The interface an app extension uses to respond to a request from a host app.
- iOS 8.0+
- macOS 10.10+
- UIKit for Mac 13.0+Beta
- tvOS 9.0+
- watchOS 2.0+
NSExtension protocol provides a life cycle hook into an app extension. An extension’s principal object can implement this protocol and use
begin to keep track of the request from a host app.