Improve the fling logic

SkiaSharp.Extended.Controls/samples/SkiaSharpDemo/GestureSurfacePage.xaml.cs

@@ -31,17 +31,28 @@ public GestureSurfacePage()
 
 			BindingContext = this;
 
-			//gestureSurface.FlingDetected += (sender, e) =>
-			//{
-			//	gestureSurface.InvalidateSurface();
-			//};
+			gestureSurface.FlingDetected += (sender, e) =>
+			{
+				var easing = Easing.SinOut;
+
+				var ratio = e.VelocityX / e.VelocityY;
+
+				gestureSurface.AbortAnimation("Fling");
+				var animation = new Animation(v => Transform(new SKPoint((float)(v * ratio), (float)v), SKPoint.Empty, 1, 0), e.VelocityY * 0.01f, 0, easing);
+				animation.Commit(gestureSurface, "Fling", 16, 1000);
+			};
+
 			gestureSurface.TransformDetected += (sender, e) =>
 			{
+				gestureSurface.AbortAnimation("Fling");
+
 				Transform(e.Center, e.PreviousCenter, e.ScaleDelta, e.RotationDelta);
 			};
 
 			gestureSurface.DoubleTapDetected += (sender, e) =>
 			{
+				gestureSurface.AbortAnimation("Fling");
+
 				Transform(e.Location, e.Location, 1.5f, 0);
 			};
 		}

SkiaSharp.Extended.Controls/source/SKGestureSurfaceView.FlingTracker.cs

@@ -1,12 +1,13 @@
-using System.Collections.Generic;
+using System;
+using System.Collections.Generic;
 
 namespace SkiaSharp.Extended.Controls
 {
 	public partial class SKGestureSurfaceView
 	{
 		private class FlingTracker
 		{
-			private const long maxTicks = 200 * 10000;  // Use only events from the last 200 ms
+			private const long thresholdTicks = 200 * TimeSpan.TicksPerMillisecond;  // Use only events from the last 200 ms
 
 			private const int maxSize = 2;
 
@@ -50,12 +51,13 @@ public SKPoint CalculateVelocity(long id, long now)
 				var nowItem = array[1];
 
 				// use last 2 events
-				if (now - lastItem.Time < maxTicks)
+				if (now - lastItem.TimeTicks < thresholdTicks)
 				{
-					velocityX = (nowItem.X - lastItem.X) * 10000000 / (nowItem.Time - lastItem.Time);
-					velocityY = (nowItem.Y - lastItem.Y) * 10000000 / (nowItem.Time - lastItem.Time);
+					velocityX = (nowItem.X - lastItem.X) * TimeSpan.TicksPerSecond / (nowItem.TimeTicks - lastItem.TimeTicks);
+					velocityY = (nowItem.Y - lastItem.Y) * TimeSpan.TicksPerSecond / (nowItem.TimeTicks - lastItem.TimeTicks);
 				}
 
+				// return the velocity in pixels per second
 				return new SKPoint(velocityX, velocityY);
 			}
 		}

SkiaSharp.Extended.Controls/source/SKGestureSurfaceView.FlingTrackerEvent.cs

@@ -4,18 +4,18 @@ public partial class SKGestureSurfaceView
 	{
 		private struct FlingTrackerEvent
 		{
-			public FlingTrackerEvent(float x, float y, long time)
+			public FlingTrackerEvent(float x, float y, long timeTicks)
 			{
 				X = x;
 				Y = y;
-				Time = time;
+				TimeTicks = timeTicks;
 			}
 
 			public float X { get; }
 
 			public float Y { get; }
 
-			public long Time { get; }
+			public long TimeTicks { get; }
 		}
 	}
 }

SkiaSharp.Extended.Controls/source/SKGestureSurfaceView.cs

@@ -1,23 +1,24 @@
 using System;
 using System.Collections.Generic;
 using System.Linq;
+using System.Threading;
 using SkiaSharp.Views.Forms;
 
 namespace SkiaSharp.Extended.Controls
 {
 	public partial class SKGestureSurfaceView : SKDynamicSurfaceView
 	{
-		private const int shortTap = 125;
-		private const int shortClick = 250;
-		private const int delayTap = 200;
-		private const int longTap = 500;
-		private const int touchSlop = 8;
-		private const int flingVelocity = 200;
+		private const long shortTapTicks = 125 * TimeSpan.TicksPerMillisecond;
+		private const long shortClickTicks = 250 * TimeSpan.TicksPerMillisecond;
+		private const int delayTapMilliseconds = 200;
+		private const long longTapTicks = 500 * TimeSpan.TicksPerMillisecond;
+		private const int touchSlopPixels = 8;
+		private const int flingVelocityThreshold = 200; // pixels per second
 
 		private readonly Dictionary<long, TouchEvent> touches = new Dictionary<long, TouchEvent>();
 		private readonly FlingTracker flingTracker = new FlingTracker();
 		private SKPoint initialTouch = SKPoint.Empty;
-		private System.Threading.Timer multiTapTimer;
+		private Timer multiTapTimer;
 		private int tapCount = 0;
 		private TouchMode touchMode = TouchMode.None;
 		private PinchValue previousValues;
@@ -226,16 +227,16 @@ private bool OnTouchReleased(SKTouchEventArgs e)
 			{
 				// check to see if it was a fling
 				var velocity = flingTracker.CalculateVelocity(e.Id, ticks);
-				if (Math.Abs(velocity.X) > flingVelocity || Math.Abs(velocity.Y) > flingVelocity)
+				if (Math.Abs(velocity.X * velocity.Y) > (flingVelocityThreshold * flingVelocityThreshold))
 				{
 					var args = new SKFlingDetectedEventArgs(velocity.X, velocity.Y);
 					OnFlingDetected(args);
 					handled = args.Handled;
 				}
 
 				// when tapping, the finger never goes to exactly the same location
-				var isAround = SKPoint.Distance(releasedTouch.Location, initialTouch) < touchSlop;
-				if (isAround && (ticks - releasedTouch.Tick) < (e.DeviceType == SKTouchDeviceType.Mouse ? shortClick : longTap) * 10000)
+				var isAround = SKPoint.Distance(releasedTouch.Location, initialTouch) < touchSlopPixels;
+				if (isAround && (ticks - releasedTouch.Tick) < (e.DeviceType == SKTouchDeviceType.Mouse ? shortClickTicks : longTapTicks))
 				{
 					// add a timer to detect the type of tap (single or multi)
 					void TimerHandler(object state)
@@ -260,9 +261,9 @@ void TimerHandler(object state)
 						multiTapTimer?.Dispose();
 						multiTapTimer = null;
 					};
-					multiTapTimer = new System.Threading.Timer(TimerHandler, location, delayTap, -1);
+					multiTapTimer = new Timer(TimerHandler, location, delayTapMilliseconds, -1);
 				}
-				else if (isAround && (ticks - releasedTouch.Tick) >= longTap * 10000)
+				else if (isAround && (ticks - releasedTouch.Tick) >= longTapTicks)
 				{
 					// if the finger was down for a long time, then it is a long tap
 					if (!handled)