class PostListViewModel:BaseViewModel(){
    // ...

    val loadingVisibility: MutableLiveData<Int> = MutableLiveData()

    // ...

    private fun onRetrievePostListStart(){
        loadingVisibility.value = View.VISIBLE
    }

    private fun onRetrievePostListFinish(){
        loadingVisibility.value = View.GONE
    }

    private fun onRetrievePostListSuccess(){

    }

    private fun onRetrievePostListError(){

    }
}