itemsPerPageExposed example

    if (!$form_state->isValueEmpty(['pager_options', 'expose', 'items_per_page']) && !$form_state->isValueEmpty(['pager_options', 'items_per_page'])) {
      $items_per_page = $form_state->getValue(['pager_options', 'items_per_page']);
      if (array_search($items_per_page, $options) === FALSE) {
        $form_state->setErrorByName('pager_options][expose][items_per_page_options', $this->t("The <em>Exposed items per page</em> field's options must include the value from the <em>Items per page</em> field (@items_per_page).",
          ['@items_per_page' => $items_per_page])
        );
      }
    }
  }

  public function query() {
    if ($this->itemsPerPageExposed()) {
      $query = $this->view->getRequest()->query;
      $items_per_page = $query->get('items_per_page');
      if ((int) $items_per_page > 0) {
        $this->options['items_per_page'] = $items_per_page;
      }
      elseif ($items_per_page == 'All' && $this->options['expose']['items_per_page_options_all']) {
        $this->options['items_per_page'] = 0;
      }
    }
    if ($this->isOffsetExposed()) {
      $query = $this->view->getRequest()->query;