App structure: @main, App, Scene, WindowGroup

App structure: @main, App, Scene, WindowGroup

@main marks the one type that is the program's entry point. It is a Swift feature, not a SwiftUI one, and a target can have only one. In a SwiftUI app that type conforms to App, and its body returns one or more scenes. A Scene is SwiftUI's version of UIScene: a unit of user interface whose lifecycle the system manages. WindowGroup is the scene you will use most. On iPhone it is the app's single window. On iPad and Mac the user can open several windows from it, and each window gets its own copy of the root view and that view's state.

Lifecycle events arrive as data rather than as delegate callbacks: read @Environment(\.scenePhase) instead of implementing sceneDidEnterBackground. If a library still needs an app delegate, you attach one with @UIApplicationDelegateAdaptor. Document-based apps use DocumentGroup, and iOS 27 adds new document protocols (ReadableDocument, WritableDocument).

@main
struct MyApp: App {
    @Environment(\.scenePhase) private var scenePhase

    var body: some Scene {
        WindowGroup {                  // one window on iPhone; many on iPad and Mac
            ContentView()
        }
        .onChange(of: scenePhase) { _, phase in
            print("scene is now \(phase)")      // active, inactive, or background
        }
    }
}

In a new project, follow the path from @main to the first Text on screen. Add the onChange(of: scenePhase) shown above, then send the app to the background and bring it back, and watch what is printed.

1. What does @main do, and how many can one target have?
2. What is a Scene, and which UIKit concept does it correspond to?
3. On iPad, what happens to the root view's @State when the user opens a second window?

• App organization
• App
• Scene
• WWDC26: What’s new in SwiftUI