Resolve #3521173 "Process attachments cssjs"

Draft MR until the htmx dependency is merged. At that point we can add and run tests.

Closes #3521173

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

+<?php
+
+declare(strict_types=1);
+
+namespace Drupal\Core\EventSubscriber;
+
+use Drupal\Component\Utility\Html;
+use Drupal\Core\Render\AttachmentsResponseProcessorInterface;
+use Drupal\Core\Render\HtmlResponse;
+use Symfony\Component\EventDispatcher\EventSubscriberInterface;
+use Symfony\Component\HttpFoundation\RequestStack;
+use Symfony\Component\HttpKernel\Event\RequestEvent;
+use Symfony\Component\HttpKernel\Event\ResponseEvent;
+use Symfony\Component\HttpKernel\KernelEvents;
+
+/**
+ * Adds assets on responses to HTMX requests.
+ *
+ * @see \Drupal\Core\Render\Hypermedia\HtmxResponseAttachmentsProcessor
+ */
+class HtmxResponseSubscriber implements EventSubscriberInterface {
+
+  /**
+   * Constructs a HtmxResponseSubscriber object.
+   */
+  public function __construct(
+    protected readonly RequestStack $requestStack,
+    protected \Closure $attachmentsProcessor,
+  ) {}
+
+  /**
+   * Add assemble and attachments add HTMX attributes.
+   *
+   * @see \Drupal\Core\EventSubscriber\HtmlResponseSubscriber::onRespond
+   */
+  public function onRespond(ResponseEvent $event): void {
+    $response = $event->getResponse();
+    $requestHeaders = $this->requestStack->getCurrentRequest()->headers;
+    if (!($response instanceof HtmlResponse && $requestHeaders->has('HX-Request'))) {
+      // Only operate on HTML responses from an HTMX request.
+      return;
+    }
+    $processedResponse = $this->getAttachementsProcessor()->processAttachments($response);
+    if (!($processedResponse instanceof HtmlResponse)) {
+      throw new \TypeError("ResponseEvent::setResponse() requires an HtmlResponse object");
+    }
+    $event->setResponse($processedResponse);
+  }
+
+  protected function getAttachementsProcessor(): AttachmentsResponseProcessorInterface {
+    return ($this->attachmentsProcessor)();
+  }
+
+  /**
+   * Sets \Drupal\Component\Utility\Html::$isAjax to TRUE.
+   *
+   * @param \Symfony\Component\HttpKernel\Event\RequestEvent $event
+   *   The request event, which contains the current request.
+   */
+  public function onRequest(RequestEvent $event) {
+    $requestHeaders = $event->getRequest()->headers;
+    // Pass to the Html class that the current request is an Ajax request.
+    if ($requestHeaders->has('HX-Request')) {
+      Html::setIsAjax(TRUE);
+    }
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public static function getSubscribedEvents(): array {
+    return [
+      // Our method needs to run before the HtmlResponseSubscriber(weight 0).
+      KernelEvents::RESPONSE => ['onRespond', 100],
+      KernelEvents::REQUEST => ['onRequest', 50],
+    ];
+  }
+
+}