class StatusMessages

class StatusMessages extends RenderElement {
    
    /**
     * {@inheritdoc}
     *
     * Generate the placeholder in a #pre_render callback, because the hash salt
     * needs to be accessed, which may not yet be available when this is called.
     */
    public function getInfo() {
        return [
            // May have a value of 'status' or 'error' when only displaying messages
            // of that specific type.
'#display' => NULL,
            '#pre_render' => [
                get_class() . '::generatePlaceholder',
            ],
            '#include_fallback' => FALSE,
        ];
    }
    
    /**
     * #pre_render callback to generate a placeholder.
     *
     * @param array $element
     *   A renderable array.
     *
     * @return array
     *   The updated renderable array containing the placeholder.
     */
    public static function generatePlaceholder(array $element) {
        $build = [
            '#lazy_builder' => [
                get_class() . '::renderMessages',
                [
                    $element['#display'],
                ],
            ],
            '#create_placeholder' => TRUE,
        ];
        // Directly create a placeholder as we need this to be placeholdered
        // regardless if this is a POST or GET request.
        // @todo remove this when https://www.drupal.org/node/2367555 lands.
        $build = \Drupal::service('render_placeholder_generator')->createPlaceholder($build);
        if ($element['#include_fallback']) {
            return [
                'fallback' => [
                    '#markup' => '<div data-drupal-messages-fallback class="hidden"></div>',
                ],
                'messages' => $build,
            ];
        }
        return $build;
    }
    
    /**
     * #lazy_builder callback; replaces placeholder with messages.
     *
     * @param string|null $type
     *   Limit the messages returned by type. Defaults to NULL, meaning all types.
     *   Passed on to \Drupal\Core\Messenger\Messenger::deleteByType(). These
     *   values are supported:
     *   - NULL
     *   - 'status'
     *   - 'warning'
     *   - 'error'
     *
     * @return array
     *   A renderable array containing the messages.
     *
     * @see \Drupal\Core\Messenger\Messenger::deleteByType()
     */
    public static function renderMessages($type = NULL) {
        $render = [];
        if (isset($type)) {
            $messages = [
                $type => \Drupal::messenger()->deleteByType($type),
            ];
        }
        else {
            $messages = \Drupal::messenger()->deleteAll();
        }
        if ($messages) {
            // Render the messages.
            $render = [
                '#theme' => 'status_messages',
                '#message_list' => $messages,
                '#status_headings' => [
                    'status' => t('Status message'),
                    'error' => t('Error message'),
                    'warning' => t('Warning message'),
                ],
            ];
        }
        return $render;
    }

}