private function getTimeConstraint(TestSession $session, QtiComponent $source, $navigationMode)
    {
        $constraint = new QtiTimeConstraint(
            $source,
            $session->getTimerDuration($source->getIdentifier()),
            $navigationMode,
            true,
            true,
            $session->getTimerTarget()
        );
        $constraint->setTimer($session->getTimer());
        return $constraint;
    }