public function attach($className, callable $callback = null)
    {
        // If the user has passed through a callable item then we want
        // to attach the raw result of that call.
        if (is_callable($className)) {
            return ['raw' => $className];
        }

        if ( ! class_exists($className)) {
            throw new AttachmentClassNotFound("No class found with the name '{$className}'.");
        }

        return ['class' => new $className, 'callback' => $callback];
    }