[Xcode/Swift] iOSアプリのライフサイクル完全理解：UIKit と SwiftUI を比較

// AppDelegate.swift
@main
class AppDelegate: UIResponder, UIApplicationDelegate {
    func application(_ application: UIApplication,
                     didFinishLaunchingWithOptions launchOptions: [UIApplication.LaunchOptionsKey: Any]?) -> Bool {
        // SDK 初期化・ログ設定など（開店準備）
        return true
    }

    // シーンを使う設定
    func application(_ application: UIApplication,
                     configurationForConnecting connectingSceneSession: UISceneSession,
                     options: UIScene.ConnectionOptions) -> UISceneConfiguration {
        UISceneConfiguration(name: "Default Configuration", sessionRole: connectingSceneSession.role)
    }
}

// SceneDelegate.swift
class SceneDelegate: UIResponder, UIWindowSceneDelegate {
    var window: UIWindow?

    func sceneWillEnterForeground(_ scene: UIScene) { /* シャッター開ける直前 */ }
    func sceneDidBecomeActive(_ scene: UIScene) { /* 接客開始 */ }
    func sceneWillResignActive(_ scene: UIScene) { /* 一時休止・電話着信など */ }
    func sceneDidEnterBackground(_ scene: UIScene) {
        // 事務作業：軽い保存やタスク終了
        UserDefaults.standard.synchronize()
    }
}

@main
struct CounterApp: App {
    @StateObject private var store = CounterStore()

    var body: some Scene {
        WindowGroup {
            ContentView()
                .environmentObject(store)
        }
    }
}

struct ContentView: View {
    @State private var items: [String] = []

    var body: some View {
        List(items, id: \.self, rowContent: Text.init)
            .task { items = await API.fetchItems() }  // 表示時に1回
            .onDisappear { /* タイマー停止・監視解除など */ }
    }
}

struct RootScene: View {
    @Environment(\.scenePhase) private var scenePhase
    @EnvironmentObject private var store: CounterStore

    var body: some View {
        ContentView()
            .onChange(of: scenePhase) { old, new in
                if new == .background { store.save() }    // 裏に回ったら保存
                if new == .active { store.refreshIfNeeded() }
            }
    }
}

struct AvatarView: View {
    @State private var userID = "me"
    @State private var image: Image?

    var body: some View {
        VStack {
            image ?? Image(systemName: "person.circle")
        }
        .task(id: userID) { image = await API.avatar(for: userID) } // userID 変更時だけ再実行
    }
}

let vc = UIHostingController(rootView: ContentView())
window.rootViewController = vc

struct MailVC: UIViewControllerRepresentable {
    func makeUIViewController(context: Context) -> MFMailComposeViewController { .init() }
    func updateUIViewController(_ uiViewController: MFMailComposeViewController, context: Context) {}
}

// モデル
final class CounterStore {
    private let key = "count"
    var count: Int = UserDefaults.standard.integer(forKey: "count")

    // 保存
    func save() {
        UserDefaults.standard.set(count, forKey: key)
    }
}

// SceneDelegate で保存フック
class SceneDelegate: UIResponder, UIWindowSceneDelegate {
    var window: UIWindow?
    let store = CounterStore()

    func scene(_ scene: UIScene, willConnectTo session: UISceneSession, options connectionOptions: UIScene.ConnectionOptions) {
        let vc = CounterViewController(store: store)
        window = UIWindow(windowScene: scene as! UIWindowScene)
        window?.rootViewController = vc
        window?.makeKeyAndVisible()
    }

    func sceneDidEnterBackground(_ scene: UIScene) {
        // アプリがバックグラウンドに入るときに保存
        store.save()
    }
}

// 画面
final class CounterViewController: UIViewController {
    private let store: CounterStore
    init(store: CounterStore) {
        self.store = store; super.init(nibName: nil, bundle: nil)
    }

    required init?(coder: NSCoder) {
        fatalError()
    }

    override func viewDidLoad() {
        super.viewDidLoad()
        view.backgroundColor = .systemBackground
        // + / - ボタンとラベルを配置（Auto Layout 省略）
    }

    @objc private func increment() {
        store.count += 1 /* ラベル更新 */
    }
    @objc private func decrement() {
        store.count -= 1 /* ラベル更新 */
    }
}

final class CounterStore: ObservableObject {
    @Published var count: Int = UserDefaults.standard.integer(forKey: "count")
    private let key = "count"

    func save() {
        UserDefaults.standard.set(count, forKey: key)
    }
    func refreshIfNeeded() {
        /* 必要ならフェッチ等 */
    }
}

@main
struct CounterApp: App {
    @StateObject private var store = CounterStore()
    @Environment(\.scenePhase) private var scenePhase

    var body: some Scene {
        WindowGroup {
            CounterView().environmentObject(store)
        }
        .onChange(of: scenePhase) { _, new in
            if new == .background { store.save() }
        }
    }
}

struct CounterView: View {
    @EnvironmentObject private var store: CounterStore

    var body: some View {
        VStack(spacing: 16) {
            Text("\(store.count)").font(.system(size: 48, weight: .bold))
            HStack {
                Button("-") { store.count -= 1 }.buttonStyle(.bordered)
                Button("+") { store.count += 1 }.buttonStyle(.borderedProminent)
            }
        }
        .padding()
    }
}

// さらに簡単にするなら @AppStorage("count") var count = 0 を使えば保存処理すら不要。