Why repository doesn't notify viewmodel about updated value of variable in spite of implementing LiveData?

Question

I am fetching data from Cloud Firestore and displaying in a recyclerview using MVVM architecture along with LiveData. When I observe the changes in viewmodel and notify the adapter about the dataset change, the changed data isn't updating in the recyclerview.

Do you see anything wrong in the code?

UPDATE: I have updated code as suggested my the first answer. But still no success. I observed that even after the repository fetches the data from firestore, it doesn't updates the viewmodel about it. I have added the repository class too. Do you see any problem there?

HomeActivity.java

public class HomeActivity extends AppCompatActivity {

    RecyclerView recyclerView;
    StandardPlansAdapter adapter;
    HomeActivityViewModel viewModel;

    @Override
    protected void onCreate(final Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);

        recyclerView = findViewById(R.id.recyclerViewHome);

        viewModel = new ViewModelProvider(this).get(HomeActivityViewModel.class);
        adapter = new StandardPlansAdapter(viewModel.getStandardPlans().getValue());
        recyclerView.setHasFixedSize(true);
        recyclerView.setLayoutManager(new LinearLayoutManager(this));
        recyclerView.setAdapter(adapter);

        viewModel.getStandardPlans().observe(this, plans -> {
            adapter.setStandardPlans(plans);
            adapter.notifyDataSetChanged();
        });
    }
}

StandardPlansAdapter.java

public class StandardPlansAdapter extends RecyclerView.Adapter<StandardPlansAdapter.StandardPlansViewHolder> {

    private ArrayList<Plan> standardPlans;

    public StandardPlansAdapter(ArrayList<Plan> standardPlans) {
        this.standardPlans = standardPlans;
    }

    @NonNull
    @Override
    public StandardPlansViewHolder onCreateViewHolder(@NonNull ViewGroup parent, int viewType) {
        View view = LayoutInflater.from(parent.getContext())
                .inflate(R.layout.home_rv_layout, parent, false);
        return new StandardPlansViewHolder(view);
    }

    @Override
    public void onBindViewHolder(@NonNull StandardPlansViewHolder holder, int position) {
        Plan plan = standardPlans.get(position);
        holder.textView.setText(plan.getName());
    }

    @Override
    public int getItemCount() {
        return standardPlans.size();
    }

    class StandardPlansViewHolder extends RecyclerView.ViewHolder {

        TextView textView;

        public StandardPlansViewHolder(@NonNull View itemView) {
            super(itemView);
            textView = itemView.findViewById(R.id.tvPlanName);
        }
    }

    public void setStandardPlans(ArrayList<Plan> standardPlans) {
        this.standardPlans = standardPlans;
    }

}

HomeActivityViewModel.java

public class HomeActivityViewModel extends ViewModel {

    private StandardPlansRepository repository;
    private MutableLiveData<ArrayList<Plan>> standardPlans;

    public HomeActivityViewModel() {
        super();
        repository = StandardPlansRepository.getInstance();
        standardPlans = repository.getStandardPlans();
    }

    public LiveData<ArrayList<Plan>> getStandardPlans() {
        return standardPlans;
    }
}

StandardPlansRepository.java

public class StandardPlansRepository {

    private static StandardPlansRepository instance;
    private ArrayList<Plan> standardPlans = new ArrayList<>();

    public static StandardPlansRepository getInstance() {
        if (instance == null) {
            instance = new StandardPlansRepository();
        }
        return instance;
    }

    public MutableLiveData<ArrayList<Plan>> getStandardPlans() {
        setStandardPlans();
        MutableLiveData<ArrayList<Plan>> plans = new MutableLiveData<>();
        plans.setValue(standardPlans);
        return plans;
    }

    private void setStandardPlans() {
        documentReference.get().addOnSuccessListener(documentSnapshot -> {
            if (documentSnapshot.exists()) {
                StandardPlans plans = documentSnapshot.toObject(StandardPlans.class);
                if (plans != null) {
                    standardPlans = plans.getStandard_plans();
                }
            } else {
                Log.e("rahul", "Document doesn't exist");
            }
        }).addOnFailureListener(e -> {
            Log.e("rahul", e.getMessage());
            e.printStackTrace();
        });
    }
}

Answer 1

Your Observer is calling notifyDataSetChanged without updating the actual data set, which is the private member standardPlans inside your StandardPlansAdapter

Add a public setStandardPlans method to your adapter:

public class StandardPlansAdapter extends RecyclerView.Adapter<StandardPlansAdapter.StandardPlansViewHolder> {
    public setStandardPlans(ArrayList<Plan> standardPlans) {
        this.standardPlans = standardPlans;
    }
}

And then call the setter before notifying in your Observer callback

viewModel.getStandardPlans().observe(this, (Observer<List<Plan>>) plans -> { 
    adapter.setStandardPlans(plans);   //You update the dataset first
    adapter.notifyDataSetChanged();    //And then notify the adapter of the update
})

EDIT

Also, in your onCreate method I noticed that you are attaching the observer before initializing the adapter. If your observer callback is ever fired before the adapter initialization due to a race condition you'll get a NullPointerExceptionand the Activity will crash since you're dereferencing the adapter in your callback.

adapter = new StandardPlansAdapter(viewModel.getStandardPlans().getValue()); //Initialize adapter first  
viewModel.getStandardPlans().observe(this, (Observer<List<Plan>>) plans -> adapter.notifyDataSetChanged()); //Potential Null pointer otherwise

EDIT2

You seem to have an inaccurate understanding of how LiveData works.

 public MutableLiveData<ArrayList<Plan>> getStandardPlans() {
     setStandardPlans();
     MutableLiveData<ArrayList<Plan>> plans = new MutableLiveData<>();
     plans.setValue(standardPlans);
     return plans;
 }

You cannot just manually create a new instance of LiveData here, set its value and expect the observers to be notified.

1. You're not supposed to create instances of LiveData and set values on them.
2. You are NOT updating the livedata value inside your document's onSuccessListener callback. (plans.setValue) How can you possibly expect to be notified of updates if you're not setting the LiveData's value there? And, if this logic is to work, you have to make your plans variable a class variable in your repository, not a local variable in your getStandardPlans method.

In any case you're not supposed to use live data this way

For example, if you were working with Room databases, you'd be getting LiveData on your tables via the Room Library observable queries, not manually initializing a LiveData instance and setting values on it.

Firestore API does not give you LiveData observable queries like the Room library. (It does give you observable queries). So if you wanted to observe those queries via LiveData and ViewModels, you'd have to extend LiveData and attach to Firestore callbacks inside it.

getSavedAddresses extends LiveData