Автор: Xavier Pacheco

unit Scwndprc;

interface

uses Forms, Messages;

const
  DDGM_FOOMSG = WM_USER;

implementation

uses Windows, SysUtils, Dialogs;

var
  WProc: Pointer;

function NewWndProc(Handle: hWnd; Msg, wParam, lParam: Longint): Longint;
  stdcall;
{ This is a Win32 API-level window procedure. It handles the messages }
{ received by the Application window. }
begin
  if Msg = DDGM_FOOMSG then
    { If it's our user-defined message, then alert the user. }
    ShowMessage(Format('Message seen by WndProc! Value is: $%x', [Msg]));
  { Pass message on to old window procedure }
  Result := CallWindowProc(WProc, Handle, Msg, wParam, lParam);
end;

initialization
  { Set window procedure of Application window. }
  WProc := Pointer(SetWindowLong(Application.Handle, gwl_WndProc,
    Integer(@NewWndProc)));
end.

unit Main;

interface

uses
  SysUtils, WinTypes, WinProcs, Messages, Classes, Graphics, Controls,
  Forms, Dialogs, StdCtrls;

type
  TMainForm = class(TForm)
    SendBtn: TButton;
    PostBtn: TButton;
    procedure SendBtnClick(Sender: TObject);
    procedure PostBtnClick(Sender: TObject);
    procedure FormCreate(Sender: TObject);
    procedure FormDestroy(Sender: TObject);
  private
    OldWndProc: Pointer;
    WndProcPtr: Pointer;
    procedure WndMethod(var Msg: TMessage);
    procedure HandleAppMessage(var Msg: TMsg; var Handled: Boolean);
  end;

var
  MainForm: TMainForm;

implementation

{$R *.DFM}

uses ScWndPrc;

procedure TMainForm.HandleAppMessage(var Msg: TMsg; var Handled: Boolean);
{ OnMessage handler for Application object. }
begin
  if Msg.Message = DDGM_FOOMSG then
    { if it's the user-defined message, then alert the user. }
    ShowMessage(Format('Message seen by OnMessage! Value is: $%x',
      [Msg.Message]));
end;

procedure TMainForm.WndMethod(var Msg: TMessage);
begin
  if Msg.Msg = DDGM_FOOMSG then
    { if it's the user-defined message, then alert the user. }
    ShowMessage(Format('Message seen by WndMethod! Value is: $%x', [Msg.Msg]));
  with Msg do
    { Pass message on to old window procedure. }
    Result := CallWindowProc(OldWndProc, Application.Handle, Msg, wParam,
      lParam);
end;

procedure TMainForm.SendBtnClick(Sender: TObject);
begin
  SendMessage(Application.Handle, DDGM_FOOMSG, 0, 0);
end;

procedure TMainForm.PostBtnClick(Sender: TObject);
begin
  PostMessage(Application.Handle, DDGM_FOOMSG, 0, 0);
end;

procedure TMainForm.FormCreate(Sender: TObject);
begin
  Application.OnMessage := HandleAppMessage; // set OnMessage handler
  WndProcPtr := MakeObjectInstance(WndMethod); // make window proc
  { Set window procedure of application window. }
  OldWndProc := Pointer(SetWindowLong(Application.Handle, GWL_WNDPROC,
    Integer(WndProcPtr)));
end;

procedure TMainForm.FormDestroy(Sender: TObject);
begin
  { Restore old window procedure for Application window }
  SetWindowLong(Application.Handle, GWL_WNDPROC, Longint(OldWndProc));
  { Free our user-created window procedure }
  FreeObjectInstance(WndProcPtr);
end;

end.