Avalonia/src/Avalonia.Input/Pointer.cs

using System;
using System.Collections.Generic;
using System.Linq;
using Avalonia.Interactivity;
using Avalonia.VisualTree;

namespace Avalonia.Input
{
    public class Pointer : IPointer, IDisposable
    {
        private static int s_NextFreePointerId = 1000;
        public static int GetNextFreeId() => s_NextFreePointerId++;
        
        public Pointer(int id, PointerType type, bool isPrimary)
        {
            Id = id;
            Type = type;
            IsPrimary = isPrimary;
        }

        public int Id { get; }

        IInputElement FindCommonParent(IInputElement control1, IInputElement control2)
        {
            if (control1 == null || control2 == null)
                return null;
            var seen = new HashSet<IInputElement>(control1.GetSelfAndVisualAncestors().OfType<IInputElement>());
            return control2.GetSelfAndVisualAncestors().OfType<IInputElement>().FirstOrDefault(seen.Contains);
        }

        protected virtual void PlatformCapture(IInputElement element)
        {
            
        }
        
        public void Capture(IInputElement control)
        {
            if (Captured != null)
                Captured.DetachedFromVisualTree -= OnCaptureDetached;
            var oldCapture = control;
            Captured = control;
            PlatformCapture(control);
            if (oldCapture != null)
            {
                var commonParent = FindCommonParent(control, oldCapture);
                foreach (var notifyTarget in oldCapture.GetSelfAndVisualAncestors().OfType<IInputElement>())
                {
                    if (notifyTarget == commonParent)
                        break;
                    notifyTarget.RaiseEvent(new PointerCaptureLostEventArgs(notifyTarget, this));
                }
            }

            if (Captured != null)
                Captured.DetachedFromVisualTree += OnCaptureDetached;
        }

        IInputElement GetNextCapture(IVisual parent) =>
            parent as IInputElement ?? parent.GetVisualAncestors().OfType<IInputElement>().FirstOrDefault();
        
        private void OnCaptureDetached(object sender, VisualTreeAttachmentEventArgs e)
        {
            Capture(GetNextCapture(e.Parent));
        }


        public IInputElement Captured { get; private set; }
            
        public PointerType Type { get; }
        public bool IsPrimary { get; }
        public void Dispose() => Capture(null);
    }
}