I have seen many implementations of notification in OffsitePaymentGatewayBase Like WorldPay, QuickPay
Can I Extend the OnsitePaymentGatewayBase class and use onNotify() Method for receiving Payment notification?
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I have raised this question to slack for drupal commerce and they responded. the Response is below
Slack Answer: your payment gateway must implement https://git.drupalcode.org/project/commerce/-/blob/8.x-2.x/modules/payment/src/Plugin/Commerce/PaymentGateway/SupportsNotificationsInterface.php the interface for offsite payment gateway is already extending that. that's why it's available for every offsite gateway, but needs to be added explicitly for onsites