Added method for accessing the graph directly

streams/api/src/main/java/org/eclipse/microprofile/reactive/streams/CompletionRunner.java

@@ -19,6 +19,7 @@
 
 package org.eclipse.microprofile.reactive.streams;
 
+import org.eclipse.microprofile.reactive.streams.spi.Graph;
 import org.eclipse.microprofile.reactive.streams.spi.ReactiveStreamsEngine;
 
 import java.util.concurrent.CompletionStage;
@@ -58,6 +59,10 @@ public CompletionStage<T> run() {
    * @return A completion stage that will be redeemed with the result of the stream, or an error if the stream fails.
    */
   public CompletionStage<T> run(ReactiveStreamsEngine engine) {
-    return engine.buildCompletion(graphBuilder.build(false, false));
+    return engine.buildCompletion(toGraph());
+  }
+
+  Graph toGraph() {
+    return graphBuilder.build(false, false);
   }
 }

streams/api/src/main/java/org/eclipse/microprofile/reactive/streams/GraphAccessor.java

@@ -0,0 +1,78 @@
+/*******************************************************************************
+ * Copyright (c) 2018 Contributors to the Eclipse Foundation
+ *
+ * See the NOTICE file(s) distributed with this work for additional
+ * information regarding copyright ownership.
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * You may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *     http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ ******************************************************************************/
+
+package org.eclipse.microprofile.reactive.streams;
+
+import org.eclipse.microprofile.reactive.streams.spi.Graph;
+
+/**
+ * Exists to allow access to the {@code toGraph} methods on each builder, so that these methods don't have to be
+ * exposed publicly in the API.
+ *
+ * This is intended only for use by implementations of the API, to get direct access to the graphs without having to
+ * build a publisher, processor or subscriber. This is particularly useful for cases where an implementation would
+ * like to do additional manipulation using its own API to the stream, but have those manipulations fused to the
+ * graph being manipulated.
+ */
+public class GraphAccessor {
+
+  private GraphAccessor() {
+  }
+
+  /**
+   * Build the graph for the given {@link PublisherBuilder}.
+   *
+   * @param publisherBuilder The builder to build the graph for.
+   * @return The built graph.
+   */
+  public static Graph buildGraphFor(PublisherBuilder<?> publisherBuilder) {
+    return publisherBuilder.toGraph();
+  }
+
+  /**
+   * Build the graph for the given {@link ProcessorBuilder}.
+   *
+   * @param processorBuilder The builder to build the graph for.
+   * @return The built graph.
+   */
+  public static Graph buildGraphFor(ProcessorBuilder<?, ?> processorBuilder) {
+    return processorBuilder.toGraph();
+  }
+
+  /**
+   * Build the graph for the given {@link SubscriberBuilder}.
+   *
+   * @param subscriberBuilder The builder to build the graph for.
+   * @return The built graph.
+   */
+  public static Graph buildGraphFor(SubscriberBuilder<?, ?> subscriberBuilder) {
+    return subscriberBuilder.toGraph();
+  }
+
+  /**
+   * Build the graph for the given {@link CompletionRunner}.
+   *
+   * @param completionRunner The runner to build the graph for.
+   * @return The built graph.
+   */
+  public static Graph buildGraphFor(CompletionRunner<?> completionRunner) {
+    return completionRunner.toGraph();
+  }
+
+}

streams/api/src/main/java/org/eclipse/microprofile/reactive/streams/ProcessorBuilder.java

@@ -19,6 +19,7 @@
 
 package org.eclipse.microprofile.reactive.streams;
 
+import org.eclipse.microprofile.reactive.streams.spi.Graph;
 import org.eclipse.microprofile.reactive.streams.spi.ReactiveStreamsEngine;
 import org.eclipse.microprofile.reactive.streams.spi.Stage;
 import org.reactivestreams.Processor;
@@ -432,7 +433,11 @@ public Processor<T, R> buildRs() {
    * @return A {@link Processor} that will run this stream.
    */
   public Processor<T, R> buildRs(ReactiveStreamsEngine engine) {
-    return engine.buildProcessor(graphBuilder.build(true, true));
+    return engine.buildProcessor(toGraph());
+  }
+
+  Graph toGraph() {
+    return graphBuilder.build(true, true);
   }
 
   private <S> ProcessorBuilder<T, S> addStage(Stage stage) {

streams/api/src/main/java/org/eclipse/microprofile/reactive/streams/SubscriberBuilder.java

@@ -19,6 +19,7 @@
 
 package org.eclipse.microprofile.reactive.streams;
 
+import org.eclipse.microprofile.reactive.streams.spi.Graph;
 import org.eclipse.microprofile.reactive.streams.spi.ReactiveStreamsEngine;
 import org.eclipse.microprofile.reactive.streams.spi.Stage;
 
@@ -62,7 +63,11 @@ public CompletionSubscriber<T, R> build() {
    * @return A {@link CompletionSubscriber} that will run this stream.
    */
   public CompletionSubscriber<T, R> build(ReactiveStreamsEngine engine) {
-    return engine.buildSubscriber(graphBuilder.build(true, false));
+    return engine.buildSubscriber(toGraph());
+  }
+
+  Graph toGraph() {
+    return graphBuilder.build(true, false);
   }
 
   ReactiveStreamsGraphBuilder getGraphBuilder() {