When required init method is mandatory? What is that?

Ans :

Write the required modifier before the definition of a class initializer to indicate that every subclass of the class must implement that initializer.

You must also write the required modifier before every subclass implementation of a required initializer, to indicate that the initializer requirement applies to further subclasses in the chain. You do not write the override modifier when overriding a required designated initializer.

“The use of the required modifier ensures that you provide an explicit or inherited implementation of the initializer requirement on all subclasses of the conforming class, such that they also conform to the protocol.”

How we stop notification to be sent to device who is uninstalled?

Ans :

The push message service has a feedback channel which reports error messages when the application has been removed or the device no longer accepts the push messages.

"The Apple Push Notification service includes a feedback service to give you information about failed remote notifications. When a remote notification cannot be delivered because the intended app does not exist on the device, the feedback service adds that device’s token to its list. Remote notifications that expire before being delivered are not considered a failed delivery and don’t impact the feedback service. By using this information to stop sending remote notifications that will fail to be delivered, you reduce unnecessary message overhead and improve overall system performance."