public class ListItemViewModel extends ViewModel {
    private MediatorLiveData<ItemList> mList;
    private MeliRepository meliRepository;

    /* Empty Contructor.
     * To have a ViewModel class with non-empty constructor,
     * I have to create a Factory class which would create instance of you ViewModel and
     * that Factory class has to implement ViewModelProvider.Factory interface.
     */
    public ListItemViewModel(){
        meliRepository = new MeliRepository();
    }

    public LiveData<ItemList> getItemList(String query){
       if(mList == null){
           mList = new MediatorLiveData<>();
           LoadItems(query);
       }
    }

    private void LoadItems(String query){
        String queryToSearch = TextUtils.isEmpty(query) ? "IPOD" : query;

        mList.addSource(
                meliRepository.getItemsByQuery(queryToSearch),
                list -> mList.setValue(list)
        );
    }
}