Add flushing support

This commit add flushing support thanks to the FlushingDataBuffer
wrapper that allows to identify the elements that should trigger a
flush.

spring-web-reactive/src/test/java/org/springframework/http/server/reactive/FlushingIntegrationTests.java

@@ -0,0 +1,82 @@
+/*
+ * Copyright 2002-2016 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
+ *
+ *      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.springframework.http.server.reactive;
+
+import org.junit.Before;
+import org.junit.Test;
+import static org.springframework.web.client.reactive.HttpRequestBuilders.get;
+import static org.springframework.web.client.reactive.WebResponseExtractors.bodyStream;
+import reactor.core.publisher.Flux;
+import reactor.core.publisher.Mono;
+import reactor.core.test.TestSubscriber;
+
+import org.springframework.core.io.buffer.DataBuffer;
+import org.springframework.core.io.buffer.FlushingDataBuffer;
+import org.springframework.http.client.reactive.ReactorHttpClientRequestFactory;
+import org.springframework.web.client.reactive.WebClient;
+
+/**
+ * @author Sebastien Deleuze
+ */
+public class FlushingIntegrationTests extends AbstractHttpHandlerIntegrationTests {
+
+	private WebClient webClient;
+
+	@Before
+	public void setup() throws Exception {
+		super.setup();
+		this.webClient = new WebClient(new ReactorHttpClientRequestFactory());
+	}
+
+	@Test
+	public void testFlushing() throws Exception {
+		Mono<String> result = this.webClient
+				.perform(get("http://localhost:" + port))
+				.extract(bodyStream(String.class))
+				.take(2)
+				.reduce((s1, s2) -> s1 + s2);
+
+		TestSubscriber
+				.subscribe(result)
+				.await()
+				.assertValues("data0data1");
+	}
+
+
+	@Override
+	protected HttpHandler createHttpHandler() {
+		return new FlushingHandler();
+	}
+
+	private static class FlushingHandler implements HttpHandler {
+
+		@Override
+		public Mono<Void> handle(ServerHttpRequest request, ServerHttpResponse response) {
+			Flux<DataBuffer> responseBody = Flux
+					.interval(50)
+					.take(2)
+					.concatWith(Flux.never())
+					.map(l -> {
+						byte[] data = ("data" + l).getBytes();
+						DataBuffer buffer = response.bufferFactory().allocateBuffer(data.length);
+						buffer.write(data);
+						return new FlushingDataBuffer(buffer);
+					});
+			return response.writeWith(responseBody);
+		}
+	}
+}