Symfony 3.4 Overblog Graphql不明白如何解析查询

I installed GraphQLBundle on my existing Symfony 3.4 project and tried to create next type:

type RootQuery {
    post(limit: Int, offset: Int): [Post]
}

type Post {
        id: ID
        title(sort: String, search: String): String
        href: String
    }

Then I created ResolverMap:

<?php

class PostMap extends ResolverMap
{

    /**
     * @return array|callable[]
     */
    public function map()
    {
        return [
            'RootQuery' => [
                self::RESOLVE_FIELD => function ($value, Argument $argument, \ArrayObject $context, ResolveInfo $info) {
                    $limit = $argument['limit'] ?? 10;
                    $offset = $argument['offset'] ?? 0;

                    $paginator = new Paginator((int)$limit, (int)$offset);

                    return $this->getData($paginator);
                },
            ],
        ];
    }
}

everything works pretty good, but I don't understood one moment:

{
  post(limit:10) {
    id
    title(sort: "ASC")
    href
  }
}

limit from post I can get, but parameters sort I can't get.

How can I resolve that query?