Counter.php

<?php

namespace Drupal\views\Plugin\views\field;

use Drupal\Core\Form\FormStateInterface;
use Drupal\views\ResultRow;

/**
 * Field handler to show a counter of the current row.
 *
 * @ingroup views_field_handlers
 *
 * @ViewsField("counter")
 */
class Counter extends FieldPluginBase {
    use UncacheableFieldHandlerTrait;
    
    /**
     * {@inheritdoc}
     */
    public function usesGroupBy() {
        return FALSE;
    }
    
    /**
     * {@inheritdoc}
     */
    protected function defineOptions() {
        $options = parent::defineOptions();
        $options['counter_start'] = [
            'default' => 1,
        ];
        return $options;
    }
    
    /**
     * {@inheritdoc}
     */
    public function buildOptionsForm(&$form, FormStateInterface $form_state) {
        $form['counter_start'] = [
            '#type' => 'textfield',
            '#title' => $this->t('Starting value'),
            '#default_value' => $this->options['counter_start'],
            '#description' => $this->t('Specify the number the counter should start at.'),
            '#size' => 2,
        ];
        parent::buildOptionsForm($form, $form_state);
    }
    
    /**
     * {@inheritdoc}
     */
    public function query() {
        // do nothing -- to override the parent query.
    }
    
    /**
     * {@inheritdoc}
     */
    public function getValue(ResultRow $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->usePager()) {
            $count += $pager->getItemsPerPage() * $pager->getCurrentPage() + $pager->getOffset();
        }
        // Add the counter for the current site.
        $count += $this->view->row_index + 1;
        return $count;
    }

}