// App.swift — Swift 6 actor-isolated entry point
@main
struct MyApp: App {
    @StateObject private var store = StoreService()

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

import StoreKit

@MainActor
final class StoreService: ObservableObject {
    @Published private(set) var products: [Product] = []
    @Published private(set) var purchasedProductIDs: Set<String> = []

    private var transactionObserver: Task<Void, Never>?

    init() {
        transactionObserver = Task { await observeTransactions() }
    }

    deinit {
        transactionObserver?.cancel()
    }

    // MARK: - Product Loading

    func loadProducts(ids: [String]) async {
        do {
            products = try await Product.products(for: ids)
            // Products come back unordered — sort by price or custom order
            products.sort { $0.price < $1.price }
        } catch {
            // StoreKitError.networkError — retry with backoff
            // StoreKitError.notEntitled — sandbox/config issue
        }
    }

    // MARK: - Transaction Observer (MUST run for app lifetime)

    func observeTransactions() async {
        for await result in Transaction.updates {
            await process(result)
        }
    }

    // MARK: - Purchase

    func purchase(_ product: Product,
                  options: Set<Product.PurchaseOption> = []) async throws -> Transaction? {
        let result = try await product.purchase(options: options)
        switch result {
        case .success(let verification):
            let transaction = try checkVerified(verification)
            await updateEntitlements(transaction)
            await transaction.finish()   // CRITICAL — unfinished = re-delivered on next launch
            return transaction
        case .userCancelled:
            return nil
        case .pending:
            // Awaiting Ask to Buy or billing fix — show "payment pending" UI
            return nil
        @unknown default:
            return nil
        }
    }

    // MARK: - Entitlement Refresh

    func refreshEntitlements() async {
        purchasedProductIDs.removeAll()
        for await result in Transaction.currentEntitlements {
            guard case .verified(let transaction) = result else { continue }
            guard transaction.revocationDate == nil else { continue }  // Apple refunded
            purchasedProductIDs.insert(transaction.productID)
        }
    }

    // MARK: - Restore

    func restore() async throws {
        // AppStore.sync() triggers re-validation + re-delivery of current entitlements
        // Do NOT use SKPaymentQueue.restoreCompletedTransactions — it is deprecated
        try await AppStore.sync()
        await refreshEntitlements()
    }

    // MARK: - Private

    private func process(_ result: VerificationResult<Transaction>) async {
        await updateEntitlements(try? checkVerified(result))
    }

    private func updateEntitlements(_ transaction: Transaction?) async {
        guard let transaction else { return }
        if transaction.revocationDate == nil {
            purchasedProductIDs.insert(transaction.productID)
        } else {
            purchasedProductIDs.remove(transaction.productID)
        }
    }

    private func checkVerified<T>(_ result: VerificationResult<T>) throws -> T {
        switch result {
        case .unverified:
            // JWS signature invalid — tampered receipt or configuration error
            throw StoreError.failedVerification
        case .verified(let value):
            return value
        }
    }
}

enum StoreError: LocalizedError {
    case failedVerification
    var errorDescription: String? { "Purchase could not be verified." }
}

struct SubscriptionStatus {
    let isActive: Bool
    let willAutoRenew: Bool
    let expirationDate: Date?
    let isInBillingRetry: Bool
    let scheduledDowngradeProductID: String?
}

func subscriptionStatus(for product: Product) async -> SubscriptionStatus? {
    guard let subscription = product.subscription,
          let statusArray = try? await subscription.status else { return nil }

    // statusArray contains one entry per subscription in the group
    for status in statusArray {
        guard case .verified(let renewalInfo) = status.renewalInfo,
              case .verified(let transaction) = status.transaction else { continue }

        let isActive = status.state == .subscribed || status.state == .inGracePeriod
        return SubscriptionStatus(
            isActive: isActive,
            willAutoRenew: renewalInfo.willAutoRenew,
            expirationDate: transaction.expirationDate,
            isInBillingRetry: status.state == .inBillingRetryPeriod,
            scheduledDowngradeProductID: renewalInfo.autoRenewProductID != product.id
                ? renewalInfo.autoRenewProductID : nil
        )
    }
    return nil
}

func introOfferDetails(for product: Product) async -> Product.SubscriptionOffer? {
    guard let subscription = product.subscription,
          await subscription.isEligibleForIntroOffer == true else { return nil }
    return subscription.introductoryOffer
}

// Render based on paymentMode
func introLabel(_ offer: Product.SubscriptionOffer) -> String {
    switch offer.paymentMode {
    case .freeTrial:
        return "Free for \(offer.period.localizedDescription)"
    case .payAsYouGo:
        return "\(offer.displayPrice)/\(offer.period.value) \(offer.period.unit) for \(offer.periodCount) periods"
    case .payUpFront:
        return "\(offer.displayPrice) for \(offer.periodCount) periods"
    @unknown default:
        return offer.displayPrice
    }
}

// Server returns: keyID, nonce, signature, timestamp
let offerID = "annual_winback_50"
let signature = try await fetchPromoSignature(productID: product.id, offerID: offerID)

let purchaseOption = Product.PurchaseOption.promotionalOffer(
    offerID: offerID,
    keyID: signature.keyID,
    nonce: signature.nonce,
    signature: signature.data,
    timestamp: signature.timestamp
)
let transaction = try await storeService.purchase(product, options: [purchaseOption])

@MainActor
final class PaywallViewModel: ObservableObject {
    enum PurchaseState: Equatable {
        case idle, loading, purchasing, purchased, failed(String)
    }

    @Published var products: [Product] = []
    @Published var purchaseState: PurchaseState = .idle
    @Published var selectedProduct: Product?
    @Published var introOffer: Product.SubscriptionOffer?

    private let store: StoreService

    init(store: StoreService) {
        self.store = store
    }

    func load(productIDs: [String]) async {
        purchaseState = .loading
        await store.loadProducts(ids: productIDs)
        products = store.products
        selectedProduct = products.first(where: { $0.type == .autoRenewable })
        if let selected = selectedProduct {
            introOffer = await introOfferDetails(for: selected)
        }
        purchaseState = .idle
    }

    func purchase() async {
        guard let product = selectedProduct else { return }
        purchaseState = .purchasing
        do {
            guard try await store.purchase(product) != nil else {
                // userCancelled or pending — no error, just reset
                purchaseState = .idle
                return
            }
            purchaseState = .purchased
        } catch {
            purchaseState = .failed(error.localizedDescription)
        }
    }

    func restore() async {
        purchaseState = .loading
        do {
            try await store.restore()
            purchaseState = store.purchasedProductIDs.isEmpty ? .idle : .purchased
        } catch {
            purchaseState = .failed(error.localizedDescription)
        }
    }
}

1. Decode JWS: split by ".", base64url-decode payload
2. Verify signature using Apple's public key from WWDR certificate chain
3. Check: environment, bundleID, productID, expirationDate, revocationDate
4. Use App Store Server API for real-time status (not polled receipts)

func purchaseConsumable(_ product: Product) async throws {
    guard let transaction = try await store.purchase(product) else { return }
    // Deliver immediately before finish — if app crashes between deliver+finish,
    // transaction re-delivers on next launch via Transaction.updates
    await deliverConsumable(transaction.productID, quantity: transaction.purchasedQuantity)
    await transaction.finish()
}

// Idempotency: store transaction.id in your DB — re-delivery must not double-grant
func deliverConsumable(_ productID: String, quantity: Int) async {
    // Check if transaction.id already processed before crediting
}

// In XCTest — use SKTestSession to script scenarios
import StoreKitTest

class SubscriptionTests: XCTestCase {
    var session: SKTestSession!

    override func setUp() async throws {
        session = try SKTestSession(configurationFileNamed: "Products")
        session.resetToDefaultState()
        session.disableDialogs = true
        session.timeRate = .monthlyRenewalEveryThirtySeconds
    }

    func testSubscriptionRenews() async throws {
        let store = StoreService()
        // purchase → wait 30s → verify renewal transaction delivered
    }
}