How to load batches of data in a recycler view using firestore?

Question

I wanted to know how to load more data in recylcer view using firestore.

 Query query = FirebaseFirestore.getInstance()
            .collection("ie").limit(5);  
adapter=new InterviewAdapter(this,query);    
recyclerView.setAdapter(adapter);
recyclerView.setLayoutManager(new LinearLayoutManager(this));

Adapter class looks like this:

public class InterviewAdapter extends FireStoreAdapter<InterviewAdapter.ViewHolder> {

public interface OnInterviewSelectedListener {

    void onInterviewSelected(DocumentSnapshot interview);

}

private InterviewAdapter.OnInterviewSelectedListener mListener;

public InterviewAdapter(Query query, OnInterviewSelectedListener listener) {
    super(query);
    mListener = listener;
}

@Override
public InterviewAdapter.ViewHolder onCreateViewHolder(ViewGroup parent, int viewType) {
    LayoutInflater inflater = LayoutInflater.from(parent.getContext());
    return new InterviewAdapter.ViewHolder(inflater.inflate(R.layout.ie, parent, false));
}

@Override
public void onBindViewHolder(InterviewAdapter.ViewHolder holder, int position) {
    holder.bind(getSnapshot(position), mListener);
}

static class ViewHolder extends RecyclerView.ViewHolder {

    TextView title,companyName,username,views,isHired;

    public ViewHolder(View itemView) {
        super(itemView);
        title= (TextView) itemView.findViewById(R.id.title);
        companyName= (TextView) itemView.findViewById(R.id.companyName);
        username= (TextView) itemView.findViewById(R.id.username);
        views= (TextView) itemView.findViewById(R.id.views);
        isHired= (TextView) itemView.findViewById(R.id.isHired);
    }

    public void bind(final DocumentSnapshot snapshot,
                     final OnInterviewSelectedListener listener) {


        InterviewExperience experience;
        String companyName=snapshot.getString("companyName");
        boolean isHired=Boolean.valueOf(snapshot.getBoolean("isHired"));
        String username=snapshot.getString("username");
        long views=new Double(Double.valueOf(snapshot.getDouble("views"))).longValue();

        String id=snapshot.getId();


        String title=snapshot.getString("title");
        experience=new InterviewExperience(id,title,companyName,username,isHired,views,null,null);


        this.title.setText(experience.getTitle());
        this.companyName.setText("Company Name: "+experience.getCompanyName());
        this.isHired.setText("Hired: "+experience.isHired());
        this.views.setText("Views: "+experience.getViews()+"");
        this.username.setText("Created By: "+experience.getUsername());



        // Click listener
        itemView.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View view) {
                if (listener != null) {
                    listener.onInterviewSelected(snapshot);
                }
            }
        });
    }

}
}

public abstract class FireStoreAdapter<VH extends RecyclerView.ViewHolder>
    extends RecyclerView.Adapter<VH>
    implements EventListener<QuerySnapshot> {

private static final String TAG = "FirestoreAdapter";

private Query mQuery;
private ListenerRegistration mRegistration;

private ArrayList<DocumentSnapshot> mSnapshots = new ArrayList<>();

public FireStoreAdapter(Query query) {
    mQuery = query;
}

@Override
public void onEvent(QuerySnapshot documentSnapshots, FirebaseFirestoreException e) {
    if (e != null) {
        Log.w(TAG, "onEvent:error", e);
        onError(e);
        return;
    }

    // Dispatch the event
    Log.d(TAG, "onEvent:numChanges:" + documentSnapshots.getDocumentChanges().size());
    for (DocumentChange change : documentSnapshots.getDocumentChanges()) {
        switch (change.getType()) {
            case ADDED:
                onDocumentAdded(change);
                break;
            case MODIFIED:
                onDocumentModified(change);
                break;
            case REMOVED:
                onDocumentRemoved(change);
                break;
        }
    }

    onDataChanged();
}

public void startListening() {
    if (mQuery != null && mRegistration == null) {
        mRegistration = mQuery.addSnapshotListener(this);
    }
}

public void stopListening() {
    if (mRegistration != null) {
        mRegistration.remove();
        mRegistration = null;
    }

    mSnapshots.clear();
    notifyDataSetChanged();
}

public void setQuery(Query query) {
    // Stop listening
    stopListening();

    // Clear existing data
    mSnapshots.clear();
    notifyDataSetChanged();

    // Listen to new query
    mQuery = query;
    startListening();
}

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

protected DocumentSnapshot getSnapshot(int index) {
    return mSnapshots.get(index);
}

protected void onDocumentAdded(DocumentChange change) {
    mSnapshots.add(change.getNewIndex(), change.getDocument());
    notifyItemInserted(change.getNewIndex());
}

protected void onDocumentModified(DocumentChange change) {
    if (change.getOldIndex() == change.getNewIndex()) {
        // Item changed but remained in same position
        mSnapshots.set(change.getOldIndex(), change.getDocument());
        notifyItemChanged(change.getOldIndex());
    } else {
        // Item changed and changed position
        mSnapshots.remove(change.getOldIndex());
        mSnapshots.add(change.getNewIndex(), change.getDocument());
        notifyItemMoved(change.getOldIndex(), change.getNewIndex());
    }
}

protected void onDocumentRemoved(DocumentChange change) {
    mSnapshots.remove(change.getOldIndex());
    notifyItemRemoved(change.getOldIndex());
}

protected void onError(FirebaseFirestoreException e) {};

protected void onDataChanged() {}
}

I used Firestore Adapter code which was given in samples of firestore documentation. Can anyone tell how to use the query object to load more data?

How to load the next 5 items in the recycler view when users scrolls to the end of the list?

Answer 1

You can paginate your Query's result using Query's methods like, startAt(), startAfter(), endAt(), endBefore() with a specified DocumentSnapshot.

If I considered your collection is called "interviews", you can add a method to your FireStoreAdapter like this:

private void paginate(final DocumentSnapshot last, final int limit) {
    final Query subset;
    if (last == null) {
        subset = db.collection("interviews")
                .limit(limit);
    } else {
        subset = db.collection("interviews")
                .startAfter(last)
                .limit(limit);
    }
    setQuery(subset);
}

You can perserve the last DocumentSnapshot within onEvent():

final List<DocumentChange> changes = documentSnapshots.getDocumentChanges();
final DocumentSnapshot lastDocument = changes.get(changes.size() - 1).getDocument();

Finally, when users scrolls to the end of the list:

paginate(lastDocument, 5);

And onDocumentAdded() will take care of it. Be carfure NOT to use startAt() because it will not execlude the last one (that already at the end of your list, and will duplicate it).