Make RouterFunctionDsl extensible

Issue: SPR-16897

Author: sdeleuze

spring-webflux/src/main/kotlin/org/springframework/web/reactive/function/server/RouterFunctionDsl.kt

@@ -1,5 +1,5 @@
 /*
- * Copyright 2002-2017 the original author or authors.
+ * Copyright 2002-2018 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.
@@ -50,7 +50,7 @@ import reactor.core.publisher.Mono
  * @see RouterFunctionDsl
  * @since 5.0
  */
-fun router(routes: RouterFunctionDsl.() -> Unit) = RouterFunctionDsl().apply(routes).router()
+fun router(routes: RouterFunctionDsl.() -> Unit) = RouterFunctionDsl(routes).invoke()
 
 /**
  * Provide a [RouterFunction] Kotlin DSL in order to be able to write idiomatic Kotlin code.
@@ -60,7 +60,7 @@ fun router(routes: RouterFunctionDsl.() -> Unit) = RouterFunctionDsl().apply(rou
  * @since 5.0
  * @see <a href="https://youtrack.jetbrains.com/issue/KT-15667">Kotlin issue about supporting ::foo for member functions</a>
  */
-open class RouterFunctionDsl {
+open class RouterFunctionDsl(private val init: RouterFunctionDsl.() -> Unit) : () -> RouterFunction<ServerResponse> {
 
 	private val routes = mutableListOf<RouterFunction<ServerResponse>>()
 
@@ -131,8 +131,8 @@ open class RouterFunctionDsl {
 	 * common path (prefix), header, or other request predicate.
 	 * @see RouterFunctions.nest
 	 */
-	fun RequestPredicate.nest(r: RouterFunctionDsl.() -> Unit) {
-		routes += RouterFunctions.nest(this, RouterFunctionDsl().apply(r).router())
+	fun RequestPredicate.nest(init: RouterFunctionDsl.() -> Unit) {
+		routes += RouterFunctions.nest(this, RouterFunctionDsl(init).invoke())
 	}
 
 	/**
@@ -143,8 +143,8 @@ open class RouterFunctionDsl {
 	 * @see RouterFunctions.nest
 	 * @see RequestPredicates.path
 	*/
-	fun String.nest(r: RouterFunctionDsl.() -> Unit) {
-		routes += RouterFunctions.nest(path(this), RouterFunctionDsl().apply(r).router())
+	fun String.nest(init: RouterFunctionDsl.() -> Unit) {
+		routes += RouterFunctions.nest(path(this), RouterFunctionDsl(init).invoke())
 	}
 
 	/**
@@ -423,9 +423,11 @@ open class RouterFunctionDsl {
 
 	/**
 	 * Return a composed routing function created from all the registered routes.
+	 * @since 5.1
 	 */
-	internal fun router(): RouterFunction<ServerResponse> {
+	override fun invoke(): RouterFunction<ServerResponse> {
+		init()
 		return routes.reduce(RouterFunction<ServerResponse>::and)
 	}
 
-}
+}