Add a JSON response subscriber. This is largely the same as the HTML one, but...

Add a JSON response subscriber.  This is largely the same as the HTML one, but forces the appropriate mime type for responses.

core/lib/Drupal/Core/DrupalKernel.php

@@ -15,6 +15,7 @@
 use Symfony\Component\HttpKernel\EventListener\RouterListener;
 
 use Drupal\Core\EventSubscriber\HtmlSubscriber;
+use Drupal\Core\EventSubscriber\JsonSubscriber;
 use Drupal\Core\EventSubscriber\AccessSubscriber;
 use Drupal\Core\EventSubscriber\PathSubscriber;
 use Drupal\Core\EventSubscriber\LegacyControllerSubscriber;
@@ -85,6 +86,7 @@ protected function getDispatcher() {
     // @todo Add a subscriber to handle other things, too, like our Ajax
     // replacement system.
     $dispatcher->addSubscriber(new HtmlSubscriber());
+    $dispatcher->addSubscriber(new JsonSubscriber());
 
     return $dispatcher;
   }

core/lib/Drupal/Core/EventSubscriber/JsonSubscriber.php

+<?php
+
+namespace Drupal\Core\EventSubscriber;
+
+use Symfony\Component\HttpFoundation\Response;
+use Symfony\Component\HttpKernel\KernelEvents;
+use Symfony\Component\HttpKernel\Event\GetResponseEvent;
+use Symfony\Component\HttpKernel\Exception\NotFoundHttpException;
+use Symfony\Component\HttpKernel\Exception\AccessDeniedHttpException;
+use Symfony\Component\Routing\Exception\ResourceNotFoundException;
+use Symfony\Component\Routing\Exception\MethodNotAllowedException;
+use Symfony\Component\EventDispatcher\EventSubscriberInterface;
+
+/**
+ * @file
+ *
+ * Definition of Drupal\Core\EventSubscriber\HtmlSubscriber;
+ */
+
+/**
+ * Main subscriber for JSON-type HTTP responses.
+ */
+class JsonSubscriber implements EventSubscriberInterface {
+
+  /**
+   * Determines if we are dealing with an JSON-style response.
+   *
+   * @param GetResponseEvent $event
+   *   The Event to process.
+   * @return boolean
+   *   True if it is an event we should process as JSON, False otherwise.
+   */
+  protected function isJsonRequestEvent(GetResponseEvent $event) {
+    return in_array('application/json', $event->getRequest()->getAcceptableContentTypes());
+  }
+
+  protected function createJsonResponse() {
+    $response = new Response();
+    $response->headers->set('Content-Type', 'application/json; charset=utf-8');
+
+    return $response;
+  }
+
+  /**
+   * Processes an AccessDenied exception into an HTTP 403 response.
+   *
+   * @param GetResponseEvent $event
+   *   The Event to process.
+   */
+  public function onAccessDeniedException(GetResponseEvent $event) {
+    if ($this->isJsonRequestEvent($event) && $event->getException() instanceof AccessDeniedHttpException) {
+      $response = $this->createJsonResponse();
+      $response->setStatusCode(403, 'Access Denied');
+      $event->setResponse($response);
+    }
+  }
+
+  /**
+   * Processes a NotFound exception into an HTTP 404 response.
+   *
+   * @param GetResponseEvent $event
+   *   The Event to process.
+   */
+  public function onNotFoundHttpException(GetResponseEvent $event) {
+    if ($this->isJsonRequestEvent($event) && $event->getException() instanceof NotFoundHttpException) {
+      $response = $this->createJsonResponse();
+      $response->setStatusCode(404, 'Not Found');
+      $event->setResponse($response);
+    }
+  }
+
+  /**
+   * Processes a MethodNotAllowed exception into an HTTP 405 response.
+   *
+   * @param GetResponseEvent $event
+   *   The Event to process.
+   */
+  public function onMethodAllowedException(GetResponseEvent $event) {
+    if ($this->isJsonRequestEvent($event) && $event->getException() instanceof MethodNotAllowedException) {
+      $response = $this->createJsonResponse();
+      $response->setStatusCode(405, 'Method Not Allowed');
+      $event->setResponse($response);
+    }
+  }
+
+  /**
+   * Processes a successful controller into an HTTP 200 response.
+   *
+   * Some controllers may not return a response object but simply the body of
+   * one.  The VIEW event is called in that case, to allow us to mutate that
+   * body into a Response object.  In particular we assume that the return
+   * from an JSON-type response is a JSON string, so just wrap it into a
+   * Response object.
+   *
+   * @param GetResponseEvent $event
+   *   The Event to process.
+   */
+  public function onView(GetResponseEvent $event) {
+    if ($this->isJsonRequestEvent($event)) {
+      $page_callback_result = $event->getControllerResult();
+
+      $response = $this->createJsonResponse();
+      $response->setContent($page_callback_result);
+
+      $event->setResponse($response);
+    }
+  }
+
+  /**
+   * Registers the methods in this class that should be listeners.
+   *
+   * @return array
+   *   An array of event listener definitions.
+   */
+  static function getSubscribedEvents() {
+    $events[KernelEvents::EXCEPTION][] = array('onNotFoundHttpException');
+    $events[KernelEvents::EXCEPTION][] = array('onAccessDeniedException');
+    $events[KernelEvents::EXCEPTION][] = array('onMethodAllowedException');
+
+    $events[KernelEvents::VIEW][] = array('onView');
+
+    return $events;
+  }
+}