Merge pull request AvaloniaUI#7964 from AvaloniaUI/fixes/7840-datetimepicker-touch-scroll

Fix Date/Time picker touch scrolling

Author: grokys

src/Avalonia.Controls/DateTimePickers/DateTimePickerPanel.cs

@@ -1,7 +1,9 @@
 using System;
 using System.Globalization;
 using System.Linq;
+using Avalonia.Controls.Presenters;
 using Avalonia.Input;
+using Avalonia.Input.GestureRecognizers;
 using Avalonia.Interactivity;
 using Avalonia.Media;
 using Avalonia.VisualTree;
@@ -60,6 +62,7 @@ public class DateTimePickerPanel : Panel, ILogicalScrollable
         private Vector _offset;
         private bool _hasInit;
         private bool _suppressUpdateOffset;
+        private ScrollContentPresenter? _parentScroller;
 
         public DateTimePickerPanel()
         {
@@ -255,6 +258,8 @@ public Vector Offset
                 _suppressUpdateOffset = true;
                 SelectedValue = (int)newSel * Increment + MinimumValue;
                 _suppressUpdateOffset = false;
+
+                System.Diagnostics.Debug.WriteLine($"Offset: {_offset} ItemHeight: {ItemHeight}");
             }
         }
 
@@ -270,7 +275,7 @@ public Vector Offset
 
         public Size Extent => _extent;
 
-        public Size Viewport => new Size(0, ItemHeight);
+        public Size Viewport => Bounds.Size;
 
         public event EventHandler ScrollInvalidated;
 
@@ -341,6 +346,20 @@ protected override Size ArrangeOverride(Size finalSize)
             return finalSize;
         }
 
+        protected override void OnAttachedToVisualTree(VisualTreeAttachmentEventArgs e)
+        {
+            base.OnAttachedToVisualTree(e);
+            _parentScroller = this.GetVisualParent() as ScrollContentPresenter;
+            _parentScroller?.AddHandler(Gestures.ScrollGestureEndedEvent, OnScrollGestureEnded);
+        }
+
+        protected override void OnDetachedFromVisualTree(VisualTreeAttachmentEventArgs e)
+        {
+            base.OnDetachedFromVisualTree(e);
+            _parentScroller?.RemoveHandler(Gestures.ScrollGestureEndedEvent, OnScrollGestureEnded);
+            _parentScroller = null;
+        }
+
         protected override void OnKeyDown(KeyEventArgs e)
         {
             switch (e.Key)
@@ -554,5 +573,15 @@ public void RaiseScrollInvalidated(EventArgs e)
         {
             ScrollInvalidated?.Invoke(this, e);
         }
+
+        private void OnScrollGestureEnded(object? sender, ScrollGestureEndedEventArgs e)
+        {
+            var snapY = Math.Round(Offset.Y / ItemHeight) * ItemHeight;
+
+            if (snapY != Offset.Y)
+            {
+                Offset = Offset.WithY(snapY);
+            }
+        }
     }
 }

src/Avalonia.Controls/Presenters/ScrollContentPresenter.cs

@@ -371,7 +371,7 @@ private void OnScrollGesture(object sender, ScrollGestureEventArgs e)
                     {
                         var logicalUnits = delta.Y / logicalScrollItemSize.Y;
                         delta = delta.WithY(delta.Y - logicalUnits * logicalScrollItemSize.Y);
-                        dy = logicalUnits * scrollable!.ScrollSize.Height;
+                        dy = logicalUnits;
                     }
                     else
                         dy = delta.Y;
@@ -389,7 +389,7 @@ private void OnScrollGesture(object sender, ScrollGestureEventArgs e)
                     {
                         var logicalUnits = delta.X / logicalScrollItemSize.X;
                         delta = delta.WithX(delta.X - logicalUnits * logicalScrollItemSize.X);
-                        dx = logicalUnits * scrollable!.ScrollSize.Width;
+                        dx = logicalUnits;
                     }
                     else
                         dx = delta.X;