android - Why LiveData observer is being triggered twice for a newly attached observer

Question

My understanding on LiveData is that, it will trigger observer on the current state change of data, and not a series of history state change of data.

Currently, I have a MainFragment, which perform Room write operation, to change non-trashed data, to trashed data.

I also another TrashFragment, which observes to trashed data.

Consider the following scenario.

1. There are currently 0 trashed data.
2. MainFragment is the current active fragment. TrashFragment is not created yet.
3. MainFragment added 1 trashed data.
4. Now, there are 1 trashed data
5. We use navigation drawer, to replace MainFragment with TrashFragment.
6. TrashFragment's observer will first receive onChanged, with 0 trashed data
7. Again, TrashFragment's observer will secondly receive onChanged, with 1 trashed data

What is out of my expectation is that, item (6) shouldn't happen. TrashFragment should only receive latest trashed data, which is 1.

Here's my codes

---

TrashFragment.java

public class TrashFragment extends Fragment {
    @Override
    public void onCreate(Bundle savedInstanceState) {
        noteViewModel = ViewModelProviders.of(getActivity()).get(NoteViewModel.class);
    }

    @Override
    public View onCreateView(LayoutInflater inflater, ViewGroup container, Bundle savedInstanceState) {
        ...

        noteViewModel.getTrashedNotesLiveData().removeObservers(this);
        noteViewModel.getTrashedNotesLiveData().observe(this, notesObserver);

MainFragment.java

public class MainFragment extends Fragment {
    @Override
    public void onCreate(Bundle savedInstanceState) {
        noteViewModel = ViewModelProviders.of(getActivity()).get(NoteViewModel.class);
    }

    @Override
    public View onCreateView(LayoutInflater inflater, ViewGroup container, Bundle savedInstanceState) {
        ...

        noteViewModel.getNotesLiveData().removeObservers(this);
        noteViewModel.getNotesLiveData().observe(this, notesObserver);

NoteViewModel .java

public class NoteViewModel extends ViewModel {
    private final LiveData<List<Note>> notesLiveData;
    private final LiveData<List<Note>> trashedNotesLiveData;

    public LiveData<List<Note>> getNotesLiveData() {
        return notesLiveData;
    }

    public LiveData<List<Note>> getTrashedNotesLiveData() {
        return trashedNotesLiveData;
    }

    public NoteViewModel() {
        notesLiveData = NoteplusRoomDatabase.instance().noteDao().getNotes();
        trashedNotesLiveData = NoteplusRoomDatabase.instance().noteDao().getTrashedNotes();
    }
}

---

Code which deals with Room

public enum NoteRepository {
    INSTANCE;

    public LiveData<List<Note>> getTrashedNotes() {
        NoteDao noteDao = NoteplusRoomDatabase.instance().noteDao();
        return noteDao.getTrashedNotes();
    }

    public LiveData<List<Note>> getNotes() {
        NoteDao noteDao = NoteplusRoomDatabase.instance().noteDao();
        return noteDao.getNotes();
    }
}

@Dao
public abstract class NoteDao {
    @Transaction
    @Query("SELECT * FROM note where trashed = 0")
    public abstract LiveData<List<Note>> getNotes();

    @Transaction
    @Query("SELECT * FROM note where trashed = 1")
    public abstract LiveData<List<Note>> getTrashedNotes();

    @Insert(onConflict = OnConflictStrategy.REPLACE)
    public abstract long insert(Note note);
}

@Database(
        entities = {Note.class},
        version = 1
)
public abstract class NoteplusRoomDatabase extends RoomDatabase {
    private volatile static NoteplusRoomDatabase INSTANCE;

    private static final String NAME = "noteplus";

    public abstract NoteDao noteDao();

    public static NoteplusRoomDatabase instance() {
        if (INSTANCE == null) {
            synchronized (NoteplusRoomDatabase.class) {
                if (INSTANCE == null) {
                    INSTANCE = Room.databaseBuilder(
                            NoteplusApplication.instance(),
                            NoteplusRoomDatabase.class,
                            NAME
                    ).build();
                }
            }
        }

        return INSTANCE;
    }
}

Any idea how I can prevent from receiving onChanged twice, for a same data?

---

Demo

I created a demo project to demonstrate this problem.

As you can see, after I perform write operation (Click on ADD TRASHED NOTE button) in MainFragment, when I switch to TrashFragment, I expect onChanged in TrashFragment will only be called once. However, it is being called twice.

Demo project can be downloaded from https://github.com/yccheok/live-data-problem

Answer 1

I have introduced just one change in your code:

noteViewModel = ViewModelProviders.of(this).get(NoteViewModel.class);

instead of:

noteViewModel = ViewModelProviders.of(getActivity()).get(NoteViewModel.class);

in Fragment's onCreate(Bundle) methods. And now it works seamlessly.

In your version you obtained a reference of NoteViewModel common to both Fragments (from Activity). ViewModel had Observer registered in previous Fragment, I think. Therefore LiveData kept reference to both Observer's (in MainFragment and TrashFragment) and called both values.

So I guess the conclusion might be, that you should obtain ViewModel from ViewModelProviders from:

• Fragment in Fragment
• Activity in Activity

Btw.

noteViewModel.getTrashedNotesLiveData().removeObservers(this);

is not necessary in Fragments, however I would advise putting it in onStop.