LoadBlock.php

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

use Magento\Framework\App\Action\HttpGetActionInterface;
use Magento\Framework\App\Action\HttpPostActionInterface as HttpPostActionInterface;
use Magento\Backend\App\Action;
use Magento\Backend\Model\View\Result\ForwardFactory;
use Magento\Framework\View\Result\PageFactory;
use Magento\Framework\Controller\Result\RawFactory;
use Magento\Sales\Controller\Adminhtml\Order\Create as CreateAction;

class LoadBlock extends CreateAction implements HttpPostActionInterface, HttpGetActionInterface
{
    /**
     * @var RawFactory
     */
    protected $resultRawFactory;

    /**
     * @param Action\Context $context
     * @param \Magento\Catalog\Helper\Product $productHelper
     * @param \Magento\Framework\Escaper $escaper
     * @param PageFactory $resultPageFactory
     * @param ForwardFactory $resultForwardFactory
     * @param RawFactory $resultRawFactory
     */
    public function __construct(
        Action\Context $context,
        \Magento\Catalog\Helper\Product $productHelper,
        \Magento\Framework\Escaper $escaper,
        PageFactory $resultPageFactory,
        ForwardFactory $resultForwardFactory,
        RawFactory $resultRawFactory
    ) {
        $this->resultRawFactory = $resultRawFactory;
        parent::__construct(
            $context,
            $productHelper,
            $escaper,
            $resultPageFactory,
            $resultForwardFactory
        );
    }

    /**
     * Loading page block
     *
     * @return \Magento\Backend\Model\View\Result\Redirect|\Magento\Framework\Controller\Result\Raw
     */
    public function execute()
    {
        $request = $this->getRequest();
        try {
            $this->_initSession()->_processData();
        } catch (\Magento\Framework\Exception\LocalizedException $e) {
            $this->_reloadQuote();
            $this->messageManager->addErrorMessage($e->getMessage());
        } catch (\Exception $e) {
            $this->_reloadQuote();
            $this->messageManager->addExceptionMessage($e, $e->getMessage());
        }

        $asJson = $request->getParam('json');
        $block = $request->getParam('block');

        /** @var \Magento\Framework\View\Result\Page $resultPage */
        $resultPage = $this->resultPageFactory->create();
        if ($asJson) {
            $resultPage->addHandle('sales_order_create_load_block_json');
        } else {
            $resultPage->addHandle('sales_order_create_load_block_plain');
        }

        if ($block) {
            $blocks = explode(',', $block);
            if ($asJson && !in_array('message', $blocks)) {
                $blocks[] = 'message';
            }

            foreach ($blocks as $block) {
                $resultPage->addHandle('sales_order_create_load_block_' . $block);
            }
        }

        $result = $resultPage->getLayout()->renderElement('content');
        if ($request->getParam('as_js_varname')) {
            $this->_objectManager->get(\Magento\Backend\Model\Session::class)->setUpdateResult($result);
            return $this->resultRedirectFactory->create()->setPath('sales/*/showUpdateResult');
        }
        return $this->resultRawFactory->create()->setContents($result);
    }
}