Improve custom project request arrangement

This commit improves the use case of configuring a custom ProjectRequest
by enforcing consistently a particular type using a generic.

As a result, `ProjectGenerationInvoker` is no longer exposed as a bean
as it is the responsibility of the custom `ProjectGenerationController`
to provide one that matches the requested `ProjectRequest` type.

See gh-990

initializr-docs/src/main/asciidoc/configuration-guide.adoc

@@ -1088,13 +1088,19 @@ include::{code-examples}/doc/generator/project/CustomProjectGenerationController
 ----
 
 If you inherit from `WebProjectRequest`, defaults can be automatically applied from the
-metadata as shown above but you may also chose to ignore that. If you define a `@Bean`
-for that controller, the auto-configuration will back-off and use yours instead.
+metadata as shown above but you may also chose to ignore that.
 
 The next step is to make sure that those additional attributes are made available in the
 `ProjectGenerationContext`. The idiomatic way of doing this is to create your own
 interface that extends from `ProjectDescription` and expose your custom attributes. To
-make sure your view of `ProjectGeneration` is made available in the
+make sure your view of `ProjectDescription` is made available in the
 `ProjectGenerationContext`, a custom `ProjectRequestToDescriptionConverter` should be
-defined. When such a bean exists in the context it replaces the default that the
-auto-configuration provides.
+defined and could reuse `DefaultProjectRequestToDescriptionConverter` to apply general
+rules for standard fields.
+
+Finally, you should wire up everything:
+
+[source,java,indent=0,subs="verbatim,quotes,attributes"]
+----
+include::{code-examples}/doc/generator/project/CustomProjectGenerationConfigurationExample.java[tag=code]
+----

initializr-docs/src/main/java/io/spring/initializr/doc/generator/project/CustomProjectGenerationConfigurationExample.java

@@ -0,0 +1,59 @@
+/*
+ * Copyright 2012-2019 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.initializr.doc.generator.project;
+
+import io.spring.initializr.generator.project.MutableProjectDescription;
+import io.spring.initializr.generator.project.ProjectDescription;
+import io.spring.initializr.metadata.InitializrMetadata;
+import io.spring.initializr.metadata.InitializrMetadataProvider;
+import io.spring.initializr.web.controller.ProjectGenerationController;
+import io.spring.initializr.web.project.ProjectGenerationInvoker;
+import io.spring.initializr.web.project.ProjectRequestToDescriptionConverter;
+
+import org.springframework.context.ApplicationContext;
+import org.springframework.context.annotation.Bean;
+import org.springframework.context.annotation.Configuration;
+
+/**
+ * Configuration example of a custom {@link ProjectGenerationController}.
+ *
+ * @author Stephane Nicoll
+ */
+@Configuration
+public class CustomProjectGenerationConfigurationExample {
+
+	// tag::code[]
+	@Bean
+	public CustomProjectGenerationController projectGenerationController(InitializrMetadataProvider metadataProvider,
+			ApplicationContext applicationContext) {
+		ProjectGenerationInvoker<CustomProjectRequest> projectGenerationInvoker = new ProjectGenerationInvoker<>(
+				applicationContext, new CustomProjectRequestToDescriptionConverter());
+		return new CustomProjectGenerationController(metadataProvider, projectGenerationInvoker);
+	}
+	// end::code[]
+
+	static class CustomProjectRequestToDescriptionConverter
+			implements ProjectRequestToDescriptionConverter<CustomProjectRequest> {
+
+		@Override
+		public ProjectDescription convert(CustomProjectRequest request, InitializrMetadata metadata) {
+			return new MutableProjectDescription();
+		}
+
+	}
+
+}

initializr-docs/src/main/java/io/spring/initializr/doc/generator/project/CustomProjectGenerationController.java

@@ -31,7 +31,7 @@ import io.spring.initializr.web.project.ProjectGenerationInvoker;
 public class CustomProjectGenerationController extends ProjectGenerationController<CustomProjectRequest> {
 
 	public CustomProjectGenerationController(InitializrMetadataProvider metadataProvider,
-			ProjectGenerationInvoker projectGenerationInvoker) {
+			ProjectGenerationInvoker<CustomProjectRequest> projectGenerationInvoker) {
 		super(metadataProvider, projectGenerationInvoker);
 	}