Read data from an Android Room database in a background Service, no exceptions but no data - android-room

Question

I am attempting to read data from an Android Room database in a background Service. There are no exceptions but no data is returned.
I wrote a function to select all rows from a table in the DAO. Calling that function from a background service succeeds, but it returns no data.
My "Contact" class holds contact information (names, phone numbers, emails) and defines the database schema. The database holds rows of contacts, with names, phone numbers, an emails as columns.
The function that returns the LiveData in the DAO is:
#Query("SELECT * FROM contacts_table")
LiveData<List<Contact>> getAll();
where "contacts_table" is the database table holding contact information.
I called getAll as follows:
AppDatabase db = AppDatabase.getDatabase(messageSenderContext.getApplicationContext());
mContactDAO = db.contactDAO();
mAllContacts = mContactDAO.getAll();
where mContactDao is a ContactDAO (The Database Access Object for my Contact class), and mAllContacts is a LiveData>. These are private fields of the class calling getAll().
db.contactDAO() returns an object, as does mContactDAO.getAll(). But attempting to unpack the List from mAllContacts using mAllContacts.getValue() returns null.

Answer 1

This turned out to be a misuse of LiveData. That requires an Observer to actually get the data.

Answer 2

In your ROOM
#Database(entities={Contact.class}, version = 1, exportSchema = false)
public abstract class AppDatabase extends RoomDatabase {
public static final String DATABASE_NAME = "AppDatabase.db";
private static volatile AppDatabase INSTANCE;
private static final int NUMBER_OF_THREADS = 4;
public static final ExecutorService EXECUTOR_SERVICE = Executors.newFixedThreadPool(NUMBER_OF_THREADS);
public abstract ContactDAO contactDAO();
public static AppDatabase getDatabase(final Context context) {
if (INSTANCE == null) {
synchronized (AppDatabase.class) {
if (INSTANCE == null) {
INSTANCE = Room.databaseBuilder(context.getApplicationContext(),
AppDatabase.class, DATABASE_NAME)
.build();
}
}
}
return INSTANCE;
}
}
In your DAO
#Dao
public interface ContactDAO{
#Query("SELECT * FROM contacts_table")
LiveData<List<Contact>> getAll();
}
In your repository:
public class AppRepository {
private ContactDAO mContactDAO;
//constructor
public AppRepository(Application application) {
AppDatabase db = AppDatabase.getDatabase(application);
mContactDAO= db.contactDAO();
}
public LiveData<List<Contact>> getAllContacts(){
LiveData<List<Contact>> contactsList = null;
Future<LiveData<List<Contact>>> futureList = AppDatabase.EXECUTOR_SERVICE.submit(new Callable(){
#Override
public LiveData<List<Contact>> call() {
return contactDAO.getAll();
}
});
try {
contactsList = futureList.get();
} catch (ExecutionException e) {
e.printStackTrace();
} catch (InterruptedException e) {
e.printStackTrace();
}
return contactsList ;
}
}
In your ViewModel
public class ContactsViewModel extends AndroidViewModel {
private AppRepository appRepository;
private LiveData<List<Contact>> contactsList;
public ContactsViewModel(#NonNull Application application) {
super(application);
appRepository = new AppRepository(application);
}
public LiveData<List<Contacts>> list() {
return appRepository.getAllContacts();
}
}
In your activity (inside of onCreated put)
ContactsViewModel contactsViewModel = new ViewModelProvider(this).get(ContactsViewModel.class);
contactsViewModel.list().observe(getViewLifecycleOwner(), new Observer<List<Contact>>() {
#Override
public void onChanged(List<Contact> contactsList) {
//the contact list will be observed and will return data if there are changes.
//use for example to feed the adapter of a recyclerview
//below an example just to view the contacts data
for(Contact conctact : contactsList){
Log.d("TestApp>>>", "Id: + contact.getId);
Log.d("TestApp>>>", "Name: + contact.getName);
}
});