cursor-lang/CursorLang.Agent/Interop/WindowNative.cs

using System.Runtime.InteropServices;
using CursorLang.Core.Interop;

namespace CursorLang.Agent.Interop;

/// <summary>
/// The Win32 pieces a window needs when there is no framework to make one:
/// the class, the window itself, the message loop.
/// </summary>
internal static class WindowNative
{
    /// <summary>The window procedure. Windows keeps the only reference to it.</summary>
    internal delegate IntPtr WindowProc(IntPtr hWnd, uint message, IntPtr wParam, IntPtr lParam);

    internal const int WS_POPUP = unchecked((int)0x80000000);

    internal const int WS_EX_LAYERED = 0x00080000;
    internal const int WS_EX_TOOLWINDOW = 0x00000080;
    internal const int WS_EX_NOACTIVATE = 0x08000000;
    internal const int WS_EX_TRANSPARENT = 0x00000020;
    internal const int WS_EX_TOPMOST = 0x00000008;

    internal const int SW_HIDE = 0;
    internal const int SW_SHOWNOACTIVATE = 4;

    internal const uint WM_DESTROY = 0x0002;
    internal const uint WM_CLOSE = 0x0010;
    internal const uint WM_QUIT = 0x0012;
    internal const uint WM_NULL = 0x0000;
    internal const uint WM_ENDSESSION = 0x0016;

    /// <summary>WM_APP and up belong to the application; the tray takes WM_APP + 1.</summary>
    internal const uint WM_APP = 0x8000;

    /// <summary>
    /// Registers a window class. A class already there is not an error: the name is
    /// unique per window kind, and a second agent in the same process would meet its
    /// own registration.
    /// </summary>
    internal static void RegisterClass(string className, WindowProc windowProc)
    {
        var description = new WindowClass
        {
            cbSize = Marshal.SizeOf<WindowClass>(),
            lpfnWndProc = windowProc,
            hInstance = GetModuleHandle(null),
            lpszClassName = className,
        };

        if (RegisterClassEx(ref description) == 0 &&
            Marshal.GetLastWin32Error() != ErrorClassAlreadyExists)
        {
            throw new InvalidOperationException(
                $"RegisterClassExW failed for '{className}': {Marshal.GetLastWin32Error()}");
        }
    }

    private const int ErrorClassAlreadyExists = 1410;

    /// <summary>Creates a window of a registered class. It is not shown.</summary>
    internal static IntPtr CreateWindow(string className, string title, int style, int exStyle)
    {
        IntPtr window = CreateWindowEx(
            exStyle, className, title, style,
            0, 0, 0, 0,
            IntPtr.Zero, IntPtr.Zero, GetModuleHandle(null), IntPtr.Zero);

        if (window == IntPtr.Zero)
        {
            throw new InvalidOperationException(
                $"CreateWindowExW failed for '{className}': {Marshal.GetLastWin32Error()}");
        }

        return window;
    }

    /// <summary>
    /// Puts a finished picture into a layered window, together with where it goes and
    /// how see-through it is.
    /// </summary>
    /// <remarks>
    /// One call replaces moving the window, resizing it, painting it and setting its
    /// opacity, and it works while the window is still hidden. That is the point: the
    /// content is ready before anyone can see the window, so it can never be shown
    /// holding the picture of the previous time.
    /// </remarks>
    internal static bool SetContent(
        IntPtr window, PopupWindowNative.Point at, Size size, IntPtr sourceDc, byte alpha)
    {
        var source = new PopupWindowNative.Point { X = 0, Y = 0 };
        var blend = new BlendFunction
        {
            BlendOp = AC_SRC_OVER,
            SourceConstantAlpha = alpha,
            AlphaFormat = AC_SRC_ALPHA,
        };

        return UpdateLayeredWindow(
            window, IntPtr.Zero, ref at, ref size, sourceDc, ref source, 0, ref blend, ULW_ALPHA);
    }

    private const byte AC_SRC_OVER = 0;
    private const byte AC_SRC_ALPHA = 1;
    private const uint ULW_ALPHA = 0x00000002;

    /// <summary>
    /// BLENDFUNCTION. <c>BlendFlags</c> is never assigned and must stay all the same:
    /// Windows reads the structure by its layout, and dropping a byte from the middle
    /// of it would shift everything after.
    /// </summary>
    [StructLayout(LayoutKind.Sequential)]
    private struct BlendFunction
    {
        public byte BlendOp;
        public byte BlendFlags;
        public byte SourceConstantAlpha;
        public byte AlphaFormat;
    }

    [StructLayout(LayoutKind.Sequential)]
    internal struct Size
    {
        public int Width;
        public int Height;
    }

    [DllImport("user32.dll", SetLastError = true)]
    private static extern bool UpdateLayeredWindow(IntPtr hWnd, IntPtr hdcDst,
        ref PopupWindowNative.Point pptDst, ref Size psize, IntPtr hdcSrc,
        ref PopupWindowNative.Point pptSrc, uint crKey, ref BlendFunction pblend, uint dwFlags);

    [DllImport("user32.dll", CharSet = CharSet.Unicode, SetLastError = true)]
    private static extern ushort RegisterClassEx(ref WindowClass lpwcx);

    [DllImport("user32.dll", CharSet = CharSet.Unicode, SetLastError = true, EntryPoint = "CreateWindowExW")]
    private static extern IntPtr CreateWindowEx(int dwExStyle, string lpClassName, string lpWindowName,
        int dwStyle, int x, int y, int nWidth, int nHeight,
        IntPtr hWndParent, IntPtr hMenu, IntPtr hInstance, IntPtr lpParam);

    [DllImport("user32.dll", CharSet = CharSet.Unicode, EntryPoint = "DefWindowProcW")]
    internal static extern IntPtr DefWindowProc(IntPtr hWnd, uint message, IntPtr wParam, IntPtr lParam);

    [DllImport("user32.dll")]
    internal static extern bool DestroyWindow(IntPtr hWnd);

    [DllImport("user32.dll")]
    internal static extern bool ShowWindow(IntPtr hWnd, int nCmdShow);

    [DllImport("user32.dll")]
    internal static extern bool IsWindowVisible(IntPtr hWnd);

    [DllImport("user32.dll", CharSet = CharSet.Unicode, EntryPoint = "PostMessageW")]
    internal static extern bool PostMessage(IntPtr hWnd, uint message, IntPtr wParam, IntPtr lParam);

    [DllImport("user32.dll")]
    internal static extern void PostQuitMessage(int nExitCode);

    [DllImport("kernel32.dll", CharSet = CharSet.Unicode)]
    private static extern IntPtr GetModuleHandle(string? lpModuleName);

    [StructLayout(LayoutKind.Sequential, CharSet = CharSet.Unicode)]
    private struct WindowClass
    {
        public int cbSize;
        public uint style;

        [MarshalAs(UnmanagedType.FunctionPtr)]
        public WindowProc lpfnWndProc;

        public int cbClsExtra;
        public int cbWndExtra;
        public IntPtr hInstance;
        public IntPtr hIcon;
        public IntPtr hCursor;
        public IntPtr hbrBackground;
        public string? lpszMenuName;
        public string lpszClassName;
        public IntPtr hIconSm;
    }
}