How to handle a message when user opens the app by tapping on a notification alert?

In order to handle user tap on notification alert:

1. Implement MMMessageHandlingDelegate protocol and it's method didPerform(action:forMessage:completion:), i.e.:

   class MyMessageHandlingDelegate : MMMessageHandlingDelegate {
       func didPerform(action: MMNotificationAction, forMessage message: MM_MTMessage?, notificationUserInfo: [String: Any]?, completion: @escaping () -> Void) {
           if action.isTapOnNotificationAlert { // here you check if the action is indeed a tap on alert
               print("Message with text: \(message.text) was tapped")       
           }
           // don't forget to call `completion`, it's very important to tell the system that action handling is finished
           completion()
       }
   }

2. Pass the delegate object to MobileMessaging SDK:

   MobileMessaging.messageHandlindDelegate = MyMessageHandlingDelegate()