显示图像采集的设备接口

我正在使用twain 2.1连接到扫描仪,让用户更改任何选项,并等待输入,每当扫描仪界面显示问题时,光标仍会指示某些循环或工作,这是以下代码:

    HWND g_hwnd = CreateWindow ("STATIC",
                          "",
                          WS_POPUPWINDOW,
                          CW_USEDEFAULT,
                          CW_USEDEFAULT,
                          CW_USEDEFAULT,
                          CW_USEDEFAULT,
                          HWND_DESKTOP,
                          0,
                          g_hinstDLL,
                          0);

   TW_USERINTERFACE ui;
   ui.ShowUI = TRUE;
   ui.ModalUI = TRUE;
   ui.hParent = g_hwnd;
   // g_pDSM_Entry -> 
   int code = (*g_pDSM_Entry) (&g_AppID,
                           &g_SrcID,
                           DG_CONTROL,
                           DAT_USERINTERFACE,
                           MSG_ENABLEDSUIONLY,
                           (TW_MEMREF) &ui);

   if(TWRC_SUCCESS != code) {
       return;
   }
   MSG msg;
   while(GetMessage ((LPMSG) &msg, g_hwnd, 0, 0)){
       // TO-DO
   }

如何等待用户输入?对话框也不会出现,只是白色窗口?