Index.php

<?php
/***
 * Copyright © Magento, Inc. All rights reserved.
 * See COPYING.txt for license details.
 */
namespace Magento\Swagger\Controller\Index;

/**
 * Class Index
 *
 * @package Magento\Swagger\Controller\Index
 */
class Index extends \Magento\Framework\App\Action\Action
{
    /**
     * @var \Magento\Framework\View\Page\Config
     */
    private $pageConfig;

    /**
     * @var \Magento\Framework\View\Result\PageFactory
     */
    private $pageFactory;

    /**
     * @param \Magento\Framework\App\Action\Context $context
     * @param \Magento\Framework\View\Page\Config $pageConfig
     * @param \Magento\Framework\View\Result\PageFactory $pageFactory
     */
    public function __construct(
        \Magento\Framework\App\Action\Context $context,
        \Magento\Framework\View\Page\Config $pageConfig,
        \Magento\Framework\View\Result\PageFactory $pageFactory
    ) {
        parent::__construct($context);
        $this->pageConfig = $pageConfig;
        $this->pageFactory = $pageFactory;
    }

    /**
     * @return \Magento\Framework\View\Result\Page
     */
    public function execute()
    {
        $this->pageConfig->addBodyClass('swagger-section');
        return $this->pageFactory->create();
    }
}