Tells the delegate that the app has transitioned from the foreground to the background.
- watchOS 3.0+
Override this method to release shared resources, invalidate timers, and store enough app state information to restore your app to its current state if it is purged from memory. You have only a few seconds to complete these actions and return.
The system typically suspends your app shortly after this method returns; therefore, you should not call any asynchronous methods from your
application implementation. Asynchronous methods may not be able to complete before the app is suspended.
Additionally, the system may purge suspended apps at any time to make room for other apps. You are not notified when an app is purged from memory. The
application method is your last chance to perform any cleanup before the app is terminated.