AddComment.php

<?php
/**
 * Copyright © Magento, Inc. All rights reserved.
 * See COPYING.txt for license details.
 */

namespace Magento\Sales\Controller\Adminhtml\Order\Creditmemo;

use Magento\Backend\App\Action;
use Magento\Sales\Model\Order\Email\Sender\CreditmemoCommentSender;

class AddComment extends \Magento\Backend\App\Action
{
    /**
     * Authorization level of a basic admin session
     *
     * @see _isAllowed()
     */
    const ADMIN_RESOURCE = 'Magento_Sales::sales_creditmemo';

    /**
     * @var \Magento\Sales\Controller\Adminhtml\Order\CreditmemoLoader
     */
    protected $creditmemoLoader;

    /**
     * @var CreditmemoCommentSender
     */
    protected $creditmemoCommentSender;

    /**
     * @var \Magento\Framework\View\Result\PageFactory
     */
    protected $pagePageFactory;

    /**
     * @var \Magento\Framework\Controller\Result\JsonFactory
     */
    protected $resultJsonFactory;

    /**
     * @var \Magento\Framework\Controller\Result\RawFactory
     */
    protected $resultRawFactory;

    /**
     * @param Action\Context $context
     * @param \Magento\Sales\Controller\Adminhtml\Order\CreditmemoLoader $creditmemoLoader
     * @param CreditmemoCommentSender $creditmemoCommentSender
     * @param \Magento\Framework\View\Result\PageFactory $resultPageFactory
     * @param \Magento\Framework\Controller\Result\JsonFactory $resultJsonFactory
     * @param \Magento\Framework\Controller\Result\RawFactory $resultRawFactory
     */
    public function __construct(
        Action\Context $context,
        \Magento\Sales\Controller\Adminhtml\Order\CreditmemoLoader $creditmemoLoader,
        CreditmemoCommentSender $creditmemoCommentSender,
        \Magento\Framework\View\Result\PageFactory $resultPageFactory,
        \Magento\Framework\Controller\Result\JsonFactory $resultJsonFactory,
        \Magento\Framework\Controller\Result\RawFactory $resultRawFactory
    ) {
        $this->creditmemoLoader = $creditmemoLoader;
        $this->creditmemoCommentSender = $creditmemoCommentSender;
        $this->resultPageFactory = $resultPageFactory;
        $this->resultJsonFactory = $resultJsonFactory;
        $this->resultRawFactory = $resultRawFactory;
        parent::__construct($context);
    }

    /**
     * Add comment to creditmemo history
     *
     * @return \Magento\Framework\Controller\Result\Raw|\Magento\Framework\Controller\Result\Json
     */
    public function execute()
    {
        try {
            $this->getRequest()->setParam('creditmemo_id', $this->getRequest()->getParam('id'));
            $data = $this->getRequest()->getPost('comment');
            if (empty($data['comment'])) {
                throw new \Magento\Framework\Exception\LocalizedException(
                    __('The comment is missing. Enter and try again.')
                );
            }
            $this->creditmemoLoader->setOrderId($this->getRequest()->getParam('order_id'));
            $this->creditmemoLoader->setCreditmemoId($this->getRequest()->getParam('creditmemo_id'));
            $this->creditmemoLoader->setCreditmemo($this->getRequest()->getParam('creditmemo'));
            $this->creditmemoLoader->setInvoiceId($this->getRequest()->getParam('invoice_id'));
            $creditmemo = $this->creditmemoLoader->load();
            $comment = $creditmemo->addComment(
                $data['comment'],
                isset($data['is_customer_notified']),
                isset($data['is_visible_on_front'])
            );
            $comment->save();

            $this->creditmemoCommentSender->send($creditmemo, !empty($data['is_customer_notified']), $data['comment']);
            $resultPage = $this->resultPageFactory->create();
            $response = $resultPage->getLayout()->getBlock('creditmemo_comments')->toHtml();
        } catch (\Magento\Framework\Exception\LocalizedException $e) {
            $response = ['error' => true, 'message' => $e->getMessage()];
        } catch (\Exception $e) {
            $response = ['error' => true, 'message' => __('Cannot add new comment.')];
        }
        if (is_array($response)) {
            $resultJson = $this->resultJsonFactory->create();
            $resultJson->setData($response);
            return $resultJson;
        } else {
            $resultRaw = $this->resultRawFactory->create();
            $resultRaw->setContents($response);
            return $resultRaw;
        }
    }
}