class QueryParameter

class QueryParameter extends ArgumentDefaultPluginBase implements CacheableDependencyInterface {
  
  /**
   * {@inheritdoc}
   */
  protected function defineOptions() {
    $options = parent::defineOptions();
    $options['query_param'] = [
      'default' => '',
    ];
    $options['fallback'] = [
      'default' => '',
    ];
    $options['multiple'] = [
      'default' => 'and',
    ];
    return $options;
  }
  
  /**
   * {@inheritdoc}
   */
  public function buildOptionsForm(&$form, FormStateInterface $form_state) {
    parent::buildOptionsForm($form, $form_state);
    $form['query_param'] = [
      '#type' => 'textfield',
      '#title' => $this->t('Query parameter'),
      '#description' => $this->t('The query parameter to use.'),
      '#default_value' => $this->options['query_param'],
    ];
    $form['fallback'] = [
      '#type' => 'textfield',
      '#title' => $this->t('Fallback value'),
      '#description' => $this->t('The fallback value to use when the above query parameter is not present.'),
      '#default_value' => $this->options['fallback'],
    ];
    $form['multiple'] = [
      '#type' => 'radios',
      '#title' => $this->t('Multiple values'),
      '#description' => $this->t('Conjunction to use when handling multiple values. E.g. "?value[0]=a&value[1]=b".'),
      '#default_value' => $this->options['multiple'],
      '#options' => [
        'and' => $this->t('AND'),
        'or' => $this->t('OR'),
      ],
    ];
  }
  
  /**
   * {@inheritdoc}
   */
  public function getArgument() {
    $current_request = $this->view
      ->getRequest();
    // Convert a[b][c][d] into ['a', 'b', 'c', 'd'].
    $path = array_filter(preg_split('#(\\[|\\]\\[|\\])#', $this->options['query_param']));
    if ($current_request->query
      ->has($path[0])) {
      $query = $current_request->query
        ->all();
      $param = NestedArray::getValue($query, $path);
      if (is_array($param)) {
        $conjunction = $this->options['multiple'] == 'and' ? ',' : '+';
        $param = implode($conjunction, $param);
      }
      return $param;
    }
    else {
      // Otherwise, use the fixed fallback value.
      return $this->options['fallback'];
    }
  }
  
  /**
   * {@inheritdoc}
   */
  public function getCacheMaxAge() {
    return Cache::PERMANENT;
  }
  
  /**
   * {@inheritdoc}
   */
  public function getCacheContexts() {
    return [
      'url',
    ];
  }

}