TransactionRefund.php

<?php
/**
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 *
 * Adyen Payment module (https://www.adyen.com/)
 *
 * Copyright (c) 2015 Adyen BV (https://www.adyen.com/)
 * See LICENSE.txt for license details.
 *
 * Author: Adyen <magento@adyen.com>
 */

namespace Adyen\Payment\Gateway\Http\Client;

use Magento\Payment\Gateway\Http\ClientInterface;

/**
 * Class TransactionSale
 */
class TransactionRefund implements ClientInterface
{
    /**
     * @var \Adyen\Payment\Helper\Data
     */
    private $adyenHelper;

    /**
     * PaymentRequest constructor.
     * @param \Adyen\Payment\Helper\Data $adyenHelper
     */
    public function __construct(
        \Adyen\Payment\Helper\Data $adyenHelper
    ) {
        $this->adyenHelper = $adyenHelper;
    }

    /**
     * @param \Magento\Payment\Gateway\Http\TransferInterface $transferObject
     * @return null
     */
    public function placeRequest(\Magento\Payment\Gateway\Http\TransferInterface $transferObject)
    {
        $requests = $transferObject->getBody();
        $responses = [];

        foreach ($requests as $request) {
            // call lib
            $service = new \Adyen\Service\Modification(
                $this->adyenHelper->initializeAdyenClient($transferObject->getClientConfig()['storeId'])
            );
            try {
                $responses[] = $service->refund($request);
            } catch (\Adyen\AdyenException $e) {
                $responses[] = ['error' => $e->getMessage()];
            }
        }
        return $responses;
    }
}