Counter.php

<?php

/**
 * @file
 * Definition of Drupal\views\Plugin\views\field\Counter.
 */
namespace Drupal\views\Plugin\views\field;

use Drupal\Core\Annotation\Plugin;

/**
 * Field handler to show a counter of the current row.
 *
 * @ingroup views_field_handlers
 *
 * @Plugin(
 *   id = "counter"
 * )
 */
class Counter extends FieldPluginBase {
  protected function defineOptions() {
    $options = parent::defineOptions();
    $options['counter_start'] = array(
      'default' => 1,
    );
    return $options;
  }
  public function buildOptionsForm(&$form, &$form_state) {
    $form['counter_start'] = array(
      '#type' => 'textfield',
      '#title' => t('Starting value'),
      '#default_value' => $this->options['counter_start'],
      '#description' => t('Specify the number the counter should start at.'),
      '#size' => 2,
    );
    parent::buildOptionsForm($form, $form_state);
  }
  public function query() {

    // do nothing -- to override the parent query.
  }

  /**
   * Overrides Drupal\views\Plugin\views\field\FieldPluginBas::get_value()
   */
  public function get_value($values, $field = NULL) {

    // Note:  1 is subtracted from the counter start value below because the
    // counter value is incremented by 1 at the end of this function.
    $count = is_numeric($this->options['counter_start']) ? $this->options['counter_start'] - 1 : 0;
    $pager = $this->view->pager;

    // Get the base count of the pager.
    if ($pager
      ->use_pager()) {
      $count += $pager
        ->get_items_per_page() * $pager
        ->get_current_page() + $pager
        ->set_offset();
    }

    // Add the counter for the current site.
    $count += $this->view->row_index + 1;
    return $count;
  }

}