GraphQL: added example of VertxBatchLoader to the doc

Closes vert-x3#1981

Signed-off-by: Thomas Segismont <tsegismont@gmail.com>

vertx-web-graphql/src/main/asciidoc/index.adoc

@@ -199,16 +199,20 @@ First, create a batch loader:
 {@link examples.GraphQLExamples#createBatchLoader}
 ----
 
-TIP: If you work with Vert.x callback-based APIs, you may use a {@link io.vertx.ext.web.handler.graphql.dataloader.VertxBatchLoader}
-or a {@link io.vertx.ext.web.handler.graphql.dataloader.VertxMappedBatchLoader} to simplify your code.
-
 Then, configure the {@link io.vertx.ext.web.handler.graphql.GraphQLHandler} to create a `DataLoaderRegistry` for each request:
 
 [source,$lang]
 ----
 {@link examples.GraphQLExamples#dataLoaderRegistry}
 ----
 
+If you work with Vert.x APIs, you may use a {@link io.vertx.ext.web.handler.graphql.dataloader.VertxBatchLoader} or a {@link io.vertx.ext.web.handler.graphql.dataloader.VertxMappedBatchLoader} to simplify your code:
+
+[source,$lang]
+----
+{@link examples.GraphQLExamples#createVertxBatchLoader}
+----
+
 === File uploads
 
 https://github.com/jaydenseric/graphql-multipart-request-spec[GraphQL multipart request] is an interoperable multipart form field structure for `GraphQL` requests.

vertx-web-graphql/src/main/java/examples/GraphQLExamples.java

@@ -29,6 +29,7 @@
 import io.vertx.ext.web.RoutingContext;
 import io.vertx.ext.web.handler.BodyHandler;
 import io.vertx.ext.web.handler.graphql.*;
+import io.vertx.ext.web.handler.graphql.dataloader.VertxBatchLoader;
 import io.vertx.ext.web.handler.graphql.schema.VertxDataFetcher;
 import io.vertx.ext.web.handler.graphql.schema.VertxPropertyDataFetcher;
 import org.dataloader.*;
@@ -213,7 +214,10 @@ public DataFetcher<Object> getDefaultDataFetcher(FieldWiringEnvironment environm
   }
 
   public void createBatchLoader() {
-    BatchLoaderWithContext<String, Link> linksBatchLoader = this::retrieveLinksFromBackend;
+    BatchLoaderWithContext<String, Link> linksBatchLoader = (ids, env) -> {
+      // retrieveLinksFromBackend takes a list of ids and returns a CompletionStage for a list of links
+      return retrieveLinksFromBackend(ids, env);
+    };
   }
 
   private CompletionStage<List<Link>> retrieveLinksFromBackend(List<String> ids, BatchLoaderEnvironment environment) {
@@ -230,6 +234,17 @@ public void dataLoaderRegistry(GraphQL graphQL, BatchLoaderWithContext<String, L
     });
   }
 
+  public void createVertxBatchLoader() {
+    BatchLoaderWithContext<Long, String> commentsBatchLoader = VertxBatchLoader.create((ids, env) -> {
+      // findComments takes a list of ids and returns a Future for a list of links
+      return findComments(ids, env);
+    });
+  }
+
+  private Future<List<String>> findComments(List<Long> ids, BatchLoaderEnvironment env) {
+    return null;
+  }
+
   public void addApolloWsHandlerToRouter(Router router) {
     GraphQL graphQL = setupGraphQLJava();