Switch to using animation request for scroll inertia dispatch (AvaloniaUI#18997)

* switch to using animation request for scroll inertia dispatch

* run inertia calcs in dispatcher operation

Author: emmauss

src/Avalonia.Base/Input/GestureRecognizers/ScrollGestureRecognizer.cs

@@ -1,5 +1,7 @@
 using System;
 using System.Diagnostics;
+using System.Security.Cryptography;
+using Avalonia.Media;
 using Avalonia.Platform;
 using Avalonia.Threading;
 
@@ -20,19 +22,23 @@ public class ScrollGestureRecognizer : GestureRecognizer
         private bool _scrolling;
         private Point _trackedRootPoint;
         private IPointer? _tracking;
+        private Stopwatch? _stopWatch;
         private int _gestureId;
         private Point _pointerPressedPoint;
         private VelocityTracker? _velocityTracker;
 
         // Movement per second
-        private Vector _inertia;
+        private Vector? _inertia;
         private ulong? _lastMoveTimestamp;
+        private TimeSpan _lastTime;
+        private TimeSpan _inertiaStartTime;
+        private int _currentInertiaGestureId;
 
         /// <summary>
         /// Defines the <see cref="CanHorizontallyScroll"/> property.
         /// </summary>
         public static readonly DirectProperty<ScrollGestureRecognizer, bool> CanHorizontallyScrollProperty =
-            AvaloniaProperty.RegisterDirect<ScrollGestureRecognizer, bool>(nameof(CanHorizontallyScroll), 
+            AvaloniaProperty.RegisterDirect<ScrollGestureRecognizer, bool>(nameof(CanHorizontallyScroll),
                 o => o.CanHorizontallyScroll, (o, v) => o.CanHorizontallyScroll = v);
 
         /// <summary>
@@ -47,7 +53,7 @@ public class ScrollGestureRecognizer : GestureRecognizer
         /// </summary>
         public static readonly DirectProperty<ScrollGestureRecognizer, bool> IsScrollInertiaEnabledProperty =
             AvaloniaProperty.RegisterDirect<ScrollGestureRecognizer, bool>(nameof(IsScrollInertiaEnabled),
-                o => o.IsScrollInertiaEnabled, (o,v) => o.IsScrollInertiaEnabled = v);
+                o => o.IsScrollInertiaEnabled, (o, v) => o.IsScrollInertiaEnabled = v);
 
         /// <summary>
         /// Defines the <see cref="ScrollStartDistance"/> property.
@@ -102,6 +108,7 @@ protected override void PointerPressed(PointerPressedEventArgs e)
             {
                 EndGesture();
                 _tracking = e.Pointer;
+                _inertia = null;
                 _gestureId = ScrollGestureEventArgs.GetNextFreeId();
                 _trackedRootPoint = _pointerPressedPoint = point.Position;
                 _velocityTracker = new VelocityTracker();
@@ -121,7 +128,7 @@ protected override void PointerMoved(PointerEventArgs e)
                     if (CanVerticallyScroll && Math.Abs(_trackedRootPoint.Y - rootPoint.Y) > ScrollStartDistance)
                         _scrolling = true;
                     if (_scrolling)
-                    {                        
+                    {
                         // Correct _trackedRootPoint with ScrollStartDistance, so scrolling does not start with a skip of ScrollStartDistance
                         _trackedRootPoint = new Point(
                             _trackedRootPoint.X - (_trackedRootPoint.X >= rootPoint.X ? ScrollStartDistance : -ScrollStartDistance),
@@ -147,23 +154,25 @@ protected override void PointerMoved(PointerEventArgs e)
 
         protected override void PointerCaptureLost(IPointer pointer)
         {
-            if (pointer == _tracking) EndGesture();
+            if (pointer == _tracking)
+                EndGesture();
         }
 
         void EndGesture()
         {
             _tracking = null;
             if (_scrolling)
             {
+                _stopWatch?.Stop();
+                _stopWatch = null;
                 _inertia = default;
                 _scrolling = false;
                 Target!.RaiseEvent(new ScrollGestureEndedEventArgs(_gestureId));
                 _gestureId = 0;
                 _lastMoveTimestamp = null;
             }
-            
-        }
 
+        }
 
         protected override void PointerReleased(PointerReleasedEventArgs e)
         {
@@ -172,7 +181,8 @@ protected override void PointerReleased(PointerReleasedEventArgs e)
                 _inertia = _velocityTracker?.GetFlingVelocity().PixelsPerSecond ?? Vector.Zero;
 
                 e.Handled = true;
-                if (_inertia == default
+                if (_inertia == null
+                    || _inertia == Vector.Zero
                     || e.Timestamp == 0
                     || _lastMoveTimestamp == 0
                     || e.Timestamp - _lastMoveTimestamp > 200
@@ -181,53 +191,63 @@ protected override void PointerReleased(PointerReleasedEventArgs e)
                 else
                 {
                     _tracking = null;
-                    var savedGestureId = _gestureId;
-                    var st = Stopwatch.StartNew();
-                    var lastTime = TimeSpan.Zero;
-                    Target!.RaiseEvent(new ScrollGestureInertiaStartingEventArgs(_gestureId, _inertia));
-                    DispatcherTimer.Run(() =>
-                    {
-                        // Another gesture has started, finish the current one
-                        if (_gestureId != savedGestureId)
-                        {
-                            return false;
-                        }
-
-                        var elapsedSinceLastTick = st.Elapsed - lastTime;
-                        lastTime = st.Elapsed;
-
-                        var speed = _inertia * Math.Pow(InertialResistance, st.Elapsed.TotalSeconds);
-                        var distance = speed * elapsedSinceLastTick.TotalSeconds;
-                        var scrollGestureEventArgs = new ScrollGestureEventArgs(_gestureId, distance);
-                        Target!.RaiseEvent(scrollGestureEventArgs);
-
-                        if (!scrollGestureEventArgs.Handled || scrollGestureEventArgs.ShouldEndScrollGesture)
-                        {
-                            EndGesture();
-                            return false;
-                        }
-
-                        // EndGesture using InertialScrollSpeedEnd only in the direction of scrolling
-                        if (CanVerticallyScroll && CanHorizontallyScroll && Math.Abs(speed.X) < InertialScrollSpeedEnd && Math.Abs(speed.Y) <= InertialScrollSpeedEnd)
-                        {
-                            EndGesture();
-                            return false;
-                        }
-                        else if (CanVerticallyScroll && Math.Abs(speed.Y) <= InertialScrollSpeedEnd)
-                        {
-                            EndGesture();
-                            return false;
-                        }
-                        else if (CanHorizontallyScroll && Math.Abs(speed.X) < InertialScrollSpeedEnd)
-                        {
-                            EndGesture();
-                            return false;
-                        }
-
-                        return true;
-                    }, TimeSpan.FromMilliseconds(16), DispatcherPriority.Background);
+                    _stopWatch = Stopwatch.StartNew();
+                    _lastTime = _stopWatch.Elapsed;
+                    _inertiaStartTime = _lastTime;
+                    _currentInertiaGestureId = _gestureId;
+                    Target!.RaiseEvent(new ScrollGestureInertiaStartingEventArgs(_gestureId, _inertia.Value));
+                    MediaContext.Instance.RequestAnimationFrame(OnAnimationRequested);
                 }
             }
         }
+
+        private void OnAnimationRequested(TimeSpan _)
+        {
+            // Calculate the current speed and dispatch the next inertia event. This is done asynchronously so we have run the events
+            // with Input priority
+            Dispatcher.UIThread.InvokeAsync(() =>
+            {
+                // Another gesture has started, finish the current one
+                if (_gestureId != _currentInertiaGestureId || _stopWatch == null || _inertia is not Vector inertia)
+                {
+                    return;
+                }
+
+                var timeSpan = _stopWatch.Elapsed;
+                var elapsedSinceLastTick = timeSpan - _lastTime;
+                _lastTime = timeSpan;
+
+                var speed = inertia * Math.Pow(InertialResistance, (_lastTime - _inertiaStartTime).TotalSeconds);
+                var distance = speed * elapsedSinceLastTick.TotalSeconds;
+                var scrollGestureEventArgs = new ScrollGestureEventArgs(_gestureId, distance);
+                Target!.RaiseEvent(scrollGestureEventArgs);
+
+                if (!scrollGestureEventArgs.Handled || scrollGestureEventArgs.ShouldEndScrollGesture)
+                {
+                    EndGesture();
+                    return;
+                }
+
+                // EndGesture using InertialScrollSpeedEnd only in the direction of scrolling
+                if (CanVerticallyScroll && CanHorizontallyScroll && Math.Abs(speed.X) < InertialScrollSpeedEnd && Math.Abs(speed.Y) <= InertialScrollSpeedEnd)
+                {
+                    // NO-OP 
+                }
+                else if (CanVerticallyScroll && Math.Abs(speed.Y) <= InertialScrollSpeedEnd)
+                {
+                    EndGesture();
+                    return;
+                }
+                else if (CanHorizontallyScroll && Math.Abs(speed.X) < InertialScrollSpeedEnd)
+                {
+                    EndGesture();
+                    return;
+                }
+
+                // Reschedule on the next animation frame. TopLevel.RequestAnimationFrame isn't available on the Base project, so we use the global MediaContext
+                MediaContext.Instance.RequestAnimationFrame(OnAnimationRequested);
+            }, DispatcherPriority.Input);
+
+        }
     }
 }