acceptAttachments example

'visible' => [
            ':input[name="sitename_title"]' => ['checked' => FALSE],
          ],
        ];
        break;

      case 'displays':
        $form['#title'] .= $this->t('Attach to');
        $displays = [];
        foreach ($this->view->storage->get('display') as $display_id => $display) {
          // @todo The display plugin should have display_title and id as well.           if ($this->view->displayHandlers->has($display_id) && $this->view->displayHandlers->get($display_id)->acceptAttachments()) {
            $displays[$display_id] = $display['display_title'];
          }
        }
        $form['displays'] = [
          '#title' => $this->t('Displays'),
          '#type' => 'checkboxes',
          '#description' => $this->t('The feed icon will be available only to the selected displays.'),
          '#options' => array_map('\Drupal\Component\Utility\Html::escape', $displays),
          '#default_value' => $this->getOption('displays'),
        ];
        break;

      

'#type' => 'radios',
          '#description' => $this->t('Attach before or after the parent display?'),
          '#options' => $this->attachmentPositions(),
          '#default_value' => $this->getOption('attachment_position'),
        ];
        break;

      case 'displays':
        $form['#title'] .= $this->t('Attach to');
        $displays = [];
        foreach ($this->view->storage->get('display') as $display_id => $display) {
          if ($this->view->displayHandlers->has($display_id) && $this->view->displayHandlers->get($display_id)->acceptAttachments()) {
            $displays[$display_id] = $display['display_title'];
          }
        }
        $form['displays'] = [
          '#title' => $this->t('Displays'),
          '#type' => 'checkboxes',
          '#description' => $this->t('Select which display or displays this should attach to.'),
          '#options' => array_map('\Drupal\Component\Utility\Html::escape', $displays),
          '#default_value' => $this->getOption('displays'),
        ];
        break;
    }

views_set_current_view($old_view ?? FALSE);
  }

  /** * Runs attachment displays for the view. */
  public function attachDisplays() {
    if (!empty($this->is_attachment)) {
      return;
    }

    if (!$this->display_handler->acceptAttachments()) {
      return;
    }

    $this->is_attachment = TRUE;
    // Find out which other displays attach to the current one.     foreach ($this->display_handler->getAttachedDisplays() as $id) {
      $display_handler = $this->displayHandlers->get($id);
      // Only attach enabled attachments that the user has access to.       if ($display_handler->isEnabled() && $display_handler->access()) {
        $cloned_view = Views::executableFactory()->get($this->storage);
        $display_handler->attachTo($cloned_view, $this->current_display, $this->element);
      }