public function getContent()
{
if (null == $this->content) {
$content = $this->output($this->data);
if (null !== $content && !is_string($content) && !is_numeric($content) && !is_callable([$content, '__toString'])) {
throw new \InvalidArgumentException(sprintf('variable type error: %s', gettype($content)));
}
$this->content = (string) $content;
}
return $this->content;
}