CBTProc callback function

  • Article

An application-defined or library-defined callback function used with the SetWindowsHookEx function. The system calls this function before activating, creating, destroying, minimizing, maximizing, moving, or sizing a window; before completing a system command; before removing a mouse or keyboard event from the system message queue; before setting the keyboard focus; or before synchronizing with the system message queue. A computer-based training (CBT) application uses this hook procedure to receive useful notifications from the system.

The HOOKPROC type defines a pointer to this callback function. CBTProc is a placeholder for the application-defined or library-defined function name.

Syntax

LRESULT CALLBACK CBTProc(
  _In_ int    nCode,
  _In_ WPARAM wParam,
  _In_ LPARAM lParam
);

Parameters

  • nCode [in]
    Type: int

    The code that the hook procedure uses to determine how to process the message. If nCode is less than zero, the hook procedure must pass the message to the CallNextHookEx function without further processing and should return the value returned by CallNextHookEx. This parameter can be one of the following values.

    Value Meaning
    HCBT_ACTIVATE 5

    The system is about to activate a window.
    HCBT_CLICKSKIPPED 6

    The system has removed a mouse message from the system message queue. Upon receiving this hook code, a CBT application must install a WH_JOURNALPLAYBACK hook procedure in response to the mouse message.
    HCBT_CREATEWND 3

    A window is about to be created. The system calls the hook procedure before sending the WM_CREATE or WM_NCCREATE message to the window. If the hook procedure returns a nonzero value, the system destroys the window; the CreateWindow function returns NULL, but the WM_DESTROY message is not sent to the window. If the hook procedure returns zero, the window is created normally.

    At the time of the HCBT_CREATEWND notification, the window has been created, but its final size and position may not have been determined and its parent window may not have been established. It is possible to send messages to the newly created window, although it has not yet received WM_NCCREATE or WM_CREATE messages. It is also possible to change the position in the z-order of the newly created window by modifying the hwndInsertAfter member of the CBT_CREATEWND structure.
    HCBT_DESTROYWND 4

    A window is about to be destroyed.
    HCBT_KEYSKIPPED 7

    The system has removed a keyboard message from the system message queue. Upon receiving this hook code, a CBT application must install a WH_JOURNALPLAYBACK hook procedure in response to the keyboard message.
    HCBT_MINMAX 1

    A window is about to be minimized or maximized.
    HCBT_MOVESIZE 0

    A window is about to be moved or sized.
    HCBT_QS 2

    The system has retrieved a WM_QUEUESYNC message from the system message queue.
    HCBT_SETFOCUS 9

    A window is about to receive the keyboard focus.
    HCBT_SYSCOMMAND 8

    A system command is about to be carried out. This allows a CBT application to prevent task switching by means of hot keys.

     

  • wParam [in]
    Type: WPARAM

    Depends on the nCode parameter. For details, see the following Remarks section.

  • lParam [in]
    Type: LPARAM

    Depends on the nCode parameter. For details, see the following Remarks section.

Return value

Type: ****

Type: LRESULT

The value returned by the hook procedure determines whether the system allows or prevents one of these operations. For operations corresponding to the following CBT hook codes, the return value must be 0 to allow the operation, or 1 to prevent it.

  • HCBT_ACTIVATE
  • HCBT_CREATEWND
  • HCBT_DESTROYWND
  • HCBT_MINMAX
  • HCBT_MOVESIZE
  • HCBT_SETFOCUS
  • HCBT_SYSCOMMAND

For operations corresponding to the following CBT hook codes, the return value is ignored.

  • HCBT_CLICKSKIPPED
  • HCBT_KEYSKIPPED
  • HCBT_QS

Remarks

The hook procedure should not install a WH_JOURNALPLAYBACK hook procedure except in the situations described in the preceding list of hook codes.

An application installs the hook procedure by specifying the WH_CBT hook type and a pointer to the hook procedure in a call to the SetWindowsHookEx function.

The following table describes the wParam and lParam parameters for each HCBT_ hook code.

Value wParam lParam
HCBT_ACTIVATE Specifies the handle to the window about to be activated. Specifies a long pointer to a CBTACTIVATESTRUCT structure containing the handle to the active window and specifies whether the activation is changing because of a mouse click.
HCBT_CLICKSKIPPED Specifies the mouse message removed from the system message queue. Specifies a long pointer to a MOUSEHOOKSTRUCT structure containing the hit-test code and the handle to the window for which the mouse message is intended.

The HCBT_CLICKSKIPPED value is sent to a CBTProc hook procedure only if a WH_MOUSE hook is installed. For a list of hit-test codes, see WM_NCHITTEST.
HCBT_CREATEWND Specifies the handle to the new window. Specifies a long pointer to a CBT_CREATEWND structure containing initialization parameters for the window. The parameters include the coordinates and dimensions of the window. By changing these parameters, a CBTProc hook procedure can set the initial size and position of the window.
HCBT_DESTROYWND Specifies the handle to the window about to be destroyed. Is undefined and must be set to zero.
HCBT_KEYSKIPPED Specifies the virtual-key code. Specifies the repeat count, scan code, key-transition code, previous key state, and context code. The HCBT_KEYSKIPPED value is sent to a CBTProc hook procedure only if a WH_KEYBOARD hook is installed. For more information, see WM_KEYUP or WM_KEYDOWN.
HCBT_MINMAX Specifies the handle to the window being minimized or maximized. Specifies, in the low-order word, a show-window value (SW_) specifying the operation. For a list of show-window values, see the ShowWindow. The high-order word is undefined.
HCBT_MOVESIZE Specifies the handle to the window to be moved or sized. Specifies a long pointer to a RECT structure containing the coordinates of the window. By changing the values in the structure, a CBTProc hook procedure can set the final coordinates of the window.
HCBT_QS Is undefined and must be zero. Is undefined and must be zero.
HCBT_SETFOCUS Specifies the handle to the window gaining the keyboard focus. Specifies the handle to the window losing the keyboard focus.
HCBT_SYSCOMMAND Specifies a system-command value (SC_) specifying the system command. For more information about system-command values, see WM_SYSCOMMAND. Contains the same data as the lParam value of a WM_SYSCOMMAND message: If a system menu command is chosen with the mouse, the low-order word contains the x-coordinate of the cursor, in screen coordinates, and the high-order word contains the y-coordinate; otherwise, the parameter is not used.

 

For information, see WinEvents.

Requirements

Minimum supported client

Windows 2000 Professional [desktop apps only]

Minimum supported server

Windows 2000 Server [desktop apps only]

Header

 Winuser.h (include Windows.h)

See also

Reference

CallNextHookEx

CreateWindow

SetWindowsHookEx

WM_SYSCOMMAND

Conceptual

Hooks