delphi执行一个外部程序，当外部程序结束后，delphi程序立即响应

//需要引用 ShellAPI 单元;
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;