How to paginate Firestore with Android?

I read Firestore documentation and all articles on internet(stackoverflow) about Firestore pagination but no luck. I tried to implement the exact code in docs, but nothing happens. I have a basic database with items(over 1250 or more) and I want to get them progressively. By scrolling to load 15 items (to the last item in the database).

If using docs code:

// Construct query for first 25 cities, ordered by population
Query first = db.collection(\"cities\")
    .orderBy(\"population\")
    .limit(25);

first.get()
    .addOnSuccessListener(new OnSuccessListener<QuerySnapshot>() {
    @Override
    public void onSuccess(QuerySnapshot documentSnapshots) {
        // ...

        // Get the last visible document
        DocumentSnapshot lastVisible = documentSnapshots.getDocuments()
            .get(documentSnapshots.size() -1);

        // Construct a new query starting at this document,
        // get the next 25 cities.
        Query next = db.collection(\"cities\")
            .orderBy(\"population\")
            .startAfter(lastVisible)
            .limit(25);

        // Use the query for pagination
        // ...
    }
});

How to do? Documentation has not too many details.

PS: I need with recycler view (not list view) when user scrolls. Thanks

回答1:

As it is mentioned in the official documentation, the key for solving this problem is to use the startAfter() method. So you can paginate queries by combining query cursors with the limit() method. You\'ll be able to use the last document in a batch as the start of a cursor for the next batch.

To solve this pagination problem, please see my answer from this post, in which I have explained step by step, how you can load data from a Cloud Firestore database in smaller chunks and display it in a ListView on button click.

Solution:

To get the data from your Firestore database and display it in smaller chunks in a RecyclerView, please follow the steps below.

Let\'s take the above example in which I have used products. You can use products, cities or whatever you want. The principles are the same. Assuming that you want to load more products when user scrolls, I\'ll use RecyclerView.OnScrollListener.

Let\'s define first the RecyclerView and set the layout manager:

RecyclerView recyclerView = findViewById(R.id.recycler_view);
recyclerView.setLayoutManager(new LinearLayoutManager(this));

Let\'s assume we have a database structure that look like this:

Firestore-root
   |
   --- products (collection)
         |
         --- productId (document)
                |
                --- productName: \"Product Name\"

And a model class that looks like this:

public class ProductModel {
    private String productName;

    public ProductModel() {}

    public ProductModel(String productName) {this.productName = productName;}

    public String getProductName() {return productName;}
}

This how the adapter class should look like:

private class ProductAdapter extends RecyclerView.Adapter<ProductViewHolder> {
    private List<ProductModel> list;

    ProductAdapter(List<ProductModel> list) {
        this.list = list;
    }

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

    @Override
    public void onBindViewHolder(@NonNull ProductViewHolder productViewHolder, int position) {
        String productName = list.get(position).getProductName();
        productViewHolder.setProductName(productName);
    }

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

The item_product layout contains only one view, a TextView.

<TextView
    android:layout_width=\"wrap_content\"
    android:layout_height=\"wrap_content\"
    android:id=\"@+id/text_view\"
    android:textSize=\"25sp\"/>

And this is the how the holder class should look like:

private class ProductViewHolder extends RecyclerView.ViewHolder {
    private View view;

    ProductViewHolder(View itemView) {
        super(itemView);
        view = itemView;
    }

    void setProductName(String productName) {
        TextView textView = view.findViewById(R.id.text_view);
        textView.setText(productName);
    }
}

Now, let\'s define a limit as a global variable and set it to 15.

private int limit = 15;

Let\'s define now the query using this limit:

FirebaseFirestore rootRef = FirebaseFirestore.getInstance();
CollectionReference productsRef = rootRef.collection(\"products\");
Query firstQuery = productsRef.orderBy(\"productName\", Query.Direction.ASCENDING).limit(limit);

Here is the code that also does the magic in your case:

query.get().addOnCompleteListener(new OnCompleteListener<QuerySnapshot>() {
    @Override
    public void onComplete(@NonNull Task<QuerySnapshot> task) {
        if (task.isSuccessful()) {
            List<ProductModel> list = new ArrayList<>();
            for (DocumentSnapshot document : task.getResult()) {
                ProductModel productModel = document.toObject(ProductModel.class);
                list.add(productModel);
            }
            ProductAdapter productAdapter = new ProductAdapter(list);
            recyclerView.setAdapter(productAdapter);
            lastVisible = task.getResult().getDocuments().get(task.getResult().size() - 1);

            RecyclerView.OnScrollListener onScrollListener = new RecyclerView.OnScrollListener() {
                @Override
                public void onScrollStateChanged(RecyclerView recyclerView, int newState) {
                    super.onScrollStateChanged(recyclerView, newState);
                    if (newState == AbsListView.OnScrollListener.SCROLL_STATE_TOUCH_SCROLL) {
                        isScrolling = true;
                    }
                }

                @Override
                public void onScrolled(RecyclerView recyclerView, int dx, int dy) {
                    super.onScrolled(recyclerView, dx, dy);

                    LinearLayoutManager linearLayoutManager = ((LinearLayoutManager) recyclerView.getLayoutManager());
                    int firstVisibleItemPosition = linearLayoutManager.findFirstVisibleItemPosition();
                    int visibleItemCount = linearLayoutManager.getChildCount();
                    int totalItemCount = linearLayoutManager.getItemCount();

                    if (isScrolling && (firstVisibleItemPosition + visibleItemCount == totalItemCount) && !isLastItemReached) {
                        isScrolling = false;
                        Query nextQuery = productsRef.orderBy(\"productName\", Query.Direction.ASCENDING).startAfter(lastVisible).limit(limit);
                        nextQuery.get().addOnCompleteListener(new OnCompleteListener<QuerySnapshot>() {
                            @Override
                            public void onComplete(@NonNull Task<QuerySnapshot> t) {
                                if (t.isSuccessful()) {
                                    for (DocumentSnapshot d : t.getResult()) {
                                        ProductModel productModel = d.toObject(ProductModel.class);
                                        list.add(productModel);
                                    }
                                    productAdapter.notifyDataSetChanged();
                                    lastVisible = t.getResult().getDocuments().get(t.getResult().size() - 1);

                                    if (t.getResult().size() < limit) {
                                        isLastItemReached = true;
                                    }
                                }
                            }
                        });
                    }
                }
            };
            recyclerView.addOnScrollListener(onScrollListener);
        }
    }
});

In which lastVisible is a DocumentSnapshot object which represents the last visibile item from the query. In this case, every 15\'th one and it is declared as gloabl variable:

private DocumentSnapshot lastVisible;

And isScrolling and isLastItemReached are also global variables and are declared as:

private boolean isScrolling = false;
private boolean isLastItemReached = false;

回答2:

FirebaseUI-Android also recently came out with a Firestore Paginator.

I have used it in my code, and it works great - just keep in mind that it operates using .get() instead of .addSnapshotListener(), so the recycler is not in realtime.

See the docs here:

https://github.com/firebase/FirebaseUI-Android/tree/master/firestore#using-the-firestorepagingadapter