class PlaceholderGenerator

class PlaceholderGenerator implements PlaceholderGeneratorInterface {
  
  /**
   * The cache contexts manager service.
   *
   * @var \Drupal\Core\Cache\Context\CacheContextsManager
   */
  protected $cacheContextsManager;
  
  /**
   * The renderer configuration array.
   *
   * @var array
   */
  protected $rendererConfig;
  
  /**
   * Constructs a new Placeholder service.
   *
   * @param \Drupal\Core\Cache\Context\CacheContextsManager $cache_contexts_manager
   *   The cache contexts manager service.
   * @param array $renderer_config
   *   The renderer configuration array.
   */
  public function __construct(CacheContextsManager $cache_contexts_manager, array $renderer_config) {
    $this->cacheContextsManager = $cache_contexts_manager;
    $this->rendererConfig = $renderer_config;
  }
  
  /**
   * {@inheritdoc}
   */
  public function canCreatePlaceholder(array $element) {
    // If generated by a #lazy_builder callback, placeholdering is possible.
    return isset($element['#lazy_builder']) && (!isset($element['#create_placeholder']) || $element['#create_placeholder'] !== FALSE);
  }
  
  /**
   * {@inheritdoc}
   */
  public function shouldAutomaticallyPlaceholder(array $element) {
    // Auto-placeholder if the max-age, cache context or cache tag is specified
    // in the auto-placeholder conditions in the 'renderer.config' container
    // parameter.
    $conditions = $this->rendererConfig['auto_placeholder_conditions'];
    $cacheability = CacheableMetadata::createFromRenderArray($element);
    if ($cacheability->getCacheMaxAge() !== Cache::PERMANENT && $cacheability->getCacheMaxAge() <= $conditions['max-age']) {
      return TRUE;
    }
    // Optimize the contexts and let them affect the cache tags to mimic what
    // happens to the cacheability in the variation cache (RenderCache backend).
    $cacheability->addCacheableDependency($this->cacheContextsManager
      ->convertTokensToKeys($cacheability->getCacheContexts()));
    $cacheability->setCacheContexts($this->cacheContextsManager
      ->optimizeTokens($cacheability->getCacheContexts()));
    if (array_intersect($cacheability->getCacheContexts(), $conditions['contexts'])) {
      return TRUE;
    }
    if (array_intersect($cacheability->getCacheTags(), $conditions['tags'])) {
      return TRUE;
    }
    return FALSE;
  }
  
  /**
   * {@inheritdoc}
   */
  public function createPlaceholder(array $element) {
    $placeholder_render_array = array_intersect_key($element, [
      // Placeholders are replaced with markup by executing the associated
      // #lazy_builder callback, which generates a render array, and which the
      // Renderer will render and replace the placeholder with.
'#lazy_builder' => TRUE,
      // The cacheability metadata for the placeholder. The rendered result of
      // the placeholder may itself be cached, if [#cache][keys] are specified.
'#cache' => TRUE,
    ]);
    if (isset($element['#lazy_builder_preview'])) {
      $placeholder_render_array['#preview'] = $element['#lazy_builder_preview'];
    }
    // Be sure cache contexts and tags are sorted before serializing them and
    // making hash. Issue #3225328 removes sort from contexts and tags arrays
    // for performances reasons.
    if (isset($placeholder_render_array['#cache']['contexts'])) {
      sort($placeholder_render_array['#cache']['contexts']);
    }
    if (isset($placeholder_render_array['#cache']['tags'])) {
      sort($placeholder_render_array['#cache']['tags']);
    }
    // Generate placeholder markup. Note that the only requirement is that this
    // is unique markup that isn't easily guessable. The #lazy_builder callback
    // and its arguments are put in the placeholder markup solely to simplify<<<
    // debugging.
    $callback = $placeholder_render_array['#lazy_builder'][0];
    $arguments = UrlHelper::buildQuery($placeholder_render_array['#lazy_builder'][1]);
    $token = Crypt::hashBase64(serialize($placeholder_render_array));
    $placeholder_markup = '<drupal-render-placeholder callback="' . Html::escape($callback) . '"';
    if ($arguments !== '') {
      $placeholder_markup .= ' arguments="' . Html::escape($arguments) . '"';
    }
    $placeholder_markup .= ' token="' . Html::escape($token) . '"></drupal-render-placeholder>';
    // Build the placeholder element to return.
    $placeholder_element = [];
    $placeholder_element['#markup'] = Markup::create($placeholder_markup);
    $placeholder_element['#attached']['placeholders'][$placeholder_markup] = $placeholder_render_array;
    return $placeholder_element;
  }

}