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I have MainActivity which contains TabLayout with tabs: each tab is a fragment and each one has a RecyclerView. When I tap FAB in Main Activity, NewReminderActivity is opened.

I use Architecture Components: Entity(Reminder), DAO, Room, ViewModel, LiveData and Repository.

The question is:

• Which methods should I use to deliver a new created reminder item into the fragment (which contains as mentioned above a RecyclerView?

I have some ideas, but could you help me please and give me a right direction for implementing:

1) I guess, I should deliver data to MainActivity, then from MainActivity to the fragment and use ViewModel as mentioned in https://developer.android.com/topic/libraries/architecture/viewmodel.html#sharing , am I right?

2) I guess I should use setResult() in NewReminderActivity, am I right?

If you are using Room, there is no reason for you to use setResult to transfer new item to any of these previous Fragments/Activities, because Room manages invalidation automatically.

@Dao
public interface MyDao 
 @Query("SELECT * FROM ITEM")
 LiveData<List<Item>> getItemsWithChanges();

 @Insert(onConflict = OnConflictStrategy.REPLACE)
 void insertItem(Item item);

Then

public class MyViewModel extends ViewModel 
 private final LiveData<List<Item>> items;

 public LiveData<List<Item>> getItems() 
 return items;
 

 public MyViewModel(MyDao myDao) 
 items = myDao.getItemsWithChanges();
 

Then

public class MyFragment extends Fragment 
 MyViewModel myViewModel;

 @Override
 protected void onViewCreated(View view, Bundle savedInstanceState) 
 super.onViewCreated(view, savedInstanceState);
 myViewModel = ViewModelProviders.of(getActivity(), viewModelFactory).get(MyViewModel.class);
 myViewModel.getItems().observe(getViewLifecycleOwner(), (items) -> 
 if(items != null) 
 adapter.submitList(items);
 
 );
 

In which case all you need to do in your second Activity is insert the new item, then finish out:

// imagine this is on background thread
myDao.insertItem(item);
runOnUiThread(() -> 
 finish();
);

And all your RecyclerViews will update with the new item (if they are part of the results as the condition matches it).