UIApplicationStateDidChangeNotification

UIApplicationS refers to the core application object in Apple's Cocoa and Cocoa Touch frameworks, responsible for managing the application's run loop, event handling, and overall lifecycle. When an application launches, an instance of UIApplication (or a subclass thereof) is created and becomes the central coordinator. It receives events from the operating system, such as touch gestures or system notifications, and dispatches them to the appropriate parts of the application.

The UIApplication object also manages the application's state, including transitions between active, inactive, background, and suspended states. It provides methods for interacting with the system, such as requesting location services, handling background tasks, and managing application-wide settings. Developers can access the shared UIApplication instance using the `shared` class property. This object is fundamental to how iOS and macOS applications operate, serving as the bridge between the application code and the underlying operating system services. It ensures that events are processed efficiently and that the application behaves correctly throughout its execution.