public class NewProductModel implements NewProductContract.Model {

  private Context context;

  public NewProductModel(Context context) {
    this.context = context;
  }

  @Override
  public void addProduct(Product product, OnAddProductListener listener) {
    ProductApiInterface api = ProductApi.buildInstance();
    Call<Product> callProducts = api.addProduct(product);
    callProducts.enqueue(new Callback<Product>() {
      @Override
      public void onResponse(Call<Product> call, Response<Product> response) {
        Product product = response.body();
        listener.onAddProductSuccess(product);
      }
      @Override
      public void onFailure(Call<Product> call, Throwable t) {
        listener.onAddProductError("Se ha producido un error al conectar con el servidor");
        t.printStackTrace();
      }
    });
  }
  . . .
  . . .