Initial impl. of scrolling with arrow keys.

- Currently only vertical implemented
- Doesn't handle partially visible items at end of list

Avalonia.sln

@@ -161,6 +161,8 @@ EndProject
 Global
 	GlobalSection(SharedMSBuildProjectFiles) = preSolution
 		src\Shared\RenderHelpers\RenderHelpers.projitems*{fb05ac90-89ba-4f2f-a924-f37875fb547c}*SharedItemsImports = 4
+		src\Shared\PlatformSupport\PlatformSupport.projitems*{4488ad85-1495-4809-9aa4-ddfe0a48527e}*SharedItemsImports = 4
+		src\Shared\PlatformSupport\PlatformSupport.projitems*{7b92af71-6287-4693-9dcb-bd5b6e927e23}*SharedItemsImports = 4
 		src\Shared\PlatformSupport\PlatformSupport.projitems*{e4d9629c-f168-4224-3f51-a5e482ffbc42}*SharedItemsImports = 13
 		src\Skia\Avalonia.Skia\Avalonia.Skia.projitems*{2f59f3d0-748d-4652-b01e-e0d954756308}*SharedItemsImports = 13
 		src\Shared\PlatformSupport\PlatformSupport.projitems*{db070a10-bf39-4752-8456-86e9d5928478}*SharedItemsImports = 4
@@ -168,11 +170,13 @@ Global
 		src\Skia\Avalonia.Skia\Avalonia.Skia.projitems*{925dd807-b651-475f-9f7c-cbeb974ce43d}*SharedItemsImports = 4
 		samples\TestApplicationShared\TestApplicationShared.projitems*{78345174-5b52-4a14-b9fd-d5f2428137f0}*SharedItemsImports = 13
 		src\Shared\PlatformSupport\PlatformSupport.projitems*{54f237d5-a70a-4752-9656-0c70b1a7b047}*SharedItemsImports = 4
+		samples\TestApplicationShared\TestApplicationShared.projitems*{ff69b927-c545-49ae-8e16-3d14d621aa12}*SharedItemsImports = 4
 		src\Shared\RenderHelpers\RenderHelpers.projitems*{3c4c0cb4-0c0f-4450-a37b-148c84ff905f}*SharedItemsImports = 13
 		src\Shared\PlatformSupport\PlatformSupport.projitems*{811a76cf-1cf6-440f-963b-bbe31bd72a82}*SharedItemsImports = 4
 		src\Shared\PlatformSupport\PlatformSupport.projitems*{88060192-33d5-4932-b0f9-8bd2763e857d}*SharedItemsImports = 4
 		src\Shared\RenderHelpers\RenderHelpers.projitems*{47be08a7-5985-410b-9ffc-2264b8ea595f}*SharedItemsImports = 4
 		src\Skia\Avalonia.Skia\Avalonia.Skia.projitems*{47be08a7-5985-410b-9ffc-2264b8ea595f}*SharedItemsImports = 4
+		samples\TestApplicationShared\TestApplicationShared.projitems*{8c923867-8a8f-4f6b-8b80-47d9e8436166}*SharedItemsImports = 4
 		samples\TestApplicationShared\TestApplicationShared.projitems*{e3a1060b-50d0-44e8-88b6-f44ef2e5bd72}*SharedItemsImports = 4
 		src\Shared\RenderHelpers\RenderHelpers.projitems*{bd43f7c0-396b-4aa1-bad9-dfde54d51298}*SharedItemsImports = 4
 		src\Skia\Avalonia.Skia\Avalonia.Skia.projitems*{bd43f7c0-396b-4aa1-bad9-dfde54d51298}*SharedItemsImports = 4

src/Avalonia.Controls/IVirtualizingPanel.cs

@@ -1,9 +1,6 @@
 // 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.
 
-using System;
-using Avalonia.Controls.Primitives;
-
 namespace Avalonia.Controls
 {
     /// <summary>
@@ -25,11 +22,21 @@ namespace Avalonia.Controls
         /// <summary>
         /// Gets a value indicating whether the panel is full.
         /// </summary>
+        /// <remarks>
+        /// This property should return false until enough children are added to fill the space
+        /// passed into the last measure in the direction of scroll. It should be updated
+        /// immediately after a child is added or removed.
+        /// </remarks>
         bool IsFull { get; }
 
         /// <summary>
         /// Gets the number of items that can be removed while keeping the panel full.
         /// </summary>
+        /// <remarks>
+        /// This property should return the number of children that are completely out of the
+        /// panel's current bounds in the direction of scroll. It should be updated after an
+        /// arrange.
+        /// </remarks>
         int OverflowCount { get; }
 
         /// <summary>

src/Avalonia.Controls/Presenters/ItemVirtualizer.cs

@@ -6,6 +6,7 @@ using System.Collections;
 using System.Collections.Specialized;
 using Avalonia.Controls.Primitives;
 using Avalonia.Controls.Utils;
+using Avalonia.Input;
 using Avalonia.VisualTree;
 
 namespace Avalonia.Controls.Presenters
@@ -145,6 +146,17 @@ namespace Avalonia.Controls.Presenters
         {
         }
 
+        /// <summary>
+        /// Gets the next control in the specified direction.
+        /// </summary>
+        /// <param name="direction">The movement direction.</param>
+        /// <param name="from">The control from which movement begins.</param>
+        /// <returns>The control.</returns>
+        public virtual IControl GetControlInDirection(FocusNavigationDirection direction, IControl from)
+        {
+            return null;
+        }
+
         /// <summary>
         /// Called when the items for the presenter change, either because 
         /// <see cref="ItemsPresenterBase.Items"/> has been set, the items collection has been
@@ -168,5 +180,10 @@ namespace Avalonia.Controls.Presenters
         {
             return false;
         }
+
+        /// <summary>
+        /// Invalidates the current scroll.
+        /// </summary>
+        protected void InvalidateScroll() => ((ILogicalScrollable)Owner).InvalidateScroll();
     }
 }

src/Avalonia.Controls/Presenters/ItemVirtualizerSimple.cs

@@ -5,8 +5,8 @@ using System;
 using System.Collections;
 using System.Collections.Specialized;
 using System.Linq;
-using Avalonia.Controls.Primitives;
 using Avalonia.Controls.Utils;
+using Avalonia.Input;
 
 namespace Avalonia.Controls.Presenters
 {
@@ -87,7 +87,7 @@ namespace Avalonia.Controls.Presenters
         public override void UpdateControls()
         {
             CreateAndRemoveContainers();
-            ((ILogicalScrollable)Owner).InvalidateScroll();
+            InvalidateScroll();
         }
 
         /// <inheritdoc/>
@@ -134,7 +134,47 @@ namespace Avalonia.Controls.Presenters
                 VirtualizingPanel.Children.Clear();
             }
 
-            ((ILogicalScrollable)Owner).InvalidateScroll();
+            InvalidateScroll();
+        }
+
+        public override IControl GetControlInDirection(FocusNavigationDirection direction, IControl from)
+        {
+            var generator = Owner.ItemContainerGenerator;
+            var itemIndex = generator.IndexFromContainer(from);
+
+            if (itemIndex == -1)
+            {
+                return null;
+            }
+
+            var newItemIndex = -1;
+
+            if (VirtualizingPanel.ScrollDirection == Orientation.Vertical)
+            {
+                switch (direction)
+                {
+                    case FocusNavigationDirection.Up:
+                        newItemIndex = itemIndex - 1;
+                        break;
+                    case FocusNavigationDirection.Down:
+                        newItemIndex = itemIndex + 1;
+                        break;
+                }
+            }
+
+            if (newItemIndex >= 0 && newItemIndex < ItemCount)
+            {
+
+                if (newItemIndex < FirstIndex || newItemIndex >= NextIndex)
+                {
+                    OffsetValue += newItemIndex - itemIndex;
+                    InvalidateScroll();
+                }
+
+                return generator.ContainerFromIndex(newItemIndex);
+            }
+
+            return null;
         }
 
         /// <summary>

src/Avalonia.Controls/Presenters/ItemsPresenter.cs

@@ -5,7 +5,6 @@ using System;
 using System.Collections.Specialized;
 using Avalonia.Controls.Primitives;
 using Avalonia.Input;
-using Avalonia.VisualTree;
 using static Avalonia.Utilities.MathUtilities;
 
 namespace Avalonia.Controls.Presenters
@@ -73,11 +72,17 @@ namespace Avalonia.Controls.Presenters
         Size ILogicalScrollable.PageScrollSize => new Size(0, 1);
 
         /// <inheritdoc/>
-        bool ILogicalScrollable.BringIntoView(IVisual target, Rect targetRect)
+        bool ILogicalScrollable.BringIntoView(IControl target, Rect targetRect)
         {
             return _virtualizer?.BringIntoView(target, targetRect) ?? false;
         }
 
+        /// <inheritdoc/>
+        IControl ILogicalScrollable.GetControlInDirection(FocusNavigationDirection direction, IControl from)
+        {
+            return _virtualizer?.GetControlInDirection(direction, from);
+        }
+
         /// <inheritdoc/>
         protected override void PanelCreated(IPanel panel)
         {

src/Avalonia.Controls/Presenters/ScrollContentPresenter.cs

@@ -118,10 +118,11 @@ namespace Avalonia.Controls.Presenters
             }
 
             var scrollable = Child as ILogicalScrollable;
+            var control = target as IControl;
 
-            if (scrollable?.IsLogicalScrollEnabled == true)
+            if (scrollable?.IsLogicalScrollEnabled == true && control != null)
             {
-                return scrollable.BringIntoView(target, targetRect);
+                return scrollable.BringIntoView(control, targetRect);
             }
 
             var transform = target.TransformToVisual(Child);

src/Avalonia.Controls/Primitives/ILogicalScrollable.cs

@@ -2,7 +2,7 @@
 // Licensed under the MIT license. See licence.md file in the project root for full license information.
 
 using System;
-using Avalonia.VisualTree;
+using Avalonia.Input;
 
 namespace Avalonia.Controls.Primitives
 {
@@ -56,6 +56,14 @@ namespace Avalonia.Controls.Primitives
         /// <param name="target">The target visual.</param>
         /// <param name="targetRect">The portion of the target visual to bring into view.</param>
         /// <returns>True if the scroll offset was changed; otherwise false.</returns>
-        bool BringIntoView(IVisual target, Rect targetRect);
+        bool BringIntoView(IControl target, Rect targetRect);
+
+        /// <summary>
+        /// Gets the next control in the specified direction.
+        /// </summary>
+        /// <param name="direction">The movement direction.</param>
+        /// <param name="from">The control from which movement begins.</param>
+        /// <returns>The control.</returns>
+        IControl GetControlInDirection(FocusNavigationDirection direction, IControl from);
     }
 }

src/Avalonia.Controls/StackPanel.cs

@@ -73,6 +73,18 @@ namespace Avalonia.Controls
         /// <param name="from">The control from which movement begins.</param>
         /// <returns>The control.</returns>
         IInputElement INavigableContainer.GetControl(FocusNavigationDirection direction, IInputElement from)
+        {
+            var fromControl = from as IControl;
+            return (fromControl != null) ? GetControlInDirection(direction, fromControl) : null;
+        }
+
+        /// <summary>
+        /// Gets the next control in the specified direction.
+        /// </summary>
+        /// <param name="direction">The movement direction.</param>
+        /// <param name="from">The control from which movement begins.</param>
+        /// <returns>The control.</returns>
+        protected virtual IInputElement GetControlInDirection(FocusNavigationDirection direction, IControl from)
         {
             var horiz = Orientation == Orientation.Horizontal;
             int index = Children.IndexOf((IControl)from);

src/Avalonia.Controls/VirtualizingStackPanel.cs

@@ -3,6 +3,9 @@
 
 using System;
 using System.Collections.Specialized;
+using Avalonia.Controls.Presenters;
+using Avalonia.Controls.Primitives;
+using Avalonia.Input;
 using Avalonia.Layout;
 using Avalonia.VisualTree;
 
@@ -107,6 +110,21 @@ namespace Avalonia.Controls
             }
         }
 
+        protected override IInputElement GetControlInDirection(FocusNavigationDirection direction, IControl from)
+        {
+            var logicalScrollable = Parent as ILogicalScrollable;
+            var fromControl = from as IControl;
+
+            if (logicalScrollable?.IsLogicalScrollEnabled == true && fromControl != null)
+            {
+                return logicalScrollable.GetControlInDirection(direction, fromControl);
+            }
+            else
+            {
+                return base.GetControlInDirection(direction, from);
+            }
+        }
+
         internal override void ArrangeChild(
             IControl child, 
             Rect rect,

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

@@ -5,6 +5,7 @@ using System;
 using System.Reactive.Linq;
 using Avalonia.Controls.Presenters;
 using Avalonia.Controls.Primitives;
+using Avalonia.Input;
 using Avalonia.Layout;
 using Avalonia.VisualTree;
 using Xunit;
@@ -293,7 +294,7 @@ namespace Avalonia.Controls.UnitTests
                 }
             }
 
-            public bool BringIntoView(IVisual target, Rect targetRect)
+            public bool BringIntoView(IControl target, Rect targetRect)
             {
                 throw new NotImplementedException();
             }
@@ -303,6 +304,11 @@ namespace Avalonia.Controls.UnitTests
                 AvailableSize = availableSize;
                 return new Size(150, 150);
             }
+
+            public IControl GetControlInDirection(FocusNavigationDirection direction, IControl from)
+            {
+                throw new NotImplementedException();
+            }
         }
     }
 }

tests/Avalonia.Controls.UnitTests/VirtualizingStackPanelTests.cs

@@ -1,6 +1,9 @@
 // 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.
 
+using Avalonia.Controls.Primitives;
+using Avalonia.Input;
+using Avalonia.LogicalTree;
 using Moq;
 using Xunit;
 
@@ -42,6 +45,7 @@ namespace Avalonia.Controls.UnitTests
 
                 target.Measure(new Size(100, 100));
 
+                Assert.Equal(new Size(0, 0), target.DesiredSize);
                 Assert.Equal(new Size(0, 0), target.Bounds.Size);
 
                 Assert.False(target.IsFull);
@@ -75,6 +79,22 @@ namespace Avalonia.Controls.UnitTests
 
                 Assert.Equal(2, target.OverflowCount);
             }
+
+            [Fact]
+            public void Passes_Navigation_Request_To_ILogicalScrollable_Parent()
+            {
+                var presenter = new Mock<ILogical>().As<IControl>();
+                var scrollable = presenter.As<ILogicalScrollable>();
+                var target = (IVirtualizingPanel)new VirtualizingStackPanel();
+                var from = new Canvas();
+
+                scrollable.Setup(x => x.IsLogicalScrollEnabled).Returns(true);
+
+                ((ISetLogicalParent)target).SetParent(presenter.Object);
+                ((INavigableContainer)target).GetControl(FocusNavigationDirection.Next, from);
+
+                scrollable.Verify(x => x.GetControlInDirection(FocusNavigationDirection.Next, from));
+            }
         }
     }
 }