More context.

src/Event/StompHeaderEvent.php

@@ -27,6 +27,20 @@ class StompHeaderEvent implements StompHeaderEventInterface {
    */
   protected $user;
 
+  /**
+   * An array of data to be sent with the STOMP request, for context.
+   *
+   * @var array
+   */
+  protected $data;
+
+  /**
+   * An array of configuration used to generate $data, for context.
+   *
+   * @var array
+   */
+  protected $configuration;
+
   /**
    * The set of headers.
    *
@@ -37,9 +51,11 @@ class StompHeaderEvent implements StompHeaderEventInterface {
   /**
    * Constructor.
    */
-  public function __construct(EntityInterface $entity, AccountInterface $user) {
+  public function __construct(EntityInterface $entity, AccountInterface $user, array $data, array $configuration) {
     $this->entity = $entity;
     $this->user = $user;
+    $this->data = $data;
+    $this->configuration = $configuration;
     $this->headers = new ParameterBag();
   }
 
@@ -57,11 +73,25 @@ public function getUser() {
     return $this->user;
   }
 
+  /**
+   * {@inheritdoc}
+   */
+  public function getData() {
+    return $this->data;
+  }
+
   /**
    * {@inheritdoc}
    */
   public function getHeaders() {
     return $this->headers;
   }
 
+  /**
+   * {@inheritdoc}
+   */
+  public function getConfiguration() {
+    return $this->configuration;
+  }
+
 }

src/Event/StompHeaderEventInterface.php

@@ -37,4 +37,20 @@ public function getEntity();
    */
   public function getUser();
 
+  /**
+   * Fetch the data to be sent in the body of the request.
+   *
+   * @return array
+   *   The array of data.
+   */
+  public function getData();
+
+  /**
+   * Fetch the configuration of the action, for context.
+   *
+   * @return array
+   *   The array of configuration for the upstream action.
+   */
+  public function getConfiguration();
+
 }

src/EventGenerator/EmitEvent.php

@@ -120,7 +120,7 @@ public function execute($entity = NULL) {
 
       $event = $this->eventDispatcher->dispatch(
         StompHeaderEvent::EVENT_NAME,
-        new StompHeaderEvent($entity, $user)
+        new StompHeaderEvent($entity, $user, $data, $this->getConfiguration())
       );
 
       $message = new Message(

src/EventSubscriber/StompHeaderSubscriber.php

@@ -6,6 +6,7 @@
 use Drupal\jwt\Authentication\Provider\JwtAuth;
 
 use Drupal\Core\Messenger\MessengerInterface;
+use Drupal\Core\StringTranslation\StringTranslationTrait;
 
 use Psr\Log\LoggerInterface;
 use Symfony\Component\EventDispatcher\EventSubscriberInterface;
@@ -15,6 +16,8 @@
  */
 class StompHeaderEventSubscriber implements EventSubscriberInterface {
 
+  use StringTranslationTrait;
+
   /**
    * The JWT auth service.
    *