Capturing keystrokes without focus

Question

E.g. with winamp (on Windows at least), you can play a game fullscreen with winamp in the background, and use the media buttons* to control the sound. Winamp doesn\'t need t

Answer 1

Low-Level windows hooks is one way to do it. Here is one article and here is a little more info from MSDN.

This is a partial view of what that code can look like:

    private IntPtr LowLevelKeyboardHook(int nCode, WindowsMessages wParam, [In] KBDLLHOOKSTRUCT lParam)
    {
        bool callNext = true;

        bool isKeyDown = (wParam == WindowsMessages.KEYDOWN || wParam == WindowsMessages.SYSKEYDOWN);
        bool isKeyUp = (wParam == WindowsMessages.KEYUP || wParam == WindowsMessages.SYSKEYUP);

        if ((nCode >= 0) && (isKeyDown || isKeyUp))
        {
            // the virtual key codes and the winforms Keys have the same enumeration
            // so we can freely cast back and forth between them
            Keys key = (Keys)lParam.vkCode;

            // Do your other processing here...
        }

        // if any handler returned false, trap the message
        return (callNext) ? User32.CallNextHookEx(_mainHook, nCode, wParam, lParam) : _nullNext;
    }


    /// <summary>
    /// Registers the user's LowLevelKeyboardProc with the system in order to
    /// intercept any keyboard events before processed in the regular fashion.
    /// This can be used to log all keyboard events or ignore them.
    /// </summary>
    /// <param name="hook">Callback function to call whenever a keyboard event occurs.</param>
    /// <returns>The IntPtr assigned by the Windows's sytem that defines the callback.</returns>
    private IntPtr RegisterLowLevelHook(LowLevelKeyboardProc hook)
    {
        IntPtr handle = IntPtr.Zero;

        using (Process currentProcess = Process.GetCurrentProcess())
        using (ProcessModule currentModule = currentProcess.MainModule)
        {
            IntPtr module = Kernel32.GetModuleHandle(currentModule.ModuleName);
            handle = User32.SetWindowsHookEx(HookType.KEYBOARD_LL, hook, module, 0);
        }

        return handle;
    }

    /// <summary>
    /// Unregisters a previously registered callback from the low-level chain.
    /// </summary>
    /// <param name="hook">IntPtr previously assigned to the low-level chain.
    /// Users should have stored the value given by 
    /// <see cref="Drs.Interop.Win32.LowLevelKeyboard.RegisterLowLevelHook"/>,
    /// and use that value as the parameter into this function.</param>
    /// <returns>True if the hook was removed, false otherwise.</returns>
    private bool UnregisterLowLevelHook(IntPtr hook)
    {
        return User32.UnhookWindowsHookEx(hook);
    }

Just implement all the P/Invoke declarations needed, and it should work. I use this approach in my application and it works fine.

Answer 2

In Java there are two libraries that can do this:

• JIntellitype (Windows)
• JXGrabKey (X11)