During startup, the [UIApplicationMain](https://developer.apple.com/documentation/uikit/1622933-uiapplicationmain) function sets up several key objects and starts the app running. At the heart of every iOS app is the [UIApplication](https://developer.apple.com/documentation/uikit/uiapplication) object, whose job is to facilitate the interactions between the system and other objects in the app. Figure 2-1 shows the objects commonly found in most apps, while Table 2-1 lists the roles each of those objects plays. The first thing to notice is that iOS apps use a model-view-controller architecture. This pattern separates the app’s data and business logic from the visual presentation of that data. This architecture is crucial to creating apps that can run on different devices with different screen sizes.

Figure 2-1  Key objects in an iOS app

https://developer.apple.com/library/content/documentation/iPhone/Conceptual/iPhoneOSProgrammingGuide/TheAppLifeCycle/TheAppLifeCycle.html#//apple_ref/doc/uid/TP40007072-CH2-SW2