class PlaceholderGenerator

class PlaceholderGenerator implements PlaceholderGeneratorInterface {
    
    /**
     * The renderer configuration array.
     *
     * @var array
     */
    protected $rendererConfig;
    
    /**
     * Constructs a new Placeholder service.
     *
     * @param array $renderer_config
     *   The renderer configuration array.
     */
    public function __construct(array $renderer_config) {
        $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 = $this->rendererConfig['auto_placeholder_conditions'];
        if (isset($element['#cache']['max-age']) && $element['#cache']['max-age'] !== Cache::PERMANENT && $element['#cache']['max-age'] <= $conditions['max-age']) {
            return TRUE;
        }
        if (isset($element['#cache']['contexts']) && array_intersect($element['#cache']['contexts'], $conditions['contexts'])) {
            return TRUE;
        }
        if (isset($element['#cache']['tags']) && array_intersect($element['#cache']['tags'], $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,
        ]);
        // 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) . '" arguments="' . Html::escape($arguments) . '" token="' . Html::escape($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;
    }

}