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

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 code:

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

I forked your project and tested it a bit. From all I can tell you discovered a serious bug.

To make the reproduction and the investigation easier, I edited your project a bit. You can find updated project here: https://github.com/techyourchance/live-data-problem . I also opened a pull request back to your repo.

To make sure that this doesn't go unnoticed, I also opened an issue in Google's issue tracker:

Steps to reproduce:

1. Ensure that REPRODUCE_BUG is set to true in MainFragment
2. Install the app
3. Click on "add trashed note" button
4. Switch to TrashFragment
5. Note that there was just one notification form LiveData with correct value
6. Switch to MainFragment
7. Click on "add trashed note" button
8. Switch to TrashFragment
9. Note that there were two notifications from LiveData, the first one with incorrect value

Note that if you set REPRODUCE_BUG to false then the bug doesn't reproduce. It demonstrates that subscription to LiveData in MainFragment changed the behavior in TrashFragment.

Expected result: Just one notification with correct value in any case. No change in behavior due to previous subscriptions.

More info: I looked at the sources a bit, and it looks like notifications being triggered due to both LiveData activation and new Observer subscription. Might be related to the way ComputableLiveData offloads onActive() computation to Executor.