How to migrate from AppDelegate to SceneDelegate? And whats are the difference between them?

To migrate from AppDelegate to SceneDelegate, you need to adopt the UI Scene lifecycle, which is becoming mandatory in the latest iOS SDKs. The SceneDelegate is responsible for managing the lifecycle of individual scenes, whereas the AppDelegate was previously responsible for the entire app lifecycle.

Differences between AppDelegate and SceneDelegate:

AppDelegate:
- Manages the app's lifecycle as a whole.
- Handles app-wide events like app launch, termination, and backgrounding.
- Typically used for setting up the app's initial state.
SceneDelegate:
- Manages the lifecycle of individual scenes (windows) within the app.
- Handles scene-specific events like scene connection, disconnection, and state restoration.
- Allows for multiple scenes to be active simultaneously, supporting multi-window capabilities.

Migration Steps:

Update Info.plist:
- Define scene configurations in your app's Info.plist file. This includes specifying the storyboard or SwiftUI view for each scene.
Implement SceneDelegate:
- Create a SceneDelegate class to manage each scene's lifecycle. Implement methods like scene(_:willConnectTo:options:) to set up the scene's initial UI.
Modify AppDelegate:
- Update your AppDelegate to handle app-wide events and delegate scene-specific tasks to the SceneDelegate.
Adopt Scene Lifecycle:
- Ensure your app supports the UI Scene lifecycle by adopting the necessary protocols and methods.

For more detailed guidance, you can refer to the session Make your UIKit app more flexible (00:58) from WWDC 2025, which covers the fundamentals of scenes and their importance in building flexible applications.