materio-d8/web/modules/custom/materio_sapi/src/Controller/Base.php

<?php
// https://www.qed42.com/blog/autocomplete-drupal-8
// https://www.drupal.org/docs/8/modules/search-api/developer-documentation/executing-a-search-in-code

namespace Drupal\materio_sapi\Controller;

use Drupal\Core\Controller\ControllerBase;
use Symfony\Component\HttpFoundation\JsonResponse;
use Symfony\Component\HttpFoundation\Request;
use Drupal\Component\Utility\Tags;
use Drupal\Component\Utility\Unicode;
use Drupal\search_api\Entity\Index;

// https://drupal.stackexchange.com/questions/225008/programatically-use-search-api

/**
 * Defines a route controller for entity autocomplete form elements.
 */
class Base extends ControllerBase {

  /**
   * Handler for autocomplete request.
   */
  public function getResults(Request $request) {
    $resp = [
      'tes' => 'ok',
      'autocomplete' => $request->query->get('autocomplete'),
      'keys' => $request->query->get('keys'),
      'range' => array(
        'offset' => $request->query->get('offset'),
        'limit' => $request->query->get('limit'),
      ),
    ];

    // Get the typed string from the URL, if it exists.
    if ($keys = $resp['keys']) {
      $typed_string = Tags::explode($keys);
      $typed_string = Unicode::strtolower($keys);
      \Drupal::logger('materio_sapi')->notice($typed_string);

      $index = Index::load('database');
      $query = $index->query();

      // Change the parse mode for the search.
      $parse_mode = \Drupal::service('plugin.manager.search_api.parse_mode')
        ->createInstance('direct');
      $parse_mode->setConjunction('OR');
      $query->setParseMode($parse_mode);

      // Set fulltext search keywords and fields.
      $query->keys($typed_string);
      // $query->setFulltextFields(['name']);

      // Set additional conditions.
      // $query->addCondition('status', 1)
      //   ->addCondition('author', 1, '<>');

      // Restrict the search to specific languages.
      // $query->setLanguages(['de', 'it']);

      // Do paging.
      $query->range($resp['range']['offset'], $resp['range']['limit']);

      // Add sorting.
      $query->sort('search_api_relevance', 'DESC');

      // Set one or more tags for the query.
      // @see hook_search_api_query_TAG_alter()
      // @see hook_search_api_results_TAG_alter()
      $query->addTag('materio_sapi_search');

      $results = $query->execute();

      $resultitems = $results->getResultItems();
      $resp['count'] = $results->getResultCount();
      $resp['resultitems'] = array_keys($resultitems);
      $resp['options'] = $query->getOptions();


      $items = [];
      foreach ($results as $result) {
        // \Drupal::logger('materio_sapi')->notice(print_r($result->getField('tid')->getValues(),true));
        // \Drupal::logger('materio_sapi')->notice(print_r($result->getField('name')->getValues(),true));
        $nid = $result->getField('nid')->getValues()[0];
        $title = $result->getField('title')->getValues()[0]->getText();
        $items[] = [
          'nid' => $nid,
          'title' => $title,
        ];
      }
      $resp['items'] = $items;
    }

    return new JsonResponse($resp);
  }

}