Author: Vimil Saju 

How to subclass non Delphi windows

Answer:

Every window has a procedure associated with it that recieves all the messages that 
are sent to it. To subclass a window means to replace the procedure associated with 
the window by another user defined procedure. The main use of subclassing windows 
is to customize how a window works. 

The handle of the procedure associated with the window can be got by calling the 
GetWindowLong function. 

hproc: TFarproc;
hproc := TFarProc(GetWindowLong(hwnd, GWL_WNDPROC));

if hwnd is the handle of the function then hproc is the handle of the procedure 
associated with the window. 

Now define a procedure that will replace the original procedure associated with the 
window. 

For example see the code below. 

1   type
2     TForm1 = class(TForm)
3     private
4       hproc: TFarproc;
5     protected
6       procedure WndProc(var msg: TMessage);
7     end;


Here the procedure WndProc will replace the original window procedure. 

To replace the original procedure with the procedure WndProc you have to first call 
the function 'MakeObjectInstance'  defined in forms unit which converts a member 
procedure(Here WndProc is member of TForm1 class) to a standard procedure. 

This is because the WndProc procedure is a member function and Windows does not 
understand class member functions.  Class member functions have a "self" pointer as 
a hidden first parameter which uniquely identifies a class object (keep in mind 
that an object is a specific instantiation of a class). The API callback does not 
know how to pass "self". 

To convert the member procedure to a standard procedure call the MakeObjectInstance 
function as shown below. 

fproc: TFarProc;
fproc := MakeObjectInstance(WndProc);

after you have done this 

call the SetWindowlong function to replace the procedure of the window as shown 
below 

SetWindowlong(hwnd, GWL_WNDPROC, longword(fcurProc));

Now all messages that are sent to the window will be intercepted by the WndProc 
procedure 

The messages that are not handled by the WndProc can be sent to the original 
procedure by using the CallWindowProc function. 

Here is how to call the function. 

8   procedure TForm1.DlgProc(var msg: Tmessage);
9   begin
10    case msg.msg of
11      WM_SIZE:
12        begin
13          //user defined code
14        end;
15      WM_PAINT:
16        begin
17          //user defined code
18        end;
19    end;
20    //all unhandled messages are sent to original procedure. Here  hproc is the  
21  	//handle to the original window procedure.
22    msg.result := CallWindowProc(hproc, hwnd, msg.msg, msg.wparam, msg.lparam);
23  end;


once you have finshed you have to destroy the handle created by the 
MakeObjectInstance(WndProc) function by calling the 'FreeObjectInstance' function 
which is also defined in the forms unit. You usually call this function when the 
form is closed. Example of how to call the function is shown below. 

24  procedure TForm1.FormClose(Sender: TObject; var Action: TCloseAction);
25  begin
26    FreeObjectInstance(fproc);
27  end;