From: lancel@pdainc.com (Lance Leverich)
Subject: Re: [delphi] Windows API headache
Date: Fri, 7 Jul 1995 09:16:54 -0400

>Question:  Does anyone know how to use SetWindowLong to install a new window
>function for an existing window (subclassing), and more specifically, the
>correct form for the new window function?  I want to do this in order to
>intercept messages going to another application.

All right, here it goes...

var
   OldWinProc: Integer;


function NewWinProc(hWnd: HWND; Msg: WORD; wParam: WORD; lParam: LONGINT): 
LONGINT;
var
   MessageProcessed: Boolean;
begin
   MessageProcessed := False;
   case Msg of
      .
      .
      { if you process a message then set MessageProcessed to True }
      .
      .
   end; {end of case Msg }
   if not MessageProcessed then
      Result := CallWindowProc(OldWinProc,hWnd,Msg,wParam,lParam)
   else
      Result := 0;
end;


procedure SubClassWin(hWnd: HWND);
begin
   OldWinProc := SetWindowLong(hWnd,GWL_WNDPROC,@NewWinProc);
end;


procedure UnSubClassWin(hWnd: HWND);
begin
   SetWindowLong(hWnd,GWL_WNDPROC,OldWinProc);
end;


The syntax (function names/parameter types)  here may not be spot on, I'm 
translating on the fly from C/C++ code that I know works.  The concepts 
should be right on line, though.


Good Luck!
Lance Leverich
PDA, Inc.
lancel@pdainc.com
(913) 469-8700 ext.4110