Issue #2692523: whole bunch of changes after Wim did a thorough review:

 - Various naming and comment changes.
 - Naming consistency: CacheTagsHeaderSubscriber --> CacheableResponseSubscriber
 - HEADER const moved into CacheableResponseSubscriber - more on this later (turning into plugins).
 - instanceof check.
 - Dropped ::testServicePresence()

purge.services.yml

@@ -2,10 +2,10 @@ services:
 
   # PURGE.CACHE_TAGS_HEADER_SUBSCRIBER
   #
-  # Solely responsible for outputting the X-Cache-Tags header, which external
-  # caching systems and CDNs should store when caching Drupal's responses.
-  purge.cache_tags_header_subscriber:
-    class: Drupal\purge\EventSubscriber\CacheTagsHeaderSubscriber
+  # Solely responsible for outputting the Purge-Cache-Tags header, which external
+  # caching systems and CDNs should store for tag-based cache invalidation.
+  purge.cacheable_response_subscriber:
+    class: Drupal\purge\EventSubscriber\CacheableResponseSubscriber
     tags:
       - { name: event_subscriber }
 

src/EventSubscriber/CacheTagsHeaderSubscriber.php → src/EventSubscriber/CacheableResponseSubscriber.php

@@ -2,19 +2,27 @@
 
 /**
  * @file
- * Contains \Drupal\purge\EventSubscriber\CacheTagsHeaderSubscriber.
+ * Contains \Drupal\purge\EventSubscriber\CacheableResponseSubscriber.
  */
 
 namespace Drupal\purge\EventSubscriber;
 
 use Symfony\Component\HttpKernel\KernelEvents;
 use Symfony\Component\HttpKernel\Event\FilterResponseEvent;
 use Symfony\Component\EventDispatcher\EventSubscriberInterface;
+use Drupal\Core\Cache\CacheableResponseInterface;
 
 /**
- * Adds the X-Cache-Tags response header, to aid external caching systems.
+ * Adds the Purge-Cache-Tags response header, to aid external caching systems.
  */
-class CacheTagsHeaderSubscriber implements EventSubscriberInterface {
+class CacheableResponseSubscriber implements EventSubscriberInterface {
+
+  /**
+   * The name of the cache tags header sent.
+   *
+   * @var string
+   */
+  const HEADER = 'Purge-Cache-Tags';
 
   /**
    * {@inheritdoc}
@@ -25,12 +33,10 @@ public static function getSubscribedEvents() {
   }
 
   /**
-   * Subscribed callback to the KernelEvents::RESPONSE event.
+   * Sets the Purge-Cache-Tags header on cacheable responses.
    *
    * @param \Symfony\Component\HttpKernel\Event\FilterResponseEvent $event
    *   The event to process.
-   *
-   * @return void.
    */
   public function onRespond(FilterResponseEvent $event) {
     if (!$event->isMasterRequest()) {
@@ -39,13 +45,11 @@ public function onRespond(FilterResponseEvent $event) {
 
     // First ensure that ::getCacheableMetadata() exists on the object.
     $response = $event->getResponse();
-    if (method_exists($response, 'getCacheableMetadata')) {
+    if ($response instanceof CacheableResponseInterface) {
 
       // Only proceed setting the tags when it isn't yet set by other modules.
-      if (is_null($response->headers->get('X-Cache-Tags'))) {
-        $tags = $response->getCacheableMetadata()->getCacheTags();
-        $response->headers->set('X-Cache-Tags', implode(' ', $tags));
-      }
+      $tags = $response->getCacheableMetadata()->getCacheTags();
+      $response->headers->set(SELF::HEADER, implode(' ', $tags));
     }
   }
 

src/Tests/CacheTagsHeaderSubscriberTest.php → src/Tests/CacheableResponseSubscriberTest.php

@@ -2,35 +2,22 @@
 
 /**
  * @file
- * Contains \Drupal\purge\Tests\CacheTagsHeaderSubscriberTest.
+ * Contains \Drupal\purge\Tests\CacheableResponseSubscriberTest.
  */
 
 namespace Drupal\purge\Tests;
 
 use Drupal\purge\Tests\KernelTestBase;
 use Symfony\Component\HttpFoundation\Request;
 use Symfony\Component\HttpFoundation\Response;
+use Drupal\purge\EventSubscriber\CacheableResponseSubscriber;
 
 /**
- * Tests \Drupal\purge\EventSubscriber\CacheTagsHeaderSubscriber.
+ * Tests \Drupal\purge\EventSubscriber\CacheableResponseSubscriber.
  *
  * @group purge
  */
-class CacheTagsHeaderSubscriberTest extends KernelTestBase {
-
-  /**
-   * The name of the cache tags header exported.
-   *
-   * @var string
-   */
-  const HEADER = 'X-Cache-Tags';
-
-  /**
-   * The name of the subscribing service.
-   *
-   * @var string
-   */
-  const SERVICE = 'purge.cache_tags_header_subscriber';
+class CacheableResponseSubscriberTest extends KernelTestBase {
 
   /**
    * {@inheritdoc}
@@ -53,19 +40,11 @@ public function testHeaderPresence() {
     $request = Request::create('/system/401');
     $response = $this->container->get('http_kernel')->handle($request);
     $this->assertEqual(200, $response->getStatusCode());
-    $header = $response->headers->get(SELF::HEADER);
+    $header = $response->headers->get(CacheableResponseSubscriber::HEADER);
     $this->assertNotNull($header);
     $this->assertTrue(is_string($header));
     $this->assertTrue(strpos($header, 'config:user.role.anonymous') !== FALSE);
     $this->assertTrue(strpos($header, 'rendered') !== FALSE);
   }
 
-  /**
-   * Test service presence.
-   */
-  public function testServicePresence() {
-    $this->assertTrue($this->container->has(SELF::SERVICE));
-    $this->assertTrue($this->container->getDefinition(SELF::SERVICE)->hasTag('event_subscriber'));
-  }
-
 }