Пример обратного вызова DLL

library Lib1;

uses WinTypes, Messages, WinProcs, Toolhelp;

const

  WM_NOTIFY = WM_USER + $100;

var

  targetHWnd: HWnd;

function HookProc(wID: Word; dwData: LongInt): Bool; export;

begin
  PostMessage(targetHWnd, WM_NOTIFY, wID, dwData);
  Result := False
end;

procedure InstallHook(notifyWindow: HWnd); export;

begin
  if targetHWnd = 0 then
  begin
    if not NotifyRegister(0, HookProc, NF_NORMAL) then
    begin
      MessageBox(notifyWindow, 'Неудача NotifyRegister!',
        'Ошибка!', MB_OK + MB_ICONSTOP);
    end
    else
    begin
      targetHWnd := notifyWindow;
    end;
  end;
end;

procedure UnInstallHook; export;

begin
  if targetHWnd <> 0 then
  begin
    NotifyUnregister(0);
    targetHWnd := 0;
  end;
end;

exports

  InstallHook,
  UnInstallHook;

begin

  targetHWnd := 0;
end.