delphi执行一个外部程序，当外部程序结束后，delphi程序立即响应

procedure TForm1.Button1Click(Sender: TObject);  
var  
  SEInfo: TShellExecuteInfo;  
  ExitCode: DWORD;  
  ExecuteFile, ParamString, StartInString: string;  
begin  
  ExecuteFile := 'c:\Windows\system32\Calc.exe';  
  FillChar(SEInfo, SizeOf(SEInfo), #0);  
  SEInfo.cbSize := SizeOf(TShellExecuteInfo);  
  with SEInfo do begin  
    fMask := SEE_MASK_NOCLOSEPROCESS;  
    Wnd := Application.Handle;  
    lpFile := PChar(ExecuteFile);  
    //ParamString can contain the application parameters  
//    lpParameters := PChar(ParamString);  
    //StartInString specifies the name of the working directory.if ommited, the current directory is used.  
//    lpDirectory := PChar(StartInString);  
    nShow := SW_SHOWNORMAL;  
  end;  
  if ShellExecuteEx(@SEInfo) then  
  begin  
    repeat  
      Application.ProcessMessages;  
      GetExitCodeProcess(SEInfo.hProcess, ExitCode);  
      if ExitCode = STILL_ACTIVE then Sleep(500);  
    until (ExitCode <> STILL_ACTIVE) or Application.Terminated;  
    ShowMessage('Calculator terminated');  
  end  
  else  
    ShowMessage('Error starting Calc!');  
end;