SwiftUI Lifecycle 🚀

SwiftUI introduces a new app lifecycle that replaces AppDelegate and SceneDelegate. Instead of relying on UIKit’s UIApplication, SwiftUI apps use the App protocol to manage their lifecycle.

1️⃣ SwiftUI App Lifecycle (@main)

🔹 The entry point of a SwiftUI app is a struct conforming to the App protocol.
🔹 It uses @main to indicate the starting point of the app.

✅ Example

import SwiftUI

@main
struct MyApp: App {
    var body: some Scene {
        WindowGroup {
            ContentView()
        }
    }
}

📌 How It Works

• @main marks this struct as the app’s entry point.
• WindowGroup creates a new window for the app.
• ContentView() is the initial screen.

🔥 No AppDelegate or SceneDelegate needed! 🎉

2️⃣ App Lifecycle Methods

SwiftUI doesn’t have AppDelegate methods like applicationDidEnterBackground(_:), but it provides an alternative via @Environment(\.scenePhase).

✅ Detect App State Changes

import SwiftUI

@main
struct MyApp: App {
    @Environment(\.scenePhase) private var scenePhase
    var body: some Scene {
        WindowGroup {
            ContentView()
        }
        .onChange(of: scenePhase) { newPhase in
            switch newPhase {
            case .active:
                print("App is active 🟢")
            case .inactive:
                print("App is inactive 🟡")
            case .background:
                print("App is in background 🔴")
            @unknown default:
                print("Unknown state 🤔")
            }
        }
    }
}

📌 App Phases

• .active → App is in the foreground
• .inactive → App is about to go inactive (e.g., phone call)
• .background → App moves to the background

3️⃣ AppDelegate in SwiftUI (If Needed)

You can still use UIApplicationDelegate inside SwiftUI if required.

✅ Example: Using AppDelegate

import SwiftUI

class AppDelegate: NSObject, UIApplicationDelegate {
    func application(
        _ application: UIApplication,
        didFinishLaunchingWithOptions launchOptions: [UIApplication.LaunchOptionsKey: Any]?
    ) -> Bool {
        print("App launched 🚀")
        return true
    }
}
@main
struct MyApp: App {
    @UIApplicationDelegateAdaptor(AppDelegate.self) var appDelegate
    var body: some Scene {
        WindowGroup {
            ContentView()
        }
    }
}

📌 When to Use AppDelegate?

✅ Firebase, Push Notifications, Background Tasks
✅ Handling deep links
✅ Custom app-wide configurations

4️⃣ Lifecycle Comparison: UIKit vs SwiftUI

Feature UIKit (AppDelegate)SwiftUI (@main)Entry Point@UIApplicationMain@main in AppWindow CreationUIWindow + SceneDelegateWindowGroupApp State HandlingAppDelegate methods@Environment(\.scenePhase)Background TasksapplicationDidEnterBackgroundscenePhase == .backgroundMultiple WindowsUISceneWindowGroup

🛠️ Key Takeaways

✅ SwiftUI eliminates the need for AppDelegate & SceneDelegate
✅ Uses @main for declarative app structure
✅ @Environment(\.scenePhase) replaces AppDelegate lifecycle methods
✅ UIApplicationDelegateAdaptor allows legacy UIKit integration