added sending closing event message to settings windows (#7)

Reviewed-on: #7
Co-authored-by: Aleksandr Neychev <alexnejchev73@gmail.com>

CursorLang.Core/Services/SettingsCloseSignal.cs

@@ -0,0 +1,98 @@
+namespace CursorLang.Core.Services;
+
+/// <summary>
+/// Tells the settings window that the agent is quitting and it is to close with it.
+/// </summary>
+/// <remarks>
+/// The one thing the agent says to the settings window, and the mirror image of
+/// <see cref="SettingsSignal"/>. "Exit" in the tray menu means the application is done
+/// with, and a settings window left alone on the screen after it is a window belonging
+/// to nothing: the tray icon it was opened from is gone, and closing it would be the
+/// user's only remaining move.
+///
+/// A named event rather than a window message, because the agent has no handle to send
+/// one to: the settings window lives in a process the agent starts and deliberately
+/// does not keep hold of. The name has no <c>Global</c> prefix, so it lives in the
+/// session namespace — same reasoning as <see cref="SingleInstanceGate"/>, and the same
+/// consequence: with fast user switching each user's halves talk to their own.
+///
+/// Only the settings window creates the object; the agent opens what is already there
+/// and stays silent when there is nothing. Were it the other way round, the request
+/// would sit in an auto-reset event waiting for the next settings window to open and
+/// close it the moment it did.
+/// </remarks>
+internal sealed class SettingsCloseSignal : IDisposable
+{
+    private const string EventName = "CursorLang.CloseSettings";
+
+    private readonly string _eventName;
+
+    private EventWaitHandle? _request;
+    private RegisteredWaitHandle? _wait;
+
+    /// <summary>
+    /// Listens on the name the two halves agree on.
+    /// </summary>
+    /// <param name="nameSuffix">
+    /// A namespace of its own. Empty for the application; the tests pass one so that
+    /// they do not answer for — or worse, close — a settings window someone is using.
+    /// </param>
+    internal SettingsCloseSignal(string nameSuffix = "") => _eventName = EventName + nameSuffix;
+
+    /// <summary>The agent asks for the window to be closed.</summary>
+    /// <remarks>
+    /// Raised on a thread pool thread, wherever the wait happened to be answered — a
+    /// window obeys only its own, so the handler has to get back to it.
+    /// </remarks>
+    internal event EventHandler? CloseRequested;
+
+    /// <summary>
+    /// Asks the settings window of this session, if one is open, to close. Silence is
+    /// a normal answer: most of the time the user quits with no window on the screen.
+    /// </summary>
+    /// <returns>Whether there was anybody to hear it.</returns>
+    internal static bool RequestClose(string nameSuffix = "")
+    {
+        if (!EventWaitHandle.TryOpenExisting(EventName + nameSuffix, out EventWaitHandle? request))
+        {
+            return false;
+        }
+
+        using (request)
+        {
+            return request.Set();
+        }
+    }
+
+    /// <summary>Starts waiting for the request. Called once, by the settings window.</summary>
+    internal void Listen()
+    {
+        if (_request is not null)
+        {
+            return;
+        }
+
+        _request = new EventWaitHandle(false, EventResetMode.AutoReset, _eventName);
+
+        // As in the gate: the thread pool holds the wait, there is no reason to keep a
+        // thread of our own for a request that may never come
+        _wait = ThreadPool.RegisterWaitForSingleObject(
+            _request,
+            OnCloseSignalled,
+            state: null,
+            Timeout.Infinite,
+            executeOnlyOnce: false);
+    }
+
+    public void Dispose()
+    {
+        _wait?.Unregister(null);
+        _wait = null;
+
+        _request?.Dispose();
+        _request = null;
+    }
+
+    private void OnCloseSignalled(object? state, bool timedOut) =>
+        CloseRequested?.Invoke(this, EventArgs.Empty);
+}