                o => o.LargeChange);
+
+        /// <summary>
+        /// Defines the <see cref="SmallChange"/> property.
+        /// </summary>
+        public static readonly DirectProperty<ScrollViewer, Size> SmallChangeProperty =
+            AvaloniaProperty.RegisterDirect<ScrollViewer, Size>(
+                nameof(SmallChange),
+                o => o.SmallChange);
+
         /// <summary>
         /// Defines the HorizontalScrollBarMaximum property.
         /// </summary>
@@ -149,9 +167,13 @@ namespace Avalonia.Controls
                 nameof(VerticalScrollBarVisibility),
                 ScrollBarVisibility.Auto);
 
+        private IDisposable _childSubscription;
+        private ILogicalScrollable _logicalScrollable;
         private Size _extent;
         private Vector _offset;
         private Size _viewport;
+        private Size _largeChange;
+        private Size _smallChange = s_defaultSmallChange;
 
         /// <summary>
         /// Initializes static members of the <see cref="ScrollViewer"/> class.
@@ -228,6 +250,16 @@ namespace Avalonia.Controls
             }
         }
 
+        /// <summary>
+        /// Gets the large (page) change value for the scroll viewer.
+        /// </summary>
+        public Size LargeChange => _largeChange;
+
+        /// <summary>
+        /// Gets the small (line) change value for the scroll viewer.
+        /// </summary>
+        public Size SmallChange => _smallChange;
+
         /// <summary>
         /// Gets or sets the horizontal scrollbar visibility.
         /// </summary>
@@ -246,22 +278,6 @@ namespace Avalonia.Controls
             set { SetValue(VerticalScrollBarVisibilityProperty, value); }
         }
 
-        /// <summary>
-        /// Scrolls to the top-left corner of the content.
-        /// </summary>
-        public void ScrollToHome()
-        {
-            Offset = new Vector(double.NegativeInfinity, double.NegativeInfinity);
-        }
-
-        /// <summary>
-        /// Scrolls to the bottom-left corner of the content.
-        /// </summary>
-        public void ScrollToEnd()
-        {
-            Offset = new Vector(double.NegativeInfinity, double.PositiveInfinity);
-        }
-
         /// <summary>
         /// Gets a value indicating whether the viewer can scroll horizontally.
         /// </summary>
@@ -347,6 +363,22 @@ namespace Avalonia.Controls
         /// <inheritdoc/>
         IControl IScrollAnchorProvider.CurrentAnchor => null; // TODO: Implement
 
+        /// <summary>
+        /// Scrolls to the top-left corner of the content.
+        /// </summary>
+        public void ScrollToHome()
+        {
+            Offset = new Vector(double.NegativeInfinity, double.NegativeInfinity);
+        }
+
+        /// <summary>
+        /// Scrolls to the bottom-left corner of the content.
+        /// </summary>
+        public void ScrollToEnd()
+        {
+            Offset = new Vector(double.NegativeInfinity, double.PositiveInfinity);
+        }
+
         /// <summary>
         /// Gets the value of the HorizontalScrollBarVisibility attached property.
         /// </summary>
@@ -397,6 +429,22 @@ namespace Avalonia.Controls
             // TODO: Implement
         }
 
+        protected override bool RegisterContentPresenter(IContentPresenter presenter)
+        {
+            _childSubscription?.Dispose();
+            _childSubscription = null;
+
+            if (base.RegisterContentPresenter(presenter))
+            {
+                _childSubscription = Presenter?
+                    .GetObservable(ContentPresenter.ChildProperty)
+                    .Subscribe(ChildChanged);
+                return true;
+            }
+
+            return false;
+        }
+
         internal static Vector CoerceOffset(Size extent, Size viewport, Vector offset)
         {
             var maxX = Math.Max(extent.Width - viewport.Width, 0);
@@ -431,6 +479,28 @@ namespace Avalonia.Controls
             }
         }
 
+        private void ChildChanged(IControl child)
+        {
+            if (_logicalScrollable is object)
+            {
+                _logicalScrollable.ScrollInvalidated -= LogicalScrollInvalidated;
+                _logicalScrollable = null;
+            }
+
+            if (child is ILogicalScrollable logical)
+            {
+                _logicalScrollable = logical;
+                logical.ScrollInvalidated += LogicalScrollInvalidated;
+            }
+
+            CalculatedPropertiesChanged();
+        }
+
+        private void LogicalScrollInvalidated(object sender, EventArgs e)
+        {
+            CalculatedPropertiesChanged();
+        }
+
         private void ScrollBarVisibilityChanged(AvaloniaPropertyChangedEventArgs e)
         {
             var wasEnabled = !ScrollBarVisibility.Disabled.Equals(e.OldValue);
@@ -465,6 +535,17 @@ namespace Avalonia.Controls
             RaisePropertyChanged(VerticalScrollBarMaximumProperty, 0, VerticalScrollBarMaximum);
             RaisePropertyChanged(VerticalScrollBarValueProperty, 0, VerticalScrollBarValue);
             RaisePropertyChanged(VerticalScrollBarViewportSizeProperty, 0, VerticalScrollBarViewportSize);
+
+            if (_logicalScrollable?.IsLogicalScrollEnabled == true)
+            {
+                SetAndRaise(SmallChangeProperty, ref _smallChange, _logicalScrollable.ScrollSize);
+                SetAndRaise(LargeChangeProperty, ref _largeChange, _logicalScrollable.PageScrollSize);
+            }
+            else
+            {
+                SetAndRaise(SmallChangeProperty, ref _smallChange, s_defaultSmallChange);
+                SetAndRaise(LargeChangeProperty, ref _largeChange, Viewport);
+            }
         }
 
         protected override void OnKeyDown(KeyEventArgs e)

src/Avalonia.Themes.Default/ScrollViewer.xaml

@@ -22,6 +22,8 @@
         </ScrollContentPresenter>
         <ScrollBar Name="horizontalScrollBar"
                    Orientation="Horizontal"
+                   LargeChange="{Binding LargeChange.Width, RelativeSource={RelativeSource TemplatedParent}}"
+                   SmallChange="{Binding SmallChange.Width, RelativeSource={RelativeSource TemplatedParent}}"
                    Maximum="{TemplateBinding HorizontalScrollBarMaximum}"
                    Value="{TemplateBinding HorizontalScrollBarValue, Mode=TwoWay}"
                    ViewportSize="{TemplateBinding HorizontalScrollBarViewportSize}"
@@ -30,6 +32,8 @@
                    Focusable="False"/>
         <ScrollBar Name="verticalScrollBar"
                    Orientation="Vertical"
+                   LargeChange="{Binding LargeChange.Height, RelativeSource={RelativeSource TemplatedParent}}"
+                   SmallChange="{Binding SmallChange.Height, RelativeSource={RelativeSource TemplatedParent}}"
                    Maximum="{TemplateBinding VerticalScrollBarMaximum}"
                    Value="{TemplateBinding VerticalScrollBarValue, Mode=TwoWay}"
                    ViewportSize="{TemplateBinding VerticalScrollBarViewportSize}"

tests/Avalonia.Controls.UnitTests/Presenters/ScrollContentPresenterTests_ILogicalScrollable.cs

@@ -101,7 +101,7 @@ namespace Avalonia.Controls.UnitTests
 
             target.UpdateChild();
 
-            Assert.NotNull(scrollable.InvalidateScroll);
+            Assert.True(scrollable.HasScrollInvalidatedSubscriber);
         }
 
         [Fact]
@@ -117,7 +117,7 @@ namespace Avalonia.Controls.UnitTests
             target.Content = null;
             target.UpdateChild();
 
-            Assert.Null(scrollable.InvalidateScroll);
+            Assert.False(scrollable.HasScrollInvalidatedSubscriber);
         }
 
         [Fact]
@@ -217,7 +217,7 @@ namespace Avalonia.Controls.UnitTests
             Assert.Equal(new Rect(0, 0, 100, 100), scrollable.Bounds);
 
             scrollable.IsLogicalScrollEnabled = false;
-            scrollable.InvalidateScroll();
+            scrollable.RaiseScrollInvalidated(EventArgs.Empty);
             target.Measure(new Size(100, 100));
             target.Arrange(new Rect(0, 0, 100, 100));
 
@@ -227,7 +227,7 @@ namespace Avalonia.Controls.UnitTests
             Assert.Equal(new Rect(0, 0, 150, 150), scrollable.Bounds);
 
             scrollable.IsLogicalScrollEnabled = true;
-            scrollable.InvalidateScroll();
+            scrollable.RaiseScrollInvalidated(EventArgs.Empty);
             target.Measure(new Size(100, 100));
             target.Arrange(new Rect(0, 0, 100, 100));
 
@@ -318,12 +318,20 @@ namespace Avalonia.Controls.UnitTests
             private Size _extent;
             private Vector _offset;
             private Size _viewport;
+            private EventHandler _scrollInvalidated;
 
             public bool CanHorizontallyScroll { get; set; }
             public bool CanVerticallyScroll { get; set; }
             public bool IsLogicalScrollEnabled { get; set; } = true;
             public Size AvailableSize { get; private set; }
-            public Action InvalidateScroll { get; set; }
+
+            public bool HasScrollInvalidatedSubscriber => _scrollInvalidated != null;
+            
+            public event EventHandler ScrollInvalidated
+            {
+                add => _scrollInvalidated += value;
+                remove => _scrollInvalidated -= value;
+            }
 
             public Size Extent
             {
@@ -331,7 +339,7 @@ namespace Avalonia.Controls.UnitTests
                 set
                 {
                     _extent = value;
-                    InvalidateScroll?.Invoke();
+                    _scrollInvalidated?.Invoke(this, EventArgs.Empty);
                 }
             }
 
@@ -341,7 +349,7 @@ namespace Avalonia.Controls.UnitTests
                 set
                 {
                     _offset = value;
-                    InvalidateScroll?.Invoke();
+                    _scrollInvalidated?.Invoke(this, EventArgs.Empty);
                 }
             }
 
@@ -351,7 +359,7 @@ namespace Avalonia.Controls.UnitTests
                 set
                 {
                     _viewport = value;
-                    InvalidateScroll?.Invoke();
+                    _scrollInvalidated?.Invoke(this, EventArgs.Empty);
                 }
             }
 
@@ -376,6 +384,11 @@ namespace Avalonia.Controls.UnitTests
                 throw new NotImplementedException();
             }
 
+            public void RaiseScrollInvalidated(EventArgs e)
+            {
+                _scrollInvalidated?.Invoke(this, e);
+            }
+
             protected override Size MeasureOverride(Size availableSize)
             {
                 AvailableSize = availableSize;
@@ -388,4 +401,4 @@ namespace Avalonia.Controls.UnitTests
             }
         }
     }
-}
+}

tests/Avalonia.Controls.UnitTests/ScrollViewerTests.cs

@@ -4,6 +4,8 @@ using Avalonia.Controls.Presenters;
 using Avalonia.Controls.Primitives;
 using Avalonia.Controls.Templates;
 using Avalonia.Layout;
+using Avalonia.LogicalTree;
+using Moq;
 using Xunit;
 
 namespace Avalonia.Controls.UnitTests
@@ -86,6 +88,65 @@ namespace Avalonia.Controls.UnitTests
             Assert.Equal(new Vector(0, 40), target.Offset);
         }
 
+        [Fact]
+        public void SmallChange_Should_Be_16()
+        {
+            var target = new ScrollViewer();
+
+            Assert.Equal(new Size(16, 16), target.SmallChange);
+        }
+
+        [Fact]
+        public void LargeChange_Should_Be_Viewport()
+        {
+            var target = new ScrollViewer();
+
+            target.SetValue(ScrollViewer.ViewportProperty, new Size(104, 143));
+            Assert.Equal(new Size(104, 143), target.LargeChange);
+        }
+
+        [Fact]
+        public void SmallChange_Should_Come_From_ILogicalScrollable_If_Present()
+        {
+            var child = new Mock<Control>();
+            var logicalScroll = child.As<ILogicalScrollable>();
+
+            logicalScroll.Setup(x => x.IsLogicalScrollEnabled).Returns(true);
+            logicalScroll.Setup(x => x.ScrollSize).Returns(new Size(12, 43));
+
+            var target = new ScrollViewer
+            {
+                Template = new FuncControlTemplate<ScrollViewer>(CreateTemplate),
+                Content = child.Object,
+            };
+
+            target.ApplyTemplate();
+            ((ContentPresenter)target.Presenter).UpdateChild();
+
+            Assert.Equal(new Size(12, 43), target.SmallChange);
+        }
+
+        [Fact]
+        public void LargeChange_Should_Come_From_ILogicalScrollable_If_Present()
+        {
+            var child = new Mock<Control>();
+            var logicalScroll = child.As<ILogicalScrollable>();
+
+            logicalScroll.Setup(x => x.IsLogicalScrollEnabled).Returns(true);
+            logicalScroll.Setup(x => x.PageScrollSize).Returns(new Size(45, 67));
+
+            var target = new ScrollViewer
+            {
+                Template = new FuncControlTemplate<ScrollViewer>(CreateTemplate),
+                Content = child.Object,
+            };
+
+            target.ApplyTemplate();
+            ((ContentPresenter)target.Presenter).UpdateChild();
+
+            Assert.Equal(new Size(45, 67), target.LargeChange);
+        }
+
         private Control CreateTemplate(ScrollViewer control, INameScope scope)
         {
             return new Grid