buildContentPreviewToggle example

// Discard is not dependent on form input.       '#limit_validation_errors' => [],
    ];
    // @todo This button should be conditionally displayed, see     // https://www.drupal.org/node/2917777.     $actions['revert'] = [
      '#type' => 'submit',
      '#value' => $this->t('Revert to defaults'),
      '#submit' => ['::redirectOnSubmit'],
      '#redirect' => 'revert',
    ];
    $actions['preview_toggle'] = $this->buildContentPreviewToggle();
    return $actions;
  }

  /** * Form submission handler. */
  public function redirectOnSubmit(array $form, FormStateInterface $form_state) {
    $form_state->setRedirectUrl($this->sectionStorage->getLayoutBuilderUrl($form_state->getTriggeringElement()['#redirect']));
  }

  /** * Retrieves the section storage object. * * @return \Drupal\layout_builder\SectionStorageInterface * The section storage for the current form. */

$actions['#attributes']['role'] = 'region';
    $actions['#attributes']['aria-label'] = $this->t('Layout Builder tools');
    $actions['submit']['#value'] = $this->t('Save layout');
    $actions['#weight'] = -1000;

    $actions['discard_changes'] = [
      '#type' => 'submit',
      '#value' => $this->t('Discard changes'),
      '#submit' => ['::redirectOnSubmit'],
      '#redirect' => 'discard_changes',
    ];
    $actions['preview_toggle'] = $this->buildContentPreviewToggle();
    return $actions;
  }

  /** * Form submission handler. */
  public function redirectOnSubmit(array $form, FormStateInterface $form_state) {
    $form_state->setRedirectUrl($this->sectionStorage->getLayoutBuilderUrl($form_state->getTriggeringElement()['#redirect']));
  }

  /** * {@inheritdoc} */