Как отловить сообщения о прокрутке TScrollBar

type
{$IFDEF WIN32}
  WParameter = LongInt;
{$ELSE}
  WParameter = Word;
{$ENDIF}
  LParameter = LongInt;

  {Declare a variable to hold the window procedure we are replacing}
var
  OldWindowProc: Pointer;

function NewWindowProc(WindowHandle: hWnd;
  TheMessage: WParameter;
  ParamW: WParameter;
  ParamL: LParameter): LongInt
{$IFDEF WIN32} stdcall;
{$ELSE}; export;
{$ENDIF}
var
  TheRangeMin: integer;
  TheRangeMax: integer;
  TheRange: integer;
begin

  if TheMessage = WM_VSCROLL then
  begin
    {Get the min and max range of the horizontal scroll box}
    GetScrollRange(WindowHandle,
      SB_HORZ,
      TheRangeMin,
      TheRangeMax);
    {Get the vertical scroll box position}
    TheRange := GetScrollPos(WindowHandle,
      SB_VERT);
    {Make sure we wont exceed the range}
    if TheRange < TheRangeMin then
      TheRange := TheRangeMin
    else if TheRange > TheRangeMax then
      TheRange := TheRangeMax;
    {Set the horizontal scroll bar}
    SetScrollPos(WindowHandle,
      SB_HORZ,
      TheRange,
      true);
  end;

  if TheMessage = WM_HSCROLL then
  begin
    {Get the min and max range of the horizontal scroll box}
    GetScrollRange(WindowHandle,
      SB_VERT,
      TheRangeMin,
      TheRangeMax);
    {Get the horizontal scroll box position}
    TheRange := GetScrollPos(WindowHandle,
      SB_HORZ);
    {Make sure we wont exceed the range}
    if TheRange < TheRangeMin then
      TheRange := TheRangeMin
    else if TheRange > TheRangeMax then
      TheRange := TheRangeMax;
    {Set the vertical scroll bar}
    SetScrollPos(WindowHandle,
      SB_VERT,
      TheRange,
      true);
  end;

  { Call the old Window procedure to }
  { allow processing of the message. }
  NewWindowProc := CallWindowProc(OldWindowProc,
    WindowHandle,
    TheMessage,
    ParamW,
    ParamL);
end;

procedure TForm1.FormCreate(Sender: TObject);
begin
  { Set the new window procedure for the control }
  { and remember the old window procedure. }
  OldWindowProc := Pointer(SetWindowLong(ScrollBox1.Handle,
    GWL_WNDPROC,
    LongInt(@NewWindowProc)));
end;

procedure TForm1.FormDestroy(Sender: TObject);
begin
  { Set the window procedure back }
  { to the old window procedure. }
  SetWindowLong(ScrollBox1.Handle,
    GWL_WNDPROC,
    LongInt(OldWindowProc));
end;