WIN32 APPLICATION : CppRegisterHotkey Project Overview


The sample demonstrates how to register and unregister a hotkey for the
current application.


Step1. Build this project in VS2010.

Step2. Run CppRegisterHotkey.exe.

Step3. Click the edit control in the window, and press Alt+Ctrl+T. You will
      see "Alt,Control+T" displayed in the edit control, and the "Register"
      button becomes enabled.

Step4. Click the "Register" button, then the edit control and the "Register"
      button are disabled, and the "Unregister" button becomes enabled.

      If the hot key has already been registered, you will get an alert
      "The hotkey is already in use.". You can try other hotkey e.g Alt+M.

Step5. Press Alt+Ctrl+T even when this application is not the active window.
      The application will show up and be activated.


A. Implementing the hotkey edit control.

The edit control traps the key-down messages, and captures the key
combination. To trap the key-down messages, we subclass the edit control in

   // Subclass the hotkey edit control.
   HWND hHotKeyEdit = GetDlgItem(hWnd, IDC_EDIT_HOTKEY);
   UINT_PTR uIdSubclass = 0;
   if (!SetWindowSubclass(hHotKeyEdit, HotKeyEditProc, uIdSubclass, 0))
       ReportError(L"SetWindowSubclass in OnInitDialog");
       return FALSE;

We remove the window subclass before the edit control is destroyed by
inserting a call to RemoveWindowSubclass inside the subclass procedure

   LRESULT CALLBACK HotKeyEditProc(HWND hWnd, UINT message, WPARAM wParam,
       LPARAM lParam, UINT_PTR uIdSubclass, DWORD_PTR dwRefData)
       switch (message)
       case WM_NCDESTROY:
           if (!RemoveWindowSubclass(hWnd, HotKeyEditProc, uIdSubclass))
               ReportError(L"RemoveWindowSubclass in handling WM_NCDESTROY");
           return DefSubclassProc(hWnd, message, wParam, lParam);

HotKeyEditProc is the new procedure that processes messages for the hotkey
edit control. Every time a message is received by the new window procedure, a
subclass ID and reference data are included.

WM_SYSKEYUP and WM_KEYUP messages and stops their default message handler
by returning zero. The six messages are introduced in this MSDN article:

   LRESULT CALLBACK HotKeyEditProc(HWND hWnd, UINT message, WPARAM wParam,
       LPARAM lParam, UINT_PTR uIdSubclass, DWORD_PTR dwRefData)
       switch (message)
       case WM_SYSKEYDOWN:
       case WM_KEYDOWN:
           // Process the WM_KEYDOWN and WM_SYSKEYDOWN messages in
           // OnHotKeyEditKeyDown. WM_SYSKEYDOWN is posted to the window with
           // the keyboard focus when the user presses the F10 key (which
           // activates the menu bar) or holds down the ALT key and then presses
           // another key. It also occurs when no window currently has the
           // keyboard focus.
           OnHotKeyEditKeyDown(hWnd, static_cast<UINT>(wParam), lParam);
           return 0;

       case WM_SYSCHAR:
       case WM_CHAR:
       case WM_SYSKEYUP:
       case WM_KEYUP:
           // Stop processing.
           return 0;


OnHotKeyEditKeyDown handles the WM_KEYDOWN and WM_SYSKEYDOWN messages posted
to the edit control. It validates the hotkey combination. If no modifier key
(Ctrl / Shift / Alt) is pressed, the function returns directly. If the key
combination is valid (e.g. Ctrl, Alt+T), it displays the key combination in
the edit control, and saves the key modifiers and the virtual-key code in
the global variables: g_fsModifiers and g_vk.


B. Registering / unregistering the hotkey, and processing the WM_HOTKEY

The registration and unregistration of the hotkey are implemented in the
OnCommand function. If the IDC_BUTTON_REGISTER button is clicked, it calls
RegisterHotKey with the g_vk and g_fsModifiers values set previously to
register the hotkey.

The second parameter of RegisterHotKey is the hotkey ID. An application must
specify an id value in the range 0x0000 through 0xBFFF (100 in this code
sample). A shared DLL must specify a value in the range 0xC000 through 0xFFFF
(the range returned by the GlobalAddAtom function). To avoid conflicts with
hot-key identifiers defined by other shared DLLs, a DLL should use the
GlobalAddAtom function to obtain the hot-key identifier.

RegisterHotKey fails if the keystrokes specified for the hot key have already
been registered by another hot key.

       // Register the hotkey.
       if (RegisterHotKey(hWnd, g_idHotKey, g_fsModifiers, g_vk))
           // Update the UI.
           Button_Enable(GetDlgItem(hWnd, IDC_BUTTON_REGISTER), FALSE);
           Edit_Enable(GetDlgItem(hWnd, IDC_EDIT_HOTKEY), FALSE);
           Button_Enable(GetDlgItem(hWnd, IDC_BUTTON_UNREGISTER), TRUE);

           g_fHotkeyRegistered = TRUE;
           ReportError(L"RegisterHotKey", GetLastError());

When a key is pressed, the system looks for a match against all hot keys. Upon
finding a match, the system posts the WM_HOTKEY message to the message queue
of the window with which the hot key is associated. If the hot key is not
associated with a window, then the WM_HOTKEY message is posted to the thread
associated with the hot key. In this code sample, when WM_HOTKEY is received,
we activate the application main window and bring it into the front.

   // Handle the WM_HOTKEY message in OnHotKey
   HANDLE_MSG (hWnd, WM_HOTKEY, OnHotKey);

   //   FUNCTION: OnHotKey(HWND, int, UINT, UINT)
   //   PURPOSE: Process the WM_HOTKEY message. The WM_HOTKEY message is posted
   //   when the user presses a hot key registered by the RegisterHotKey
   //   function. The message is placed at the top of the message queue
   //   associated with the thread that registered the hot key.
   void OnHotKey(HWND hWnd, int idHotKey, UINT fuModifiers, UINT vk)
       // If the window is minimized, restore it.
       if (IsIconic(hWnd))
           ShowWindow(hWnd, SW_SHOWNORMAL);

       // Bring the window into the front, and activate the window.

If the IDC_BUTTON_UNREGISTER button is clicked, the OnCommand function calls
UnregisterHotKey to unregister the hotkey.  

       // Unregister the hotkey.
       if (UnregisterHotKey(hWnd, g_idHotKey))
           // Update the UI.
           Button_Enable(GetDlgItem(hWnd, IDC_BUTTON_REGISTER), TRUE);
           Edit_Enable(GetDlgItem(hWnd, IDC_EDIT_HOTKEY), TRUE);
           Button_Enable(GetDlgItem(hWnd, IDC_BUTTON_UNREGISTER), FALSE);

           g_fHotkeyRegistered = FALSE;
           ReportError(L"UnregisterHotKey", GetLastError());


MSDN: RegisterHotKey Function

MSDN: UnregisterHotKey Function