unit SubSecon;

interface

uses
  Windows, Messages, SysUtils, Classes, Graphics, Controls, Forms, Dialogs;

type
  TForm2 = class(TForm)
    procedure FormMouseMove(Sender: TObject; Shift: TShiftState; X,
      Y: Integer);
  private
    { Private declarations }
  public
    { Public declarations }
  end;

var
  Form2: TForm2;

implementation

{$R *.DFM}

procedure TForm2.FormMouseMove(Sender: TObject; Shift: TShiftState; X,
  Y: Integer);
begin
  Caption := Format ('Cursor in %d, %d', [X, Y]);
end;

end.

unit SubMain;

interface

uses
  Windows, Messages, SysUtils, Classes, Graphics, Controls, Forms, Dialogs,
  StdCtrls;

type
  TForm1 = class(TForm)
    BtnShow: TButton;
    BtnProc: TButton;
    BtnMeth: TButton;
    procedure BtnShowClick(Sender: TObject);
    procedure BtnMethClick(Sender: TObject);
    procedure BtnProcClick(Sender: TObject);
    procedure FormDestroy(Sender: TObject);
  private
    OldWndMeth, NewWndMeth: Pointer;
    SubControl: TWinControl;
  public
    procedure NewWinMethod (var Msg: TMessage);
  end;

var
  Form1: TForm1;

implementation

uses SubSecon;

{$R *.DFM}

var
  OldWndProc: Pointer = nil;

function NewWinProc (Handle: THandle;
  Msg, wParam, lParam: LongInt): LongInt; stdcall;
begin
  if Msg = wm_RButtonDown then
  begin
    Beep;
    SetWindowText (Handle,
      PChar (Format ('Right click in %d, %d', [
        LoWord (lParam), HiWord (lParam)])));
  end;
  // pass call to old window proc
  Result := CallWindowProc (OldWndProc, Handle,
    Msg, wParam, lParam);
end;

procedure TForm1.NewWinMethod (var Msg: TMessage);
begin
  if Msg.Msg = wm_LButtonDown then
  begin
    Beep;
    SubControl.SetTextBuf (
      PChar (Format ('Left click in %d, %d', [
        LoWord (Msg.lParam), HiWord (Msg.lParam)])));
  end
  else
    Msg.Result := CallWindowProc (OldWndMeth,
      SubControl.Handle, Msg.Msg, Msg.WParam, Msg.LParam);
end;

procedure TForm1.BtnShowClick(Sender: TObject);
begin
  Form2.Show;
end;

procedure TForm1.BtnProcClick(Sender: TObject);
begin
  OldWndProc := Pointer (SetWindowLong
    (Form2.Handle, gwl_WndProc, LongInt (@NewWinProc)));
  BtnProc.Enabled := False;
  end;

procedure TForm1.BtnMethClick(Sender: TObject);
begin
  SubControl := Form2;
  NewWndMeth := MakeObjectInstance (NewWinMethod);
  OldWndMeth := Pointer (SetWindowLong (
    SubControl.Handle, gwl_WndProc, Longint (NewWndMeth)));
  BtnMeth.Enabled := False;
    end;

procedure TForm1.FormDestroy(Sender: TObject);
begin
  if Assigned (NewWndMeth) then
    FreeObjectInstance (NewWndMeth);
end;

end.