CODEWITHSUNDEEP
androidmvvmandroid-livedata
A.M.
A.M.

Reputation: 223

ViewModelProvider.NewInstanceFactory - receive same instance

I want to re-use ViewModel and LiveData for reading nodes from Firebase. This is my code in Fragment

        FirebaseDatabaseViewModel test = ViewModelProviders.of(this, new FirebaseDatabaseViewModel.Factory(getActivity().getApplication(),"node1")).get(FirebaseDatabaseViewModel.class);
    LiveData<DataSnapshot> ldTest = test.getDataSnapshotLiveData();
    ldTest.observe(this, new Observer<DataSnapshot>() {
        @Override
        public void onChanged(@Nullable DataSnapshot dataSnapshot) {
            Log.d("MyTag", "liveData.observe TEST dataSnapshot = " + dataSnapshot);
        }
    });

    FirebaseDatabaseViewModel test2 = ViewModelProviders.of(this, new FirebaseDatabaseViewModel.Factory(getActivity().getApplication(),"node2")).get(FirebaseDatabaseViewModel.class);
    LiveData<DataSnapshot> ldTest2 = test2.getDataSnapshotLiveData();
    ldTest2.observe(this, new Observer<DataSnapshot>() {
        @Override
        public void onChanged(@Nullable DataSnapshot dataSnapshot) {
            Log.d("MyTag", "liveData.observe TEST2 dataSnapshot = " + dataSnapshot);
        }
    });
}

Here is ViewModel

public class FirebaseDatabaseViewModel extends AndroidViewModel {

private final String mRef;
private final FirebaseQueryLiveData liveData;

public FirebaseDatabaseViewModel(Application application, String ref) {
    super(application);
    this.mRef = ref;
    this.liveData = new FirebaseQueryLiveData(FirebaseDatabase.getInstance().getReference(mRef));
}

@NonNull
public LiveData<DataSnapshot> getDataSnapshotLiveData() {
    return liveData;
}

public static class Factory extends ViewModelProvider.NewInstanceFactory {

    @NonNull
    private final Application mApplication;

    private final String mRef;

    public Factory(@NonNull Application application, String ref) {
        mApplication = application;
        this.mRef = ref;
    }

    @NonNull
    @Override
    public <T extends ViewModel> T create(@NonNull Class<T> modelClass) {
        return (T) new FirebaseDatabaseViewModel(mApplication, mRef);
    }
}

}

Here is LiveData

public class FirebaseQueryLiveData extends LiveData<DataSnapshot> {

private final Query query;
private final MyValueEventListener listener = new MyValueEventListener();

public FirebaseQueryLiveData(Query query) {
    this.query = query;
}

public FirebaseQueryLiveData(DatabaseReference ref) {
    this.query = ref;
}

@Override
protected void onActive() {
    Log.d("MyTag", "onActive");
    query.addValueEventListener(listener);
}

@Override
protected void onInactive() {
    Log.d("MyTag", "onInactive");
    query.removeEventListener(listener);
}

private class MyValueEventListener implements ValueEventListener{

    @Override
    public void onDataChange(DataSnapshot dataSnapshot) {
        setValue(dataSnapshot);
    }

    @Override
    public void onCancelled(DatabaseError databaseError) {
        Log.e("MyTag", "Can't listen to query " + query, databaseError.toException());
    }
}

}

Problem is reading same node from FirebaseDatabase

D/MyTag: liveData.observe TEST dataSnapshot = DataSnapshot { key = node1, value = {.....

D/MyTag: liveData.observe TEST2 dataSnapshot = DataSnapshot { key = node1, value = {....

Second time I expected node2

Upvotes: 1

Views: 2077

Answers (1)

ianhanniballake
ianhanniballake

Reputation: 200130

The default ViewModelProvider only keeps a single ViewModel for a given class name. The only time your Factory is invoked is when there's no already existing ViewModel - in your case, you're using the same class name for both calls, so your second factory is never used.

Generally, you should consider only having a single ViewModel and have it return multiple different LiveData instances based on the key passed into it, keeping a HashMap<String,LiveData> to avoid recreating LiveData objects it already has:

public class FirebaseDatabaseViewModel extends AndroidViewModel {
  private HashMap<String, LiveData<DataSnapshot>> mLiveDataMap = new HashMap<>();

  public FirebaseDatabaseViewModel(@NonNull final Application application) {
    super(application);
  }

  public LiveData<DataSnapshot> getDataSnapshotLiveData(String ref) {
    if (!mLiveDataMap.containsKey(ref)) {
      // We don't have an existing LiveData for this ref
      // so create a new one
      mLiveDataMap.put(ref, new FirebaseQueryLiveData(
          FirebaseDatabase.getInstance().getReference(ref)));
    }
    return mLiveDataMap.get(ref);
  }
}

Upvotes: 2

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