Btukfyl

Everywhere the same information, long time operation need to run in other thread, different than UI thread, for example - operation with Database.

...and its true, if I try to save something into DB I got:

Cannot access database on the main thread since it may potentially
lock the UI for a long period of time.

Here is my question, why I'm able get all data from database from UI ? :)

(my database didn't have .allowMainThreadQueries()"

This is a little modified code from google documentation:

ViewModel:

public class LocationsViewModel extends AndroidViewModel {



    private LocationRepository locationRepository;

    private LiveData<List<LocationModel>> allLocationsLiveData;



    public LiveData<List<LocationModel>> getAllLocationsLiveData() {

        return allLocationsLiveData;

    }



    public LocationsViewModel(@NonNull Application application) {

        super(application);

        locationRepository = new LocationRepository(application);

        allLocationsLiveData = locationRepository.getAllLocation();

    }

}

Repository:

public class LocationRepository {



    private final LocationDao locationDao;

    private LiveData<List<LocationModel>> allLocation;



    LiveData<List<LocationModel>> getAllLocation() {

        return allLocation;

    }



    LocationRepository(Application application){

        LocationsDatabase db = LocationsDatabase.getDatabase(application);

        locationDao = db.locationDao();

        allLocation = locationDao.getAllLocations();

    }



}

init method from Fragment:

private void initData() {

    locationsViewModel = ViewModelProviders.of(this).get(LocationsViewModel.class);

    locationsViewModel.getAllLocationsLiveData().observe(this, new Observer<List<LocationModel>>() {

        @Override

        public void onChanged(@Nullable List<LocationModel> locations) {

            mAdapter.setLocationList(locations);

        }

    });

}

Dao:

@Dao

public interface LocationDao {

    @Insert(onConflict = OnConflictStrategy.IGNORE)

    void insert(LocationModel... locations);



    @Query("SELECT * FROM LocationModel")

    LiveData<List<LocationModel>> getAllLocations();

}

So why I'm able to getFromDatabse from UI, but need other thread for save ?

You use LiveData, which literally runs on a background thread.

It does not happen synchronously, so it does not happen on the main thread.

You subscribe for changes, and those changes are calculated on a background thread, and then only given to your Activity/Fragment on the main thread.

When you try to save something, you don't use LiveData (which you aren't supposed to either, so its all good), so it runs on the main thread. Which it shouldn't.

So you'll need to run it in a new thread, create an AsyncTask, or similar.