Drupal 8 views style plugin for a better rest serializer that leverages the Facets module.

BetterRestFacetsSerializer.php

<?php

namespace Drupal\my_module\Plugin\views\style;

use Drupal\facets_rest\Plugin\views\style\FacetsSerializer;

/**
 * The style plugin for serialized output formats.
 *
 * @ingroup views_style_plugins
 *
 * @ViewsStyle(
 *   id = "my_module_facets_serializer",
 *   title = @Translation("Better Facets Serializer"),
 *   help = @Translation("Extends existing serializer styles to provide additional view data."),
 *   display_types = {"data"}
 * )
 */
class BetterRestFacetsSerializer extends FacetsSerializer {

  /**
   * {@inheritdoc}
   */
  public function render() {
    $rows['search_results'] = $this->getSearchResults();
    $rows['facets'] = $this->options['show_facets'] ? $this->getFacets() : [];
    $rows['active_facet_items'] = $this->getActiveFacetItems($rows['facets']);
    $rows['pager'] = $this->getPagerDetails();
    $rows['exposed_filters'] = $this->getExposedHandlers('filter');
    $rows['exposed_sorts'] = $this->getExposedHandlers('sort');

    // Get the content type configured in the display or fallback to the
    // default.
    if ((empty($this->view->live_preview))) {
      $content_type = $this->displayHandler->getContentType();
    }
    else {
      $content_type = !empty($this->options['formats']) ? \reset($this->options['formats']) : 'json';
    }

    return $this->serializer->serialize($rows, $content_type, ['views_style_plugin' => $this]);
  }

  /**
   * Get an array of active facet items.
   *
   * @param array $facets
   *
   * @return array
   */
  protected function getActiveFacetItems(array $facets): array {
    $active_items = [];

    foreach ($facets as $facet) {
      if (!empty($facet['items'])) {
        foreach ($facet['items'] as $item) {
          if (!empty($item['values']['active'])) {
            $item['facet_config'] = $facet['config'];

            // We need to do some trickier to correctly set static values
            // display item values.
            if (!empty($item['facet_config']['static_values'])) {
              // Break apart the config string in value pairs.
              $static_values = \preg_split('/\R+/', $item['facet_config']['static_values']);

              foreach ($static_values as $value) {
                // Break apart key value pairs.
                $key_values = \explode('|', $value);
                // If value equates to the active item value, set display.
                if ($key_values[0] === $item['values']['value']) {
                  $item['values']['display_value'] = $key_values[1];
                }
              }
            }

            $active_items[] = $item;
          }
        }
      }
    }

    return $active_items;
  }

  /**
   * Get the search results and process facets.
   *
   * @see FacetsSerializer::render();
   *
   * @return array
   */
  protected function getSearchResults() {
    $rows = [];

    // If the Data Entity row plugin is used, this will be an array of entities
    // which will pass through Serializer to one of the registered Normalizers,
    // which will transform it to arrays/scalars. If the Data field row plugin
    // is used, $rows will not contain objects and will pass directly to the
    // Encoder.
    foreach ($this->view->result as $row_index => $row) {
      // Keep track of the current rendered row, like every style plugin has to
      // do.
      // @see \Drupal\views\Plugin\views\style\StylePluginBase::renderFields
      $this->view->row_index = $row_index;
      $rows[] = $this->view->rowPlugin->render($row);
    }

    // Remove native row index which was throwing off something in the rendering
    // and avoid confusion. This is not relevant as the actually rows are being
    // stored and rendered elsewhere. Can ask Jonathan to help clarify if need
    // be.
    unset($this->view->row_index);

    return $rows;
  }

  /**
   * Process and return.
   *
   * @see FacetsSerializer::render();
   *
   * @return array
   *
   * @throws \Drupal\facets\Exception\InvalidProcessorException
   */
  protected function getFacets() {
    // Processing facets.
    $facetsource_id = "search_api:views_rest__{$this->view->id()}__{$this->view->getDisplay()->display['id']}";
    $facets = $this->facetsManager->getFacetsByFacetSourceId($facetsource_id);
    $this->facetsManager->updateResults($facetsource_id);

    // Sort facets by weight.
    \usort($facets, function ($a, $b) {
      return $a->getWeight() > $b->getWeight();
    });

    $processed_facets = [];
    foreach ($facets as $facet) {
      $processed = $this->facetsManager->build($facet);

      // Ensure facet is prepared and flatten it.
      if (isset($processed[0], $processed[0]['config'])) {
        $processed_facets[] = $processed[0];
      }
    }

    return $processed_facets;
  }

  /**
   * Get pager and page details.
   *
   * @link https://www.drupal.org/project/drupal/issues/2982729
   *
   * @return array
   */
  protected function getPagerDetails() {
    $details = ['active' => FALSE];

    $pager = $this->view->pager;

    if ($pager) {
      $class = \get_class($pager);
      $total_items = $pager->getTotalItems();
      $items_per_page = $pager->getItemsPerPage();
      $current_page = $pager->getCurrentPage();
      $total_pages = 0;

      if (!in_array($class, ['Drupal\views\Plugin\views\pager\None', 'Drupal\views\Plugin\views\pager\Some'])) {
        $total_pages = $pager->getPagerTotal();
      }

      $details = [
        'active' => TRUE,
        'current_page' => $current_page,
        'total_items' => $total_items,
        'total_pages' => $total_pages,
        'items_per_page' => $items_per_page,
        'options' => $pager->usesOptions() ? $pager->options : FALSE,
      ];
    }

    return $details;
  }

  /**
   * Get exposed handler information and option values.
   *
   * @param string $type
   *   ID for a handler type.
   *
   * @return array
   */
  protected function getExposedHandlers(string $type) {
    $exposed = [];

    $handlers = $this->view->getHandlers($type);
    $exposed_input = $this->view->getExposedInput();

    foreach ($handlers as $id => $item) {
      if ($item['exposed']) {
        $info = ['id' => $id];

        if (isset($item['expose']['identifier'])) {
          $info['submitted_values'] = $exposed_input[$item['expose']['identifier']] ?? [];
        }

        $info += $item['expose'];
        $exposed[] = $info;
      }
    }

    return $exposed;
  }

}