Refactor View contract

View now returns Mono<Void> rather than Flux<DataBuffer> which aligns
more closely with the reactive HttpMessageConverter vs the Encoder.

The change was prompted by the upcoming implementation of a View that
delegates to an existing HttpMessageConverter e.g. for JSON, XML.

The resulting change also brings the reactive View closer in spirit to
the View from spring-webmvc which returns void.

spring-web-reactive/src/main/java/org/springframework/web/reactive/result/view/AbstractView.java

@@ -24,6 +24,7 @@ import java.util.Map;
 import org.apache.commons.logging.Log;
 import org.apache.commons.logging.LogFactory;
 import reactor.core.publisher.Flux;
+import reactor.core.publisher.Mono;
 
 import org.springframework.context.ApplicationContext;
 import org.springframework.context.ApplicationContextAware;
@@ -111,10 +112,10 @@ public abstract class AbstractView implements View, ApplicationContextAware {
 	 * @param contentType the content type selected to render with which should
 	 * match one of the {@link #getSupportedMediaTypes() supported media types}.
 	 * @param exchange the current exchange
-	 * @return
+	 * @return {@code Mono} to represent when and if rendering succeeds
 	 */
 	@Override
-	public Flux<DataBuffer> render(HandlerResult result, MediaType contentType,
+	public Mono<Void> render(HandlerResult result, MediaType contentType,
 			ServerWebExchange exchange) {
 
 		if (logger.isTraceEnabled()) {
@@ -151,8 +152,9 @@ public abstract class AbstractView implements View, ApplicationContextAware {
 	 * @param renderAttributes combined output Map (never {@code null}),
 	 * with dynamic values taking precedence over static attributes
 	 * @param exchange current exchange
+	 * @return {@code Mono} to represent when and if rendering succeeds
 	 */
-	protected abstract Flux<DataBuffer> renderInternal(Map<String, Object> renderAttributes,
+	protected abstract Mono<Void> renderInternal(Map<String, Object> renderAttributes,
 			ServerWebExchange exchange);
 
 

spring-web-reactive/src/main/java/org/springframework/web/reactive/result/view/View.java

@@ -19,6 +19,7 @@ import java.util.List;
 import java.util.Optional;
 
 import reactor.core.publisher.Flux;
+import reactor.core.publisher.Mono;
 
 import org.springframework.core.io.buffer.DataBuffer;
 import org.springframework.http.MediaType;
@@ -53,8 +54,8 @@ public interface View {
 	 * @param contentType the content type selected to render with which should
 	 * match one of the {@link #getSupportedMediaTypes() supported media types}.
 	 * @param exchange the current exchange
-	 * @return the output stream
+	 * @return {@code Mono} to represent when and if rendering succeeds
 	 */
-	Flux<DataBuffer> render(HandlerResult result, MediaType contentType, ServerWebExchange exchange);
+	Mono<Void> render(HandlerResult result, MediaType contentType, ServerWebExchange exchange);
 
 }

spring-web-reactive/src/main/java/org/springframework/web/reactive/result/view/ViewResolutionResultHandler.java

@@ -34,7 +34,6 @@ import org.springframework.core.Ordered;
 import org.springframework.core.ResolvableType;
 import org.springframework.core.annotation.AnnotationAwareOrderComparator;
 import org.springframework.core.convert.ConversionService;
-import org.springframework.core.io.buffer.DataBuffer;
 import org.springframework.ui.Model;
 import org.springframework.util.Assert;
 import org.springframework.util.StringUtils;
@@ -184,8 +183,7 @@ public class ViewResolutionResultHandler implements HandlerResultHandler, Ordere
 
 		return viewMono.then(returnValue -> {
 			if (returnValue instanceof View) {
-				Flux<DataBuffer> body = ((View) returnValue).render(result, null, exchange);
+				return ((View) returnValue).render(result, null, exchange);
-				return exchange.getResponse().writeWith(body);
 			}
 			else if (returnValue instanceof CharSequence) {
 				String viewName = returnValue.toString();
@@ -194,10 +192,7 @@ public class ViewResolutionResultHandler implements HandlerResultHandler, Ordere
 						.concatMap(resolver -> resolver.resolveViewName(viewName, locale))
 						.next()
 						.otherwiseIfEmpty(handleUnresolvedViewName(viewName))
-						.then(view -> {
+						.then(view -> view.render(result, null, exchange));
-							Flux<DataBuffer> body = view.render(result, null, exchange);
-							return exchange.getResponse().writeWith(body);
-						});
 			}
 			else {
 				// Should not happen

spring-web-reactive/src/main/java/org/springframework/web/reactive/result/view/freemarker/FreeMarkerView.java

@@ -30,6 +30,7 @@ import freemarker.template.SimpleHash;
 import freemarker.template.Template;
 import freemarker.template.Version;
 import reactor.core.publisher.Flux;
+import reactor.core.publisher.Mono;
 
 import org.springframework.beans.BeansException;
 import org.springframework.beans.factory.BeanFactoryUtils;
@@ -156,7 +157,7 @@ public class FreeMarkerView extends AbstractUrlBasedView {
 	}
 
 	@Override
-	protected Flux<DataBuffer> renderInternal(Map<String, Object> renderAttributes, ServerWebExchange exchange) {
+	protected Mono<Void> renderInternal(Map<String, Object> renderAttributes, ServerWebExchange exchange) {
 		// Expose all standard FreeMarker hash models.
 		SimpleHash freeMarkerModel = getTemplateModel(renderAttributes, exchange);
 		if (logger.isDebugEnabled()) {
@@ -170,12 +171,12 @@ public class FreeMarkerView extends AbstractUrlBasedView {
 		}
 		catch (IOException ex) {
 			String message = "Could not load FreeMarker template for URL [" + getUrl() + "]";
-			return Flux.error(new IllegalStateException(message, ex));
+			return Mono.error(new IllegalStateException(message, ex));
 		}
 		catch (Throwable ex) {
-			return Flux.error(ex);
+			return Mono.error(ex);
 		}
-		return Flux.just(dataBuffer);
+		return exchange.getResponse().writeWith(Flux.just(dataBuffer));
 	}
 
 	/**

spring-web-reactive/src/test/java/org/springframework/web/reactive/result/view/UrlBasedViewResolverTests.java

@@ -63,8 +63,8 @@ public class UrlBasedViewResolverTests {
 		}
 
 		@Override
-		protected Flux<DataBuffer> renderInternal(Map<String, Object> attributes, ServerWebExchange exchange) {
+		protected Mono<Void> renderInternal(Map<String, Object> attributes, ServerWebExchange exchange) {
-			return Flux.empty();
+			return Mono.empty();
 		}
 	}
 

spring-web-reactive/src/test/java/org/springframework/web/reactive/result/view/ViewResolutionResultHandlerTests.java

@@ -48,6 +48,7 @@ import org.springframework.http.MediaType;
 import org.springframework.http.server.reactive.MockServerHttpRequest;
 import org.springframework.http.server.reactive.MockServerHttpResponse;
 import org.springframework.http.server.reactive.ServerHttpRequest;
+import org.springframework.http.server.reactive.ServerHttpResponse;
 import org.springframework.ui.ExtendedModelMap;
 import org.springframework.ui.Model;
 import org.springframework.ui.ModelMap;
@@ -324,10 +325,11 @@ public class ViewResolutionResultHandlerTests {
 		}
 
 		@Override
-		public Flux<DataBuffer> render(HandlerResult result, MediaType mediaType, ServerWebExchange exchange) {
+		public Mono<Void> render(HandlerResult result, MediaType mediaType, ServerWebExchange exchange) {
 			String value = this.name + ": " + result.getModel().toString();
 			assertNotNull(value);
-			return Flux.just(asDataBuffer(value));
+			ServerHttpResponse response = exchange.getResponse();
+			return response.writeWith(Flux.just(asDataBuffer(value)));
 		}
 	}
 

spring-web-reactive/src/test/java/org/springframework/web/reactive/result/view/freemarker/FreeMarkerViewTests.java

@@ -19,14 +19,12 @@ import java.net.URI;
 import java.nio.ByteBuffer;
 import java.nio.charset.Charset;
 import java.util.Locale;
-import java.util.Optional;
 
 import freemarker.template.Configuration;
 import org.junit.Before;
 import org.junit.Rule;
 import org.junit.Test;
 import org.junit.rules.ExpectedException;
-import reactor.core.publisher.Flux;
 import reactor.core.test.TestSubscriber;
 
 import org.springframework.context.ApplicationContextException;
@@ -46,7 +44,6 @@ import org.springframework.web.server.session.WebSessionManager;
 
 import static org.junit.Assert.assertEquals;
 import static org.junit.Assert.assertTrue;
-import static org.mockito.Mockito.mock;
 
 /**
  * @author Rossen Stoyanchev
@@ -60,6 +57,8 @@ public class FreeMarkerViewTests {
 
 	private ServerWebExchange exchange;
 
+	private MockServerHttpResponse response;
+
 	private GenericApplicationContext context;
 
 	private Configuration freeMarkerConfig;
@@ -83,7 +82,7 @@ public class FreeMarkerViewTests {
 		fv.setApplicationContext(this.context);
 
 		MockServerHttpRequest request = new MockServerHttpRequest(HttpMethod.GET, new URI("/path"));
-		MockServerHttpResponse response = new MockServerHttpResponse();
+		this.response = new MockServerHttpResponse();
 		WebSessionManager manager = new DefaultWebSessionManager();
 		this.exchange = new DefaultServerWebExchange(request, response, manager);
 	}
@@ -127,10 +126,10 @@ public class FreeMarkerViewTests {
 		ModelMap model = new ExtendedModelMap();
 		model.addAttribute("hello", "hi FreeMarker");
 		HandlerResult result = new HandlerResult(new Object(), "", ResolvableType.NONE, model);
-		Flux<DataBuffer> flux = view.render(result, null, this.exchange);
+		view.render(result, null, this.exchange);
 
 		TestSubscriber<DataBuffer> subscriber = new TestSubscriber<>();
-		subscriber.bindTo(flux).assertValuesWith(dataBuffer ->
+		subscriber.bindTo(this.response.getBody()).assertValuesWith(dataBuffer ->
 					assertEquals("<html><body>hi FreeMarker</body></html>", asString(dataBuffer)));
 	}