ScreenExtender/PcWin/ScreenExtender/Utility/Spy/SpyAgent.cs

using System;
using System.Drawing;
using System.Runtime.InteropServices;
using System.Windows.Forms;

namespace ScreenExtender.Utility.Spy
{
    public class SpyAgent
    {
        private readonly Action<SpiedWindow> _selectedAction;
        private readonly Timer _timer;
        private SpyAgentLocator _locator;

        public SpyAgent(Action<SpiedWindow> selectedAction)
        {
            _selectedAction = selectedAction;
            _timer = new Timer { Interval = 25, Enabled = false };
            _timer.Tick += OnTimerTicked;
            BeginSpying();
        }

        private void OnTimerTicked(object sender, EventArgs e)
        {
            ShowLocator();
            if (0 == (Control.MouseButtons & MouseButtons.Left)) return;
            EndSpying();
            _selectedAction(GetHoveredWindow());
        }

        private void ShowLocator()
        {
            var window = GetHoveredWindow();

            if (window.Handle == IntPtr.Zero)
            {
                _locator.Hide();
                return;
            }

            _locator.Location = window.Area.Location;
            _locator.Size = window.Area.Size;
            _locator.TopLevel = true;
            _locator.TopMost = true;
            _locator.Show();
        }

        public void BeginSpying()
        {
            if (_locator != null)
            {
                _locator.Close();
                _locator.Dispose();
            }

            _locator = new SpyAgentLocator();
            MakePassThrough(_locator.Handle);

            _timer.Enabled = true;
        }

        public void EndSpying()
        {
            _timer.Enabled = false;

            _locator?.Close();
            _locator?.Dispose();
            _locator = null;
        }

        private class SpyAgentLocator : Form
        {
            public SpyAgentLocator()
            {
                FormBorderStyle = FormBorderStyle.None;
                BackColor = Color.OrangeRed;
                Opacity = 0.25;
                TopLevel = true;
                TopMost = true;
            }
        }

        #region Under the Hood

        private const int GWL_EXSTYLE = -20;
        private const int WS_EX_TRANSPARENT = 0x20;

        [DllImport("user32.dll")]
        private static extern IntPtr WindowFromPoint(POINT point);

        [DllImport("user32.dll")]
        private static extern IntPtr ChildWindowFromPoint(IntPtr hWndParent, POINT point);

        [DllImport("user32.dll", SetLastError = true)]
        private static extern int GetWindowLong(IntPtr hWnd, int nIndex);

        [DllImport("user32.dll", SetLastError = true)]
        private static extern int SetWindowLong(IntPtr hWnd, int nIndex, int dwNewLong);

        public static SpiedWindow GetHoveredWindow()
        {
            var handle = WindowFromPoint(Cursor.Position);
            return new SpiedWindow(handle);
        }

        private static void MakePassThrough(IntPtr handle)
        {
            var exstyle = GetWindowLong(handle, GWL_EXSTYLE);
            exstyle |= WS_EX_TRANSPARENT;
            SetWindowLong(handle, GWL_EXSTYLE, exstyle);
        }

        [StructLayout(LayoutKind.Sequential)]
        private struct POINT
        {
            public readonly int X;
            public readonly int Y;

            public POINT(int x, int y)
            {
                X = x;
                Y = y;
            }

            public POINT(Point pt)
                : this(pt.X, pt.Y)
            {
            }

            public static implicit operator Point(POINT p)
            {
                return new(p.X, p.Y);
            }

            public static implicit operator POINT(Point p)
            {
                return new(p.X, p.Y);
            }
        }

        #endregion Under the Hood
    }
}