silverstripe-subsites/code/SubsiteXHRController.php

<?php

/**
 * Section-agnostic PJAX controller.
 */
class SubsiteXHRController extends LeftAndMain
{
    /**
     * Relax the access permissions, so anyone who has access to any CMS subsite can access this controller.
     */
    public function canView($member = null)
    {
        if (parent::canView()) {
            return true;
        }

        if (Subsite::all_accessible_sites()->count()>0) {
            return true;
        }

        return false;
    }

    /**
     * Similar as above, but for the LeftAndMainSubsites - allow access if user allowed into the CMS at all.
     */
    public function canAccess()
    {
        if (Subsite::all_accessible_sites()->count()>0) {
            return true;
        }
    }

    public function getResponseNegotiator()
    {
        $negotiator = parent::getResponseNegotiator();
        $self = $this;

        // Register a new callback
        $negotiator->setCallback('SubsiteList', function () use (&$self) {
            return $self->SubsiteList();
        });

        return $negotiator;
    }

    /**
     * Provide the list of available subsites as a cms-section-agnostic PJAX handler.
     */
    public function SubsiteList()
    {
        return $this->renderWith('SubsiteList');
    }
}