F1CDHelp.exe Uses Keyboard Hook to Access Help (83233)

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The technique used in this article is straightforward and minimizes the number of global variables required. The application installs the keyboard hook and calls the RegisterWindowMessage() function to define a help message. When an application registers the help message, the common dialog box DLL notifies the application each time the user chooses the Help button in one of the common dialog boxes. The DLL sends the help message to the window procedure of the dialog box's parent window.

When the keyboard hook function detects that the F1 key is pressed, it posts a WM_COMMAND message to the appropriate window procedure. In the F1CDHELP example, the message is posted either to the main window (when no dialog box is displayed) or to the dialog box's window procedure. If the message is posted to one of the common dialog boxes, wParam is set to pshHelp; the application simulates choosing the Help button in the common dialog box. Otherwise, wParam is set to IDM_HELPCONTENTS; the application simulates selecting a menu item in the application.

The following code demonstrates installing the hook and registering the help message in response to a WM_CREATE message:

   case WM_CREATE:
      // Install the keyboard hook

      lpfnKbrdHook = MakeProcInstance((FARPROC)KeyboardHook, ghInst);
      ghKbrdHook = SetWindowsHookEx(WH_KEYBOARD, lpfnKbrdHook,
                                    ghInst, GetCurrentTask());

      // Register the help message. The common dialog box DLL sends this
      // message when the user chooses the Help file in a common
      // dialog box.
      gwHelpMsg = RegisterWindowMessage((LPSTR)HELPMSGSTRING);
      break;
				

The following code demonstrates removing the keyboard hook in response to a WM_DESTROY message:

   case WM_DESTROY:
      UnhookWindowsHookEx(ghKbrdHook);
      break;
				

The hook function receives notification about the F1 key and posts a message as appropriate:

DWORD FAR PASCAL KeyboardHook(int iCode, WPARAM wParam, LPARAM lParam)
{
   if (iCode < 0 || iCode != HC_ACTION)
      return CallNextHookEx(ghKbrdHook, iCode, wParam, lParam);

   if (wParam == VK_F1)

      // If this is a repeat or the key is being released, ignore it.
      if (lParam & 0x80000000 || lParam & 0x40000000)
         return CallNextHookEx(ghKbrdHook, iCode, wParam, lParam);
      else
      {
         if (IsWindowEnabled(ghWnd)) // F1 pressed in main window?
            PostMessage(ghWnd, WM_COMMAND, IDM_HELPCONTENTS, 0L);
         else                        // F1 pressed in a dialog box
            PostMessage(GetActiveWindow(), WM_COMMAND, pshHelp, 0L);
      }

   return CallNextHookEx(ghKbrdHook, iCode, wParam, lParam);
}
				

The second PostMessage() call above is executed when the user requests help in a specific common dialog box. This simulates choosing the Help button in the dialog box. Because the application registered the help message (during the processing of its WM_CREATE message), the common dialog box DLL will send the gwHelpMessage to the parent window procedure. An application can process this message as follows:

   default:
      if (message == gwHelpMsg)  // Help requested in a common dialog
                                 // box
      {
      // The lParam points to an OPENFILENAME or a CHOOSECOLOR data
      // structure. The application can differentiate between them by
      // checking the structure's size, which is in the first four
      // bytes (a DWORD) of the structure. This allows the application
      // to display different help for each of the common dialog boxes.

      dwStructSize = (DWORD)(*(LPDWORD)lParam);

      switch (dwStructSize)
         {
         case sizeof(OPENFILENAME):
            MessageBox((HWND)wParam, "Help requested for OpenFile",
                       gszAppName, MB_OK);
            break;

         case sizeof(CHOOSECOLOR):
            MessageBox((HWND)wParam, "Help requested for ChooseColor"
                       gszAppName, MB_OK);
            break;

         default:
            break;
         }
      break;
      }
      else // Not a help message
         return DefWindowProc(hWnd, message, wParam, lParam);