Author: Tomas Rutkauskas 

How to trap and process Windows messages before the applications' window procedure 
is executed

Answer:

The following project source demonstrates how to get Window messages before the 
application's window procedure is called. It is rare, if ever, that this needs to 
be done. In most cases assigning a procedure to the Application.OnMessage will 
accomplish the same thing.


1   program Project1;
2   
3   uses
4     Forms, messages, wintypes, winprocs, Unit1 in 'UNIT1.PAS' {Form1};
5   
6   {$R *.RES}
7   
8   var
9     OldWndProc: TFarProc;
10  
11  function NewWndProc(hWndAppl: HWnd; Msg, wParam: Word; lParam: Longint): Longint; 
12  export;
13  begin
14    result := 0; { Default WndProc return value }
15    {Handle messages here; The message number is in Msg}
16    result := CallWindowProc(OldWndProc, hWndAppl, Msg, wParam, lParam);
17  end;
18  
19  begin
20    Application.CreateForm(TForm1, Form1);
21    OldWndProc := TFarProc(GetWindowLong(Application.Handle, GWL_WNDPROC));
22    SetWindowLong(Application.Handle, GWL_WNDPROC, longint(@NewWndProc));
23    Application.Run;
24  end.