lightweight variant (#1)

Reviewed-on: #1
Co-authored-by: Aleksandr Neychev <alexnejchev73@gmail.com>

CursorLang.Core/Threading/MessageLoop.cs

@@ -0,0 +1,66 @@
+using System.Runtime.InteropServices;
+
+namespace CursorLang.Core.Threading;
+
+/// <summary>
+/// The Win32 message loop — what the application has instead of a dispatcher.
+/// </summary>
+/// <remarks>
+/// It lives in Core rather than in the agent because the things that need a pumping
+/// thread do: <see cref="MessageTimer"/> is used by the layout polling and by saving
+/// the settings, and both are Core's. The settings window has a loop of its own, run
+/// by WPF, and everything here works inside it just the same.
+/// </remarks>
+public static class MessageLoop
+{
+    /// <summary>
+    /// Pumps messages until <c>WM_QUIT</c> and returns its exit code.
+    /// </summary>
+    /// <remarks>
+    /// A -1 from GetMessage means the window handle has already gone; going round
+    /// again would spin forever, so the loop gives up instead.
+    /// </remarks>
+    public static int Run()
+    {
+        while (true)
+        {
+            int result = GetMessage(out Message message, IntPtr.Zero, 0, 0);
+            if (result is 0 or -1)
+            {
+                return result == 0 ? (int)message.wParam : 1;
+            }
+
+            TranslateMessage(ref message);
+            DispatchMessage(ref message);
+        }
+    }
+
+    /// <summary>Asks the loop on this thread to finish.</summary>
+    public static void Quit(int exitCode = 0) => PostQuitMessage(exitCode);
+
+
+    [DllImport("user32.dll", CharSet = CharSet.Unicode, EntryPoint = "GetMessageW")]
+    private static extern int GetMessage(out Message lpMsg, IntPtr hWnd, uint filterMin, uint filterMax);
+
+    [DllImport("user32.dll")]
+    private static extern bool TranslateMessage(ref Message lpMsg);
+
+    [DllImport("user32.dll", CharSet = CharSet.Unicode, EntryPoint = "DispatchMessageW")]
+    private static extern IntPtr DispatchMessage(ref Message lpMsg);
+
+    [DllImport("user32.dll")]
+    private static extern void PostQuitMessage(int exitCode);
+
+    [StructLayout(LayoutKind.Sequential)]
+    private struct Message
+    {
+        public IntPtr hwnd;
+        public uint message;
+        public IntPtr wParam;
+        public IntPtr lParam;
+        public uint time;
+        public int x;
+        public int y;
+        public uint lPrivate;
+    }
+}

CursorLang.Core/Threading/MessageTimer.cs

@@ -0,0 +1,70 @@
+using System.Runtime.InteropServices;
+
+namespace CursorLang.Core.Threading;
+
+/// <summary>
+/// A timer that ticks on the message loop — the agent's stand-in for
+/// <c>DispatcherTimer</c>.
+/// </summary>
+/// <remarks>
+/// <c>SetTimer</c> with a null window binds the timer to the thread rather than to a
+/// window, and <c>DispatchMessage</c> calls the callback straight from the loop. The
+/// upshot is the same as with a dispatcher timer: the tick arrives on the thread that
+/// owns the hook and the popup, so nothing needs marshalling and nothing races.
+///
+/// The callback lives in a field for the reason a hook procedure does: the only
+/// reference to it is held by Win32, and a collected delegate takes the process down
+/// with it at the first tick.
+/// </remarks>
+internal sealed class MessageTimer : IDisposable
+{
+    /// <summary>Windows will not go below this, and pretending otherwise misleads.</summary>
+    private const uint MinimumIntervalMilliseconds = 10;
+
+    private readonly TimerProc _callback;
+
+    private nuint _id;
+
+    internal MessageTimer() => _callback = OnTimer;
+
+    internal event EventHandler? Tick;
+
+    internal TimeSpan Interval { get; set; } = TimeSpan.FromMilliseconds(100);
+
+    internal bool IsRunning => _id != 0;
+
+    /// <summary>Starts the timer, or restarts it from zero when it is already running.</summary>
+    internal void Start()
+    {
+        Stop();
+
+        var milliseconds = (uint)Math.Clamp(
+            Math.Round(Interval.TotalMilliseconds), MinimumIntervalMilliseconds, int.MaxValue);
+
+        _id = SetTimer(IntPtr.Zero, 0, milliseconds, _callback);
+    }
+
+    internal void Stop()
+    {
+        if (_id == 0)
+        {
+            return;
+        }
+
+        KillTimer(IntPtr.Zero, _id);
+        _id = 0;
+    }
+
+    public void Dispose() => Stop();
+
+    private void OnTimer(IntPtr window, uint message, nuint id, uint time) =>
+        Tick?.Invoke(this, EventArgs.Empty);
+
+    private delegate void TimerProc(IntPtr hWnd, uint message, nuint idEvent, uint time);
+
+    [DllImport("user32.dll", SetLastError = true)]
+    private static extern nuint SetTimer(IntPtr hWnd, nuint nIDEvent, uint uElapse, TimerProc lpTimerFunc);
+
+    [DllImport("user32.dll", SetLastError = true)]
+    private static extern bool KillTimer(IntPtr hWnd, nuint uIDEvent);
+}