Drupal 8.0.0 beta 12. More info: https://www.drupal.org/node/2514176

core/modules/book/src/Cache/BookNavigationCacheContext.php

@@ -0,0 +1,71 @@
+<?php
+
+/**
+ * @file
+ * Contains \Drupal\book\Cache\BookNavigationCacheContext.
+ */
+
+namespace Drupal\book\Cache;
+
+use Drupal\Core\Cache\Context\CacheContextInterface;
+use Symfony\Component\DependencyInjection\ContainerAware;
+use Symfony\Component\HttpFoundation\RequestStack;
+
+/**
+ * Defines the book navigation cache context service.
+ *
+ * This allows for book navigation location-aware caching. It depends on:
+ * - whether the current route represents a book node at all
+ * - and if so, where in the book hierarchy we are
+ *
+ * This class is container-aware to avoid initializing the 'book.manager'
+ * service when it is not necessary.
+ */
+class BookNavigationCacheContext extends ContainerAware implements CacheContextInterface {
+
+  /**
+   * The request stack.
+   *
+   * @var \Symfony\Component\HttpFoundation\RequestStack
+   */
+  protected $requestStack;
+
+  /**
+   * Constructs a new BookNavigationCacheContext service.
+   *
+   * @param \Symfony\Component\HttpFoundation\RequestStack $request_stack
+   *   The request stack.
+   */
+  public function __construct(RequestStack $request_stack) {
+    $this->requestStack = $request_stack;
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public static function getLabel() {
+    return t("Book navigation");
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public function getContext() {
+    // Find the current book's ID.
+    $current_bid = 0;
+    if ($node = $this->requestStack->getCurrentRequest()->get('node')) {
+      $current_bid = empty($node->book['bid']) ? 0 : $node->book['bid'];
+    }
+
+    // If we're not looking at a book node, then we're not navigating a book.
+    if ($current_bid === 0) {
+      return 'book.none';
+    }
+
+    // If we're looking at a book node, get the trail for that node.
+    $active_trail = $this->container->get('book.manager')
+      ->getActiveTrailIds($node->book['bid'], $node->book);
+    return 'book.' . implode('|', $active_trail);
+  }
+
+}