Initial implementation of IScrollAnchorProvider.

On `ScrollContentPresenter`.

src/Avalonia.Controls/IScrollAnchorProvider.cs

@@ -1,9 +1,29 @@
 namespace Avalonia.Controls
 {
+    /// <summary>
+    /// Specifies a contract for a scrolling control that supports scroll anchoring.
+    /// </summary>
     public interface IScrollAnchorProvider
     {
+        /// <summary>
+        /// The currently chosen anchor element to use for scroll anchoring.
+        /// </summary>
         IControl CurrentAnchor { get; }
+
+        /// <summary>
+        /// Registers a control as a potential scroll anchor candidate.
+        /// </summary>
+        /// <param name="element">
+        /// A control within the subtree of the <see cref="IScrollAnchorProvider"/>.
+        /// </param>
         void RegisterAnchorCandidate(IControl element);
+
+        /// <summary>
+        /// Unregisters a control as a potential scroll anchor candidate.
+        /// </summary>
+        /// <param name="element">
+        /// A control within the subtree of the <see cref="IScrollAnchorProvider"/>.
+        /// </param>
         void UnregisterAnchorCandidate(IControl element);
     }
 }

src/Avalonia.Controls/Presenters/ScrollContentPresenter.cs

@@ -14,7 +14,7 @@ namespace Avalonia.Controls.Presenters
     /// <summary>
     /// Presents a scrolling view of content inside a <see cref="ScrollViewer"/>.
     /// </summary>
-    public class ScrollContentPresenter : ContentPresenter, IPresenter, IScrollable
+    public class ScrollContentPresenter : ContentPresenter, IPresenter, IScrollable, IScrollAnchorProvider
     {
         /// <summary>
         /// Defines the <see cref="CanHorizontallyScroll"/> property.
@@ -65,6 +65,8 @@ namespace Avalonia.Controls.Presenters
         private IDisposable _logicalScrollSubscription;
         private Size _viewport;
         private Dictionary<int, Vector> _activeLogicalGestureScrolls;
+        private List<IControl> _anchorCandidates;
+        private (IControl control, Rect bounds) _anchor;
 
         /// <summary>
         /// Initializes static members of the <see cref="ScrollContentPresenter"/> class.
@@ -132,6 +134,9 @@ namespace Avalonia.Controls.Presenters
             private set { SetAndRaise(ViewportProperty, ref _viewport, value); }
         }
 
+        /// <inheritdoc/>
+        IControl IScrollAnchorProvider.CurrentAnchor => _anchor.Item1;
+
         /// <summary>
         /// Attempts to bring a portion of the target visual into view by scrolling the content.
         /// </summary>
@@ -196,6 +201,24 @@ namespace Avalonia.Controls.Presenters
             return result;
         }
 
+        /// <inheritdoc/>
+        void IScrollAnchorProvider.RegisterAnchorCandidate(IControl element)
+        {
+            _anchorCandidates ??= new List<IControl>();
+            _anchorCandidates.Add(element);
+        }
+
+        /// <inheritdoc/>
+        void IScrollAnchorProvider.UnregisterAnchorCandidate(IControl element)
+        {
+            _anchorCandidates?.Remove(element);
+
+            if (_anchor.Item1 == element)
+            {
+                _anchor = default;
+            }
+        }
+
         /// <inheritdoc/>
         protected override Size MeasureOverride(Size availableSize)
         {
@@ -223,9 +246,44 @@ namespace Avalonia.Controls.Presenters
             var size = new Size(
                 CanHorizontallyScroll ? Math.Max(Child.DesiredSize.Width, finalSize.Width) : finalSize.Width,
                 CanVerticallyScroll ? Math.Max(Child.DesiredSize.Height, finalSize.Height) : finalSize.Height);
+
+            Vector TrackAnchor()
+            {
+                // If we have an anchor and its position relative to Child has changed during the
+                // arrange then that change wasn't just due to scrolling (as scrolling doesn't adjust
+                // relative positions within Child).
+                if (_anchor.control != null &&
+                    TranslateBounds(_anchor.control, Child, out var updatedBounds) &&
+                    updatedBounds.Position != _anchor.bounds.Position)
+                {
+                    return updatedBounds.Position - _anchor.bounds.Position;
+                }
+
+                return default;
+            }
+
+            // Check whether our previous anchor (if any) has moved relative to Child since the last
+            // arrange, and if so adjust the offset to bring it back into place.
+            Offset += TrackAnchor();
+
+            // Calculate the new anchor element.
+            _anchor = CalculateCurrentAnchor();
+
+            // Do the arrange.
             ArrangeOverrideImpl(size, -Offset);
+
+            // If the anchor moved during the arrange, we need to adjust the offset and do another arrange.
+            var postOffset = TrackAnchor();
+
+            if (postOffset != default)
+            {
+                Offset += postOffset;
+                ArrangeOverrideImpl(size, -Offset);
+            }
+
             Viewport = finalSize;
             Extent = Child.Bounds.Size.Inflate(Child.Margin);
+
             return finalSize;
         }
 
@@ -390,5 +448,70 @@ namespace Avalonia.Controls.Presenters
                 Offset = scrollable.Offset;
             }
         }
+
+        (IControl, Rect) CalculateCurrentAnchor()
+        {
+            if (_anchorCandidates == null)
+            {
+                return default;
+            }
+
+            var thisBounds = new Rect(Bounds.Size);
+            var bestCandidate = default(IControl);
+            var bestCandidateDistance = double.MaxValue;
+
+            // Find the anchor candidate that is scrolled closest to the top-left of this
+            // ScrollContentPresenter.
+            foreach (var element in _anchorCandidates)
+            {
+                if (element.IsVisible && 
+                    TranslateBounds(element, this, out var bounds) &&
+                    bounds.Intersects(thisBounds))
+                {
+                    var distance = (Vector)bounds.Position;
+                    var candidateDistance = Math.Abs(distance.Length);
+
+                    if (candidateDistance < bestCandidateDistance)
+                    {
+                        bestCandidate = element;
+                        bestCandidateDistance = candidateDistance;
+                    }
+                }
+            }
+
+            if (bestCandidate != null)
+            {
+                // We have a candidate, calculate its bounds relative to Child. Because these
+                // bounds aren't relative to the ScrollContentPresenter itself, if they change
+                // then we know it wasn't just due to scrolling.
+                var unscrolledBounds = TranslateBounds(bestCandidate, Child);
+                return (bestCandidate, unscrolledBounds);
+            }
+
+            return default;
+        }
+
+        private Rect TranslateBounds(IControl control, IControl to)
+        {
+            if (TranslateBounds(control, to, out var bounds))
+            {
+                return bounds;
+            }
+
+            throw new InvalidOperationException("The control's bounds could not be translated to the requested control.");
+        }
+
+        private bool TranslateBounds(IControl control, IControl to, out Rect bounds)
+        {
+            if (!control.IsVisible)
+            {
+                bounds = default;
+                return false;
+            }
+
+            var p = control.TranslatePoint(default, to);
+            bounds = p.HasValue ? new Rect(p.Value, control.Bounds.Size) : default;
+            return p.HasValue;
+        }
     }
 }

src/Avalonia.Controls/Repeater/ViewportManager.cs

@@ -236,14 +236,12 @@ namespace Avalonia.Controls
 
         public void OnElementPrepared(IControl element)
         {
-            // If we have an anchor element, we do not want the
-            // scroll anchor provider to start anchoring some other element.
-            ////element.CanBeScrollAnchor(true);
+            _scroller.RegisterAnchorCandidate(element);
         }
 
-        public void OnElementCleared(ILayoutable element)
+        public void OnElementCleared(IControl element)
         {
-            ////element.CanBeScrollAnchor(false);
+            _scroller.UnregisterAnchorCandidate(element);
         }
 
         public void OnOwnerMeasuring()

src/Avalonia.Controls/ScrollViewer.cs

@@ -316,6 +316,9 @@ namespace Avalonia.Controls
             get { return VerticalScrollBarVisibility != ScrollBarVisibility.Disabled; }
         }
 
+        /// <inheritdoc/>
+        public IControl CurrentAnchor => (Presenter as IScrollAnchorProvider)?.CurrentAnchor;
+
         /// <summary>
         /// Gets the maximum horizontal scrollbar value.
         /// </summary>
@@ -382,9 +385,6 @@ namespace Avalonia.Controls
             get { return _viewport.Height; }
         }
 
-        /// <inheritdoc/>
-        IControl IScrollAnchorProvider.CurrentAnchor => null; // TODO: Implement
-
         /// <summary>
         /// Scrolls to the top-left corner of the content.
         /// </summary>
@@ -441,14 +441,16 @@ namespace Avalonia.Controls
             control.SetValue(VerticalScrollBarVisibilityProperty, value);
         }
 
-        void IScrollAnchorProvider.RegisterAnchorCandidate(IControl element)
+        /// <inheritdoc/>
+        public void RegisterAnchorCandidate(IControl element)
         {
-            // TODO: Implement
+            (Presenter as IScrollAnchorProvider)?.RegisterAnchorCandidate(element);
         }
 
-        void IScrollAnchorProvider.UnregisterAnchorCandidate(IControl element)
+        /// <inheritdoc/>
+        public void UnregisterAnchorCandidate(IControl element)
         {
-            // TODO: Implement
+            (Presenter as IScrollAnchorProvider)?.UnregisterAnchorCandidate(element);
         }
 
         protected override bool RegisterContentPresenter(IContentPresenter presenter)