public function init()
    {
        parent::init();

        if ($this->tile !== null) {
            $this->tile->init();
        }

        foreach ($this->groups as $group) {
            $group->init();
            // index the group by id if it is not already indexed
            if (!array_key_exists($group->id, $this->groups_by_id)) {
                $this->groups_by_id[$group->id] = $group;
            }
        }
    }