public void getAllProducts(final OnLoadListener listener) {
        db.collection("products").get()
                .addOnSuccessListener(new OnSuccessListener<QuerySnapshot>() {
                    @Override
                    public void onSuccess(QuerySnapshot queryDocumentSnapshots) {
                        final ArrayList<Product> products = new ArrayList<Product>();

                        if (listener != null && !queryDocumentSnapshots.isEmpty()) {
                            for (QueryDocumentSnapshot document : queryDocumentSnapshots) {
                                products.add(document.toObject(Product.class));
                            }

                            listener.onLoad(products);
                        }
                    }
                })
                .addOnFailureListener(new OnFailureListener() {
                    @Override
                    public void onFailure(@NonNull Exception e) {
                        listener.onFailedLoad();
                    }
                });
    }

public ProductCatalog(VariableChangeListener variableChangeListener) {
        this.variableChangeListener = variableChangeListener;
        DatabaseManager dbManager = new DatabaseManager();

        dbManager.getAllProducts(this);
    }

public ProductsPresenter(ProductsActivity activity) {
        this.activity = activity;
        this.catalog = new ProductCatalog(this);
        this.showProducts();
}