SKTouchHandler.cs

using System;
using System.Linq;
using Foundation;
using UIKit;

namespace SkiaSharp.Views.Forms
{
    internal class SKTouchHandler : UIGestureRecognizer
    {
        private readonly HashSet<UITouch> _touches = new HashSet<UITouch>();

        private Action<SKTouchEventArgs> onTouchAction;
        private Func<double, double, SKPoint> scalePixels;

        public SKTouchHandler(Action<SKTouchEventArgs> onTouchAction, Func<double, double, SKPoint> scalePixels)
        {
            this.onTouchAction = onTouchAction;
            this.scalePixels = scalePixels;

            DisablesUserInteraction = false;
        }

        public bool DisablesUserInteraction { get; set; }

        public void SetEnabled(UIView view, bool enableTouchEvents)
        {
            if (view != null)
            {
                if (!view.UserInteractionEnabled || DisablesUserInteraction)
                {
                    view.UserInteractionEnabled = enableTouchEvents;
                }
                if (enableTouchEvents && view.GestureRecognizers?.Contains(this) != true)
                {
                    view.AddGestureRecognizer(this);
                }
                else if (!enableTouchEvents && view.GestureRecognizers?.Contains(this) == true)
                {
                    view.RemoveGestureRecognizer(this);
                }
            }
        }

        public void Detach(UIView view)
        {
            // clean the view
            SetEnabled(view, false);

            // remove references
            onTouchAction = null;
            scalePixels = null;
        }

        public override void TouchesBegan(NSSet touches, UIEvent evt)
        {
            base.TouchesBegan(touches, evt);

            foreach (UITouch touch in touches.Cast<UITouch>())
            {
                if (FireEvent(SKTouchAction.Pressed, touch, true))
                {
                    _touches.Add(touch);
                }
                else
                {
                    IgnoreTouch(touch, evt);
                }
            }
        }

        public override void TouchesMoved(NSSet touches, UIEvent evt)
        {
            base.TouchesMoved(touches, evt);

            foreach (UITouch touch in touches.Cast<UITouch>())
            {
                FireEvent(SKTouchAction.Moved, touch, true);
            }
        }

        public override void TouchesEnded(NSSet touches, UIEvent evt)
        {
            base.TouchesEnded(touches, evt);

            foreach (UITouch touch in touches.Cast<UITouch>())
            {
                FireEvent(SKTouchAction.Released, touch, false);
                _touches.Remove(touch);
            }
        }

        public override void TouchesCancelled(NSSet touches, UIEvent evt)
        {
            base.TouchesCancelled(touches, evt);

            foreach (UITouch touch in touches.Cast<UITouch>())
            {
                FireEvent(SKTouchAction.Cancelled, touch, false);
                _touches.Remove(touch);
            }
        }

        public override void Reset()
        {
            base.Reset();

            foreach (var touch in _touches)
            {
                FireEvent(SKTouchAction.Cancelled, touch, false);
            }

            _touches.Clear();
        }

        private bool FireEvent(SKTouchAction actionType, UITouch touch, bool inContact)
        {
            if (onTouchAction == null || scalePixels == null)
                return false;

            var id = touch.Handle.ToInt64();

            var cgPoint = touch.LocationInView(View);
            var point = scalePixels(cgPoint.X, cgPoint.Y);

            var args = new SKTouchEventArgs(id, actionType, point, inContact);
            onTouchAction(args);
            return args.Handled;
        }
    }
}