Creating simple progress bar with CFUserNotification

Hi. The following code creates and initialises a simple progress bar. How would I add a callback to update the progress bar ?

float prog = 0.5;

    CFNumberRef cfNum = CFNumberCreate(kCFAllocatorDefault,kCFNumberFloatType, &prog);

    const void* keys[] = { kCFUserNotificationAlertHeaderKey, kCFUserNotificationProgressIndicatorValueKey };

     const void* vals[] = { CFSTR("Progress Bar"), cfNum};

    CFDictionaryRef dict = CFDictionaryCreate(0, keys, vals,

                    sizeof(keys)/sizeof(*keys),

                    &kCFTypeDictionaryKeyCallBacks,

                    &kCFTypeDictionaryValueCallBacks);

    CFUserNotificationRef pDlg = NULL;

     pDlg = CFUserNotificationCreate(kCFAllocatorDefault, 0,

                         kCFUserNotificationPlainAlertLevel,

                         &nRes, dict);