Add SSE support to WebMvc.fn

This commit adds support for sending Server-Sent Events in WebMvc.fn,
through the ServerResponse.sse method that takes a SseBuilder DSL.
It also includes reference documentation.

Closes gh-25920

spring-webmvc/src/test/java/org/springframework/web/servlet/function/SseServerResponseTests.java

@@ -0,0 +1,145 @@
+/*
+ * Copyright 2002-2020 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 org.springframework.web.servlet.function;
+
+import java.io.IOException;
+import java.io.UncheckedIOException;
+import java.time.Duration;
+import java.util.Collections;
+
+import org.junit.jupiter.api.BeforeEach;
+import org.junit.jupiter.api.Test;
+
+import org.springframework.http.converter.json.MappingJackson2HttpMessageConverter;
+import org.springframework.web.servlet.ModelAndView;
+import org.springframework.web.testfixture.servlet.MockHttpServletRequest;
+import org.springframework.web.testfixture.servlet.MockHttpServletResponse;
+
+import static org.assertj.core.api.Assertions.assertThat;
+
+/**
+ * @author Arjen Poutsma
+ */
+class SseServerResponseTests {
+
+	private MockHttpServletRequest mockRequest;
+
+	private MockHttpServletResponse mockResponse;
+
+	@BeforeEach
+	void setUp() {
+		this.mockRequest = new MockHttpServletRequest("GET", "https://example.com");
+		this.mockRequest.setAsyncSupported(true);
+		this.mockResponse = new MockHttpServletResponse();
+	}
+
+	@Test
+	void sendString() throws Exception {
+		String body = "foo bar";
+		ServerResponse response = ServerResponse.sse(sse -> {
+			try {
+				sse.send(body);
+			}
+			catch (IOException ex) {
+				throw new UncheckedIOException(ex);
+			}
+		});
+
+		ServerResponse.Context context = Collections::emptyList;
+
+		ModelAndView mav = response.writeTo(this.mockRequest, this.mockResponse, context);
+		assertThat(mav).isNull();
+
+		String expected = "data:" + body + "\n\n";
+		assertThat(this.mockResponse.getContentAsString()).isEqualTo(expected);
+	}
+
+	@Test
+	void sendObject() throws Exception {
+		Person person = new Person("John Doe", 42);
+		ServerResponse response = ServerResponse.sse(sse -> {
+			try {
+				sse.send(person);
+			}
+			catch (IOException ex) {
+				throw new UncheckedIOException(ex);
+			}
+		});
+
+		ServerResponse.Context context = () -> Collections.singletonList(new MappingJackson2HttpMessageConverter());
+
+		ModelAndView mav = response.writeTo(this.mockRequest, this.mockResponse, context);
+		assertThat(mav).isNull();
+
+		String expected = "data:{\"name\":\"John Doe\",\"age\":42}\n\n";
+		assertThat(this.mockResponse.getContentAsString()).isEqualTo(expected);
+	}
+
+	@Test
+	public void builder() throws Exception {
+		String body = "foo bar";
+		ServerResponse response = ServerResponse.sse(sse -> {
+			try {
+				sse.id("id")
+						.event("name")
+						.comment("comment line 1\ncomment line 2")
+						.retry(Duration.ofSeconds(1))
+						.data("data");
+			}
+			catch (IOException ex) {
+				throw new UncheckedIOException(ex);
+			}
+		});
+
+		ServerResponse.Context context = Collections::emptyList;
+
+		ModelAndView mav = response.writeTo(this.mockRequest, this.mockResponse, context);
+		assertThat(mav).isNull();
+
+		String expected = "id:id\n" +
+				"event:name\n" +
+				":comment line 1\n" +
+				":comment line 2\n" +
+				"retry:1000\n" +
+				"data:data\n\n";
+		assertThat(this.mockResponse.getContentAsString()).isEqualTo(expected);
+	}
+
+
+	private static final class Person {
+
+		private final String name;
+
+		private final int age;
+
+
+		public Person(String name, int age) {
+			this.name = name;
+			this.age = age;
+		}
+
+		public String getName() {
+			return this.name;
+		}
+
+		public int getAge() {
+			return this.age;
+		}
+	}
+
+
+}