{#
/**
 * @file
 * Default theme implementation to display a book tree.
 *
 * Returns HTML for a wrapper for a book sub-tree.
 *
 * Available variables:
 * - items: A nested list of book items. Each book item contains:
 *   - attributes: HTML attributes for the book item.
 *   - below: The book item child items.
 *   - title: The book link title.
 *   - url: The book link URL, instance of \Drupal\Core\Url.
 *   - is_expanded: TRUE if the link has visible children within the current
 *     book tree.
 *   - is_collapsed: TRUE if the link has children within the current book tree
 *     that are not currently visible.
 *   - in_active_trail: TRUE if the link is in the active trail.
 *
 * @ingroup themeable
 */
#}
{% import _self as book_tree %}

{#
  We call a macro which calls itself to render the full tree.
  @see https://twig.symfony.com/doc/1.x/tags/macro.html
#}
{{ book_tree.book_links(items, attributes, 0) }}

{% macro book_links(items, attributes, menu_level) %}
  {% import _self as book_tree %}
  {% if items %}
    {% if menu_level == 0 %}
      <ul{{ attributes }}>
    {% else %}
      <ul>
    {% endif %}
    {% for item in items %}
      <li{{ item.attributes }}>
        {{ link(item.title, item.url) }}
        {% if item.below %}
          {{ book_tree.book_links(item.below, attributes, menu_level + 1) }}
        {% endif %}
      </li>
    {% endfor %}
    </ul>
  {% endif %}
{% endmacro %}