ever() does not work when bool.obs value changed GetXController

I have var hasActivatedIndex = false.obs; and

@override
  void onInit() {
    super.onInit();

    ever(hasActivatedIndex, (callback) {
      print('hasActivatedIndex !!== $hasActivatedIndex'); // not activated
    });
  }

with following method I changed hasActivatedIndex true or false, and hope every time this ever() will get this boolean changed state and print something

  int updateActivatedIndex({double scrollPy}){
    activatedIndex = eventPyList.indexWhere((element) {
      if (scrollPy >= element && scrollPy <= element + 20) {
        hasActivatedIndex.value = true;
        return true;
      } else {
        hasActivatedIndex.value = false;
        showEventCard = false;
        return false;
      }
    });
  
    return activatedIndex;
  }

The hasActivatedIndex.value has indeed changed to true or false, but ever() in onInit does not react to this change, why? thanks in advance for the explanation!