Support for views REST export settings

Two years and a half later I'm asking the same question: how to implement spellcheck in the context of a rest export?

For example, is it possible to include the "did you mean" suggestion inside a metadata as pager or facets do?

OK, I answer to my own question,

For those who are interested, here is some code to create a custom serializer to use within a REST Export display which shows spellcheck suggestions in a "spellcheck" metadata inside the JSON response.

modules/custom/my_custom_module/Plugin/views/style/CustomSerializer.php

<?php
namespace Drupal\my_custom_module\Plugin\views\style;
use Drupal\Core\Form\FormStateInterface;
use Drupal\rest\Plugin\views\style\Serializer;
use Symfony\Component\DependencyInjection\ContainerInterface;
use Symfony\Component\Serializer\SerializerInterface;
/**
 * The style plugin for serialized output formats.
 *
 * @ingroup views_style_plugins
 *
 * @ViewsStyle(
 *   id = "my_custom_serializer",
 *   title = @Translation( "My Custom serializer" ),
 *   help = @Translation( "Serializes views row data using the Serializer component." ),
 *   display_types = { "data" }
 * )
 */
class CustomSerializer extends Serializer {
  public static function create( ContainerInterface $container, array $configuration, $plugin_id, $plugin_definition ) {
    return new static(
      $configuration,
      $plugin_id,
      $plugin_definition,
      $container -> get( 'serializer' ),
      $container -> getParameter( 'serializer.formats' ),
      $container -> getParameter( 'serializer.format_providers' )
    );
  }
  public function __construct( array $configuration, $plugin_id, $plugin_definition, SerializerInterface $serializer, array $serializer_formats, array $serializer_format_providers ) {
    parent::__construct( $configuration, $plugin_id, $plugin_definition, $serializer, $serializer_formats, $serializer_format_providers );
  }
  protected function defineOptions() {
    $options = parent::defineOptions();
    $options[ 'spellcheck_count' ] = [ 'default' => 1 ];
    return $options;
  }
  public function buildOptionsForm( &$form, FormStateInterface $form_state ) {
    parent::buildOptionsForm( $form, $form_state );
    $form[ 'spellcheck_count' ] = [
      '#type' => 'number',
      '#title' => $this -> t( 'Spellcheck, amount of results to show' ),
      '#default_value' => $this -> options[ 'spellcheck_count' ],
    ];
  }
  public function render() {
    $rows = [];
    foreach ( $this -> view -> result as $row_index => $row ) {
      $this -> view -> row_index = $row_index;
      $row = $this -> view -> rowPlugin -> render( $row );
      //YOUR OWN CODE IF YOU WANT TO MAKE SOME MODIFICATION ON EACH ROW
      $rows[] = $row;
    }
    unset( $this -> view -> row_index) ;
    if ( empty( $this -> view -> live_preview ) ) {
      $content_type = $this -> displayHandler -> getContentType();
    } else {
      $content_type = !empty( $this -> options[ 'formats' ] ) ? reset( $this -> options[ 'formats' ] ) : 'json';
    }
    $fields = [];
    foreach ( $this -> view -> field as $field_name => $field ) {
      if ( $field -> options[ 'exclude' ] == 0 && $field -> label() ) {
        $fields[ $field_name ] = $field -> label();
      }
    }
    $current_page = 0;
    $total_items = 0;
    $total_pages = 0;
    $items_per_page = 0;
    $pager = $this -> view -> pager;
    if ( $pager ) {
      $class = get_class( $pager );
      $current_page = $pager -> getCurrentPage() ?? 0;
      $items_per_page = $pager -> getItemsPerPage();
      $total_items = $pager -> getTotalItems();
      if ( !in_array( $class, [
        'Drupal\views\Plugin\views\pager\None',
        'Drupal\views\Plugin\views\pager\Some',
      ] ) ) {
        $total_pages = $pager -> getPagerTotal();
      }
    } else {
      $current_page = intval( \Drupal::request() -> query -> get( 'page' ) );
      $items_per_page = 1;
      $total_pages = count( $rows );
      $total_items = count( $rows );
    }
    $spellcheck_suggestions = [];
    $query = $this -> view -> query;
    if ( $query instanceof \Drupal\search_api\Plugin\views\query\SearchApiQuery && $query -> getIndex() -> getServerInstance()-> supportsFeature( 'search_api_spellcheck' ) ) {
      $search_api_query = $query -> getSearchApiQuery();
      $index = $search_api_query -> getIndex();
      $backend = $index -> getServerInstance() -> getBackend();
      $connector = $backend -> getSolrConnector();
      $solarium_query = $connector -> getSelectQuery();
      $spellcheck = $solarium_query -> getSpellcheck();
      $schema_languages = $backend -> getSchemaLanguageStatistics();
      $dictionaries = [];
      foreach ( \Drupal\search_api_solr\Utility\Utility::ensureLanguageCondition( $search_api_query ) as $language_id ) {
        if ( isset( $schema_languages[ $language_id ] ) && $schema_languages[ $language_id ] ) {
          $dictionaries[] = $schema_languages[ $language_id ];
        }
      }
      if ( $dictionaries ) {
        $spellcheck -> setDictionary( $dictionaries );
      } else {
        $spellcheck -> setDictionary( \Drupal\Core\Language\LanguageInterface::LANGCODE_NOT_SPECIFIED );
      }
      $keys = $query -> getKeys();
      if ( $keys && is_array( $keys ) ) {
        $spellcheck -> setQuery( implode( ' ', array_filter( $keys, 'is_int', ARRAY_FILTER_USE_KEY ) ) );
      }
      $spellcheck -> setCount( $this -> options[ 'spellcheck_count' ] );
      $spellcheck -> setBuild( TRUE );
      $spellcheck -> setCollate( TRUE );
      $spellcheck -> setExtendedResults( TRUE );
      $spellcheck -> setCollateExtendedResults( TRUE );
      $resultset = $connector -> execute( $solarium_query );
      $spellcheck_results = $resultset -> getSpellcheck();
      $spellcheck_suggestions = [];
      if ( $spellcheck_results ) {
        foreach ( $spellcheck_results -> getSuggestions() as $suggestion ) {
          $spellcheck_suggestions[] = [
            'word' => $suggestion -> getWord(),
            'numFound' => $suggestion -> getNumFound(),
            'startOffset' => $suggestion -> getStartOffset(),
            'endOffset' => $suggestion -> getEndOffset(),
            'suggestion' => $suggestion -> getWords()
          ];
        }
      }
    }
    return $this -> serializer -> serialize( [
      'title' => $this -> view -> getTitle(),
      'rows' => $rows,
      'spellcheck' => $spellcheck_suggestions,
      'fields' => $fields,
      'pager' => [
        'current_page' => $current_page,
        'total_items' => $total_items,
        'total_pages' => $total_pages,
        'items_per_page' => $items_per_page,
      ]
    ], $content_type, [ 'views_style_plugin' => $this ] );
  }
}

For example, if I set an exposed search_api_fulltext filter (be careful that this is not working for "direct parse") and if I search for the word "spac", I obtain the following JSON response:

{"title":"My view",rows":[],"spellcheck":[{"word":"space","numFound":10,"startOffset":0,"endOffset":4,"suggestion":[{"word":"space","freq":60},{"word":"spacex","freq":7},{"word":"spain","freq":7},{"word":"scaf","freq":6},{"word":"spare","freq":6},{"word":"sdas","freq":3},{"word":"spot","freq":3},{"word":"seal","freq":2},{"word":"seas","freq":2},{"word":"spaces","freq":2}]}],"fields":{"date":"Date","description":"Description","country":"Country","source":"Source","tag":"Tag(s)","created":"Edited on"},"pager":{"current_page":0,"total_items":0,"total_pages":0,"items_per_page":50}}

It's just a first shot, I know it can be improved