lightweight variant (#1)

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

CursorLang.Core/Services/SettingsSignal.cs

@@ -0,0 +1,52 @@
+using System.Runtime.InteropServices;
+
+namespace CursorLang.Core.Services;
+
+/// <summary>
+/// Tells the agent that settings.json has changed.
+/// </summary>
+/// <remarks>
+/// The message carries nothing but the fact. The temptation to send the changed values
+/// along has to be resisted: the file would stop being the only source of truth, and
+/// the two would part company the first time somebody edits it by hand. Nothing else
+/// tells the agent — it does not watch the file — so a message that goes missing means
+/// settings it does not pick up until it is restarted.
+///
+/// Order is what makes it safe. The settings window writes the file whole, moves it
+/// into place in one step and only then signals, so by the time the agent reads there
+/// is nothing half-written to read.
+///
+/// A registered message rather than <c>WM_APP + n</c>: the identifier is unique across
+/// the system, so it cannot be confused with anything else that finds its way to that
+/// window.
+/// </remarks>
+internal static class SettingsSignal
+{
+    /// <summary>The window class the agent registers for its hidden window.</summary>
+    internal const string AgentWindowClass = "CursorLang.Agent.Window";
+
+    /// <summary>The message both sides agree on.</summary>
+    internal static uint Message { get; } = RegisterWindowMessage("CursorLang.SettingsChanged");
+
+    /// <summary>
+    /// Wakes the agent, if one is running in this session. Silence is a normal
+    /// answer: the settings window is perfectly usable with no agent behind it.
+    /// </summary>
+    internal static void NotifyAgent()
+    {
+        IntPtr agent = FindWindow(AgentWindowClass, null);
+        if (agent != IntPtr.Zero)
+        {
+            PostMessage(agent, Message, IntPtr.Zero, IntPtr.Zero);
+        }
+    }
+
+    [DllImport("user32.dll", CharSet = CharSet.Unicode, EntryPoint = "RegisterWindowMessageW")]
+    private static extern uint RegisterWindowMessage(string lpString);
+
+    [DllImport("user32.dll", CharSet = CharSet.Unicode, EntryPoint = "FindWindowW")]
+    private static extern IntPtr FindWindow(string lpClassName, string? lpWindowName);
+
+    [DllImport("user32.dll", CharSet = CharSet.Unicode, EntryPoint = "PostMessageW")]
+    private static extern bool PostMessage(IntPtr hWnd, uint message, IntPtr wParam, IntPtr lParam);
+}