public function listen(callable $finalHandler = null)
{
$callback = $this->callback;
ob_start();
$bufferLevel = ob_get_level();
$response = $callback($this->request, $this->response, $finalHandler);
if (!$response instanceof ResponseInterface) {
$response = $this->response;
}
$this->getEmitter()->emit($response, $bufferLevel);
}