lightweight variant (#1)

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

CursorLang.Core/Interop/LowLevelKeyboardHook.cs

@@ -0,0 +1,130 @@
+using System.Runtime.InteropServices;
+
+namespace CursorLang.Core.Interop;
+
+/// <summary>
+/// A system keyboard hook (WH_KEYBOARD_LL): sees key presses in every application
+/// and can keep them from going any further.
+/// </summary>
+/// <remarks>
+/// Only the low-level hook is installed: a regular WH_KEYBOARD requires injecting a
+/// DLL into other processes, which is impossible for managed code. The callback
+/// arrives on the thread that installed the hook, and that thread must pump a message
+/// loop — hence the requirement to install the hook from the user interface thread.
+/// Returning control must not be delayed: once the system timeout expires, Windows
+/// silently removes the hook.
+/// </remarks>
+internal sealed class LowLevelKeyboardHook : IDisposable
+{
+    /// <summary>
+    /// A key event handler. Returns <c>true</c> when the event must be swallowed —
+    /// then the foreground application will not see it.
+    /// </summary>
+    internal delegate bool KeyFilter(int virtualKey, bool isKeyDown);
+
+    private const int WhKeyboardLowLevel = 13;
+    private const int HcAction = 0;
+
+    private const int WmKeyDown = 0x0100;
+    private const int WmKeyUp = 0x0101;
+    private const int WmSysKeyDown = 0x0104;
+    private const int WmSysKeyUp = 0x0105;
+
+    /// <summary>The event came from SendInput rather than from a real key press.</summary>
+    private const uint LowLevelKeyHookFlagInjected = 0x10;
+
+    private readonly KeyFilter _filter;
+
+    // The delegate lives in a field not for convenience: the only reference to it is
+    // held by Win32, which the garbage collector knows nothing about, and without the
+    // field the hook stops working after a random amount of time
+    private readonly HookProc _callback;
+
+    private IntPtr _handle;
+
+    internal LowLevelKeyboardHook(KeyFilter filter)
+    {
+        _filter = filter;
+        _callback = OnHookEvent;
+    }
+
+    private delegate IntPtr HookProc(int code, IntPtr wParam, IntPtr lParam);
+
+    internal bool IsInstalled => _handle != IntPtr.Zero;
+
+    /// <summary>
+    /// Installs the hook. Returns <c>false</c> when the system refuses.
+    /// </summary>
+    internal bool Install()
+    {
+        if (_handle != IntPtr.Zero)
+        {
+            return true;
+        }
+
+        _handle = SetWindowsHookEx(WhKeyboardLowLevel, _callback, GetModuleHandle(null), 0);
+        return _handle != IntPtr.Zero;
+    }
+
+    internal void Uninstall()
+    {
+        if (_handle == IntPtr.Zero)
+        {
+            return;
+        }
+
+        UnhookWindowsHookEx(_handle);
+        _handle = IntPtr.Zero;
+    }
+
+    public void Dispose() => Uninstall();
+
+    [DllImport("user32.dll", SetLastError = true)]
+    private static extern IntPtr SetWindowsHookEx(int idHook, HookProc lpfn, IntPtr hMod, uint dwThreadId);
+
+    [DllImport("user32.dll", SetLastError = true)]
+    private static extern bool UnhookWindowsHookEx(IntPtr hhk);
+
+    [DllImport("user32.dll")]
+    private static extern IntPtr CallNextHookEx(IntPtr hhk, int code, IntPtr wParam, IntPtr lParam);
+
+    [DllImport("kernel32.dll", CharSet = CharSet.Unicode)]
+    private static extern IntPtr GetModuleHandle(string? lpModuleName);
+
+    private IntPtr OnHookEvent(int code, IntPtr wParam, IntPtr lParam)
+    {
+        if (code != HcAction)
+        {
+            return CallNextHookEx(_handle, code, wParam, lParam);
+        }
+
+        var message = (int)wParam;
+        bool isKeyDown = message is WmKeyDown or WmSysKeyDown;
+        bool isKeyUp = message is WmKeyUp or WmSysKeyUp;
+
+        var data = Marshal.PtrToStructure<KeyboardHookData>(lParam);
+
+        // Synthetic input comes from on-screen keyboards, text expanders and
+        // automation tools: overriding what they do is none of our business
+        bool injected = (data.flags & LowLevelKeyHookFlagInjected) != 0;
+
+        if ((isKeyDown || isKeyUp) && !injected && _filter((int)data.vkCode, isKeyDown))
+        {
+            // A non-zero result instead of CallNextHookEx breaks the chain: the event
+            // will reach neither the application nor the Windows caps-lock handler
+            return 1;
+        }
+
+        return CallNextHookEx(_handle, code, wParam, lParam);
+    }
+
+    [StructLayout(LayoutKind.Sequential)]
+    private struct KeyboardHookData
+    {
+        public uint vkCode;
+        public uint scanCode;
+        public uint flags;
+        public uint time;
+        public IntPtr dwExtraInfo;
+    }
+}