Preview Mode

The Contao Managed Edition comes with an additional preview.php entry point. This entry point is either accessible via a back end login, or a valid preview link.

When this entry point is accessed with a back end loingn it offers features like logging in as a front end member or showing hidden elements.

To check whether your own code (content element, front end module, etc.) is currently executed within the preview entry point you can check for the _preview request attribute.

class FoobarController extends AbstractContentElementController
{
    protected function getResponse(Template $template, ContentModel $model, Request $request): Response
    {
        if ($request->attributes->get('_preview')) {
            // Execute some code within the preview entry point
        }

        return $template->getResponse();
    }
}

{% if app.request.attributes._preview|default %}
    {# Show some content within the preview entry point #}
{% endif %}

This however, does not check for the actual preview mode, which is activated by selecting Unpublished: show in the preview toolbar - or by accessing the preview entry with a valid preview link that has this feature enabled. To check for this you can use the TokenChecker service.

use Contao\CoreBundle\Security\Authentication\Token\TokenChecker;

class FoobarController extends AbstractContentElementController
{
    public function __construct(private readonly TokenChecker $tokenChecker)
    {
    }

    protected function getResponse(Template $template, ContentModel $model, Request $request): Response
    {
        if ($this->tokenChecker->isPreviewmode()) {
            // Execute some code when the preview mode is active
        }

        return $template->getResponse();
    }
}

since 5.3 This is also available via the contao global Twig variable:

{% if contao.is_preview_mode %}
    {# Show some content only available in the preview mode #}
{% endif %}

In previous Contao versions the BE_USER_LOGGED_IN constant was used for this. This constant was true whenever a valid back end login session is present and the unpublished elements should be shown. However, with the introduction of the preview link feature of Contao 4.13 this is not necessarily the case anymore, i.e. this constant can be true even without a back end login. The constant is deprecated in any case and the aforementioned contao.security.token_checker should be used instead.