GH-422 Add test and documentation for pure function interaction

Author: olegz

spring-cloud-function-samples/function-sample-cloudevent/README.adoc

@@ -15,7 +15,25 @@ The example provides dependencies and instructions to demonstrate several distin
 The POM file defines all the necessary dependency in a segregated way, so you can choose the one you're interested in.
 
 ### Direct function invocation
-TBD
+
+By looking up user declared functions in `FunctionCatalog` you can interact (i.e., for testing purposes) with functions directly 
+while enjoying all the features of _spring-cloud-function_ such as transparent type conversion, function composition and more. 
+
+[source, java]
+----
+Message<String> binaryCloudEventMessage = MessageBuilder
+	.withPayload("{\"releaseDate\":\"24-03-2004\", \"releaseName\":\"Spring Framework\", \"version\":\"1.0\"}")
+	.setHeader("ce-specversion", "1.0")
+	.setHeader("ce-type", "com.example.springevent")
+	.setHeader("ce-source", "spring.io/spring-event")
+	.setHeader("ce-id", "123-456-9876-09")
+	.build();
+Function<Message<String>, String> asPojoMessage = catalog.lookup("asPOJOMessage");
+System.out.println(asPojoMessage.apply(binaryCloudEventMessage));
+----
+
+The test case link:src/test/java/io/spring/cloudevent/CloudeventDemoApplicationFunctionTests.java[CloudeventDemoApplicationFunctionTests]
+provides a good example on how to accomplish this.
 
 ### Function as a REST endpoint
 
@@ -114,6 +132,17 @@ entire structure of Cloud Event message as payload (see the screenshot below)._
 
 image::images\rabbit-send-structured.png[structured,700,700]
 
+You can follow similar approach with Apache Kafka or any other binder. All you need is bring a required binder dependency. 
+For example for Apache Kafka
+[source, xml]
+----
+<dependency>
+   <groupId>org.springframework.cloud</groupId>
+   <artifactId>spring-cloud-stream-binder-kafka</artifactId>
+   <version>3.1.0-SNAPSHOT</version>
+</dependency>
+----
+
 ### Function invocation via RSocket
 
 TBD

spring-cloud-function-samples/function-sample-cloudevent/pom.xml

@@ -35,7 +35,7 @@
             <version>3.1.0-SNAPSHOT</version>
         </dependency>
         <!-- end REST -->
-        
+      
         <!-- RSocket - only needed if you intend to invoke via RSocket -->
 <!--         <dependency> -->
 <!--             <groupId>org.springframework.cloud</groupId> -->
@@ -45,11 +45,11 @@
         <!-- end RSocket -->
 
         <!-- RabbitMQ - only needed if you intend to invoke via RabbitMQ -->
-        <dependency>
-            <groupId>org.springframework.cloud</groupId>
-            <artifactId>spring-cloud-stream-binder-rabbit</artifactId>
-            <version>3.1.0-SNAPSHOT</version>
-        </dependency>
+<!--         <dependency> -->
+<!--             <groupId>org.springframework.cloud</groupId> -->
+<!--             <artifactId>spring-cloud-stream-binder-rabbit</artifactId> -->
+<!--             <version>3.1.0-SNAPSHOT</version> -->
+<!--         </dependency> -->
         <!-- end RabbitMQ -->
 
          <!-- Kafka - only needed if you intend to invoke via RabbitMQ -->

spring-cloud-function-samples/function-sample-cloudevent/src/test/java/io/spring/cloudevent/CloudeventDemoApplicationFunctionTests.java

@@ -0,0 +1,64 @@
+/*
+ * Copyright 2020-2020 the original author or authors.
+ *
+ * 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
+ *
+ *      https://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 io.spring.cloudevent;
+
+import java.util.function.Function;
+
+import org.junit.jupiter.api.Test;
+import org.springframework.boot.SpringApplication;
+import org.springframework.cloud.function.context.FunctionCatalog;
+import org.springframework.context.ApplicationContext;
+import org.springframework.messaging.Message;
+import org.springframework.messaging.support.MessageBuilder;
+
+/**
+ *
+ * @author Oleg Zhurakousky
+ *
+ */
+public class CloudeventDemoApplicationFunctionTests {
+
+	@Test
+	public void demoPureFunctionInvocation() {
+		ApplicationContext context = SpringApplication.run(CloudeventDemoApplication.class);
+		FunctionCatalog catalog = context.getBean(FunctionCatalog.class);
+		Message<String> binaryCloudEventMessage = MessageBuilder
+				.withPayload("{\"releaseDate\":\"24-03-2004\", \"releaseName\":\"Spring Framework\", \"version\":\"1.0\"}")
+				.setHeader("ce-specversion", "1.0")
+				.setHeader("ce-type", "com.example.springevent")
+				.setHeader("ce-source", "spring.io/spring-event")
+				.setHeader("ce-id", "123-456-9876-09")
+				.build();
+
+		/*
+		 * NOTE how it makes no difference what the actual function signature
+		 * is (see `asPOJOMessage` and `asPOJO` specifically). Type conversion will happen
+		 * inside spring-cloud-function.
+		 */
+		Function<Message<String>, String> asPojoMessage = catalog.lookup("asPOJOMessage");
+		System.out.println(asPojoMessage.apply(binaryCloudEventMessage));
+
+		Function<Message<String>, String> asPojo = catalog.lookup("asPOJO");
+		System.out.println(asPojo.apply(binaryCloudEventMessage));
+
+		Function<Message<String>, String> asString = catalog.lookup("asString");
+		System.out.println(asString.apply(binaryCloudEventMessage));
+
+		Function<Message<String>, String> asStringMessage = catalog.lookup("asStringMessage");
+		System.out.println(asStringMessage.apply(binaryCloudEventMessage));
+	}
+}