In iOS development, which core object is created by the UIApplicationMain function when an app launches?

Introduction / Context:
In iOS development, UIApplicationMain is the entry point for most Objective C and Swift applications. Rather than writing a custom main loop, developers rely on this function to bootstrap the application, create key objects and start processing events. A common interview question checks whether candidates know which object UIApplicationMain creates and manages during app launch.

Given Data / Assumptions:

• The application is an iOS app that uses UIKit and follows the standard UIApplication model.
• The code includes a main function that calls UIApplicationMain, often generated by Xcode templates.
• There is an application delegate class specified either in code or in the Info.plist file.
• iOS uses an event driven architecture with a central application object and a main event loop.

Concept / Approach:
UIApplicationMain performs several tasks: it creates the singleton UIApplication object, sets up the main event loop, loads the application interface from storyboards or nib files and instantiates the application delegate. The central point is that UIApplicationMain is responsible for creating the UIApplication instance that represents the running app. The UIApplication object coordinates status bar appearance, handles app level events and dispatches incoming user actions. Other objects such as windows and view controllers are created as part of the launch process, but the core object directly created and managed by UIApplicationMain is the UIApplication instance.

Step-by-Step Solution:
Step 1: When the app starts, the system calls the main function generated by Xcode. Step 2: This main function calls UIApplicationMain, passing arguments such as argc, argv, the application class name and the delegate class name. Step 3: UIApplicationMain creates the UIApplication singleton object (or a subclass if specified) to represent the app at the system level. Step 4: It then creates the application delegate instance and associates it with the UIApplication object. Step 5: Finally, it starts the main event loop, receiving events from the system and delivering them to the UIApplication and the rest of the UIKit hierarchy.

Verification / Alternative check:
Official Apple documentation describes UIApplicationMain as creating the application object and the application delegate and then starting the event processing loop. In typical projects, developers do not manually instantiate UIApplication. Instead, they may subclass UIApplication if they need custom behaviour, and then pass the subclass name to UIApplicationMain. This confirms that the primary object created by UIApplicationMain is the UIApplication instance, while the delegate and UI elements are created through configuration or subsequent code paths.

Why Other Options Are Wrong:
Option B suggests that UIApplicationMain creates the root UIViewController, which is usually created by storyboards, nibs or application code, not directly by UIApplicationMain itself. Option C focuses on UIWindow, which is created by the application delegate or scene delegate, not by UIApplicationMain directly. Option D claims only the delegate is created, but documentation clearly states that the application object is also created. Option E incorrectly states that only a background thread is created and no user interface objects, which does not reflect UIKit behaviour. Only option A correctly identifies that UIApplicationMain creates the UIApplication object and starts the main event loop.

Common Pitfalls:
Beginner developers sometimes assume that view controllers or windows are the first objects created when an app launches. In reality, the application object is central and is created before most of the interface. Another pitfall is forgetting that UIApplication is a singleton and should not be instantiated directly in code. In interviews, clearly describing that UIApplicationMain creates the UIApplication object and the delegate, then starts the event loop, shows a solid understanding of the iOS app lifecycle.

Final Answer:
At app launch time, UIApplicationMain creates the UIApplication object that represents the running iOS application and sets up the main event loop.