Drupal 8: DefaultSelection Plugin subclass for filtering selectable Views by tag

ViewSelection.php

<?php

/**
 * @file
 * Contains \Drupal\example\Plugin\EntityReferenceSelection\ViewSelection.
 */

namespace Drupal\example\Plugin\EntityReferenceSelection;

use Drupal\Core\Entity\Plugin\EntityReferenceSelection\DefaultSelection;
use Drupal\Core\Form\FormStateInterface;
use Drupal\views\Views;

/**
 * Provides specific access control for the View entity type.
 *
 * @EntityReferenceSelection(
 *   id = "default:view",
 *   label = @Translation("View selection"),
 *   entity_types = {"view"},
 *   group = "default",
 *   weight = 1
 * )
 */
class ViewSelection extends DefaultSelection {

  /**
   * {@inheritdoc}
   */
  public function buildConfigurationForm(array $form, FormStateInterface $form_state) {
    $form = parent::buildConfigurationForm($form, $form_state);
    $selection_handler_settings = $this->configuration['handler_settings'];

    // Get all view tags
    $views = Views::getAllViews();
    $view_options = [];
    foreach ($views as $view) {
      $tag = $view->get('tag');
      if ($tag) {
        $view_options[$tag] = $tag;
      }
    }

    // Add tag selection to form
    $form['target_views'] = array(
      '#type' => 'checkboxes',
      '#title' => $this->t('View tags'),
      '#description' => $this->t('Filter which views should be selectable by View tags.'),
      '#options' => $view_options,
      '#default_value' => (array) $selection_handler_settings['target_views'],
      '#required' => FALSE,
      '#size' => 6,
      '#multiple' => TRUE,
      '#element_validate' => [[get_class($this), 'elementValidateFilter']],
    );

    return $form;
  }

  /**
   * {@inheritdoc}
   */
  public function validateConfigurationForm(array &$form, FormStateInterface $form_state) {
    parent::validateConfigurationForm($form, $form_state);

    // Allow all views to be selected if no tag was chosen
    if ($form_state->getValue(['settings', 'handler_settings', 'target_views']) === []) {
      $form_state->setValue(['settings', 'handler_settings', 'target_views'], NULL);
    }
  }

  /**
   * {@inheritdoc}
   */
  protected function buildEntityQuery($match = NULL, $match_operator = 'CONTAINS') {
    $query = parent::buildEntityQuery($match, $match_operator);
    $handler_settings = $this->configuration['handler_settings'];

    // Filter selectable views by tag
    if (isset($handler_settings['target_views']) && is_array($handler_settings['target_views'])) {
      if ($handler_settings['target_views'] === []) {
        $query->condition('id', NULL, '=');
        return $query;
      }
      else {
        $query->condition('tag', $handler_settings['target_views'], 'IN');
      }
    }

    return $query;
  }
}