KB140883 - How To Subclass a VB Form Using 16-Bit VB CDK Custom Control

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The following code example demonstrates how to subclass a form from a custom control using the Visual Basic Custom CDK.

This example is developed using the CIRCLE.C program example from the CIRCLE1 project supplied with the CDK package. Only the file(s) that have changed from this project are included, and it is assumed that you have the additional CDK files as well as a C compiler capable of creating a Windows 3.0 compatible dynamic link library (DLL).

The basic idea for subclassing is to examine the window structure for a window directly using the GetWindowLong function to determine the address of the original window procedure. You can then change the address of the target window's window procedure to the address of your subclass procedure using SetWindowLong. In your subclass window procedure, you handle the messages you wish and use CallWindowProc to pass along other messages to the original window procedure.

//=================== CIRCLE1 ==================
// CIRCLE.C
// An example of subclassing a Visual Basic Form
//==============================================

#define  NOCOMM
#include <windows.h>

#include <vbapi.h>
#include "circle.h"

//declare the subclass procedure
LONG FAR PASCAL _export SbClsProc(HWND,USHORT,USHORT,LONG);

//far pointer to the default procedure
FARPROC lpfnOldProc = (FARPROC) NULL ;

//get the controls parent handle(form1)
HWND    hParent ;

//----------------------------------------------------------
// Circle Control Procedure
//----------------------------------------------------------
LONG FAR PASCAL _export CircleCtlProc (HCTL hctl, HWND hwnd,
     USHORT msg, USHORT wp, LONG lp)
{
   LONG lResult ;
   switch (msg)
   {
      case WM_CREATE:
         switch (VBGetMode())
         {
            //this will only be processed during run mode
            case MODE_RUN:
            {
               hParent = GetParent (hwnd) ;
               //get the address instance to normal proc
               lpfnOldProc = (FARPROC) GetWindowLong
                              (hParent, GWL_WNDPROC) ;
               //reset the address instance to the new proc
               SetWindowLong (hParent,
                        GWL_WNDPROC, (LONG) SbClsProc) ;
            }
            break ;
         }
         break ;
   }
   // call the default VB proc
   lResult = VBDefControlProc(hctl, hwnd, msg, wp, lp);
   return lResult;

}

LONG FAR PASCAL _export SbClsProc (HWND hwnd, USHORT msg,
     USHORT wp, LONG lp)
{
   switch (msg)
   {
      case WM_SIZE:
      {
      //place size event here for example...
      }
      break;
      case WM_DESTROY:
         SetWindowLong (hwnd, GWL_WNDPROC,
                        (LONG) lpfnOldProc) ;
      break ;
   }
   // call CircleCtlProc to process any other messages
   return (CallWindowProc(lpfnOldProc, hwnd, msg, wp, lp));
}

;==========================================================
;Circle.def - module definition file for CIRCLE3.VBX control
;==========================================================
LIBRARY         CIRCLE
EXETYPE         WINDOWS
DESCRIPTION     'Visual Basic Circle Custom Control'

CODE            MOVEABLE
DATA            MOVEABLE SINGLE

HEAPSIZE        1024

EXPORTS
           WEP       @1 RESIDENTNAME
           SbClsProc @2
;------------------------------------------------------

How To Subclass a VB Form Using 16-Bit VB CDK Custom Control

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