What notification or signaling mechanisms are used to wake up or alert about events in an application?

In iOS, the following mechanisms are used to notify and wake up the application about events:

• NotificationCenter — local notifications within the app for message passing between objects.
• Delegate and Closure — callback patterns for event notification.
• Push Notifications (APNs) — remote notifications from the server to wake up the app.
• Local Notifications — notifications scheduled by the app itself.
• KVO (Key-Value Observing) — observing changes to object properties.

These mechanisms allow for effective response to events and updating the interface or app logic.