I'm following the example code that comes with SKDemo to understand where in the lifecycle of my app to detect the subscription has expired to block users from having access to functionality they shouldn't have access to anymore. This is pretty straight forward to implement and check on app launch. However, my app has a pretty low memory footprint so it's often not getting terminated over the course of several days. Still, I would like to check for a valid subscription whenever the app is foregrounded. Otherwise users might have access to subscriber-only features for numerous days before the app finally has to cold start again. Is there a way to listen to these kind of 'subscription-expired' updates or do I need to figure out myself to verify the subscription, for instance, by listening to UIApplication.willEnterForegroundNotification and then run some business logic on my end to confirm the user is still within the subscription period?