import StoreKit

enum ProductID {
    static let premium = "com.myapp.premium"
    static let gems100 = "com.myapp.gems100"
    static let monthlyPlan = "com.myapp.monthly"
    static let yearlyPlan = "com.myapp.yearly"
    static let all: [String] = [premium, gems100, monthlyPlan, yearlyPlan]
}

let products = try await Product.products(for: ProductID.all)
for product in products {
    print("\(product.displayName): \(product.displayPrice)")
}

func purchase(_ product: Product) async throws {
    let result = try await product.purchase(options: [
        .appAccountToken(userAccountToken)
    ])
    switch result {
    case .success(let verification):
        let transaction = try checkVerified(verification)
        await deliverContent(for: transaction)
        await transaction.finish()
    case .userCancelled:
        break
    case .pending:
        // Ask to Buy or deferred approval -- do not unlock content yet
        break
    @unknown default:
        break
    }
}

func checkVerified<T>(_ result: VerificationResult<T>) throws -> T {
    switch result {
    case .verified(let value): return value
    case .unverified(_, let error): throw error
    }
}

@main
struct MyApp: App {
    private var transactionListener: Task<Void, Error>?

    init() {
        transactionListener = listenForTransactions()
    }

    var body: some Scene {
        WindowGroup { ContentView() }
    }

    func listenForTransactions() -> Task<Void, Error> {
        Task.detached {
            for await result in Transaction.updates {
                guard case .verified(let transaction) = result else { continue }
                await StoreManager.shared.updateEntitlements()
                await transaction.finish()
            }
        }
    }
}

@Observable
@MainActor
class StoreManager {
    static let shared = StoreManager()
    var purchasedProductIDs: Set<String> = []
    var isPremium: Bool { purchasedProductIDs.contains(ProductID.premium) }

    func updateEntitlements() async {
        var purchased = Set<String>()
        for await result in Transaction.currentEntitlements {
            if case .verified(let transaction) = result,
               transaction.revocationDate == nil {
                purchased.insert(transaction.productID)
            }
        }
        purchasedProductIDs = purchased
    }
}

struct PremiumGatedView: View {
    @State private var state: EntitlementTaskState<VerificationResult<Transaction>?> = .loading

    var body: some View {
        Group {
            switch state {
            case .loading: ProgressView()
            case .failure: PaywallView()
            case .success(let transaction):
                if transaction != nil { PremiumContentView() }
                else { PaywallView() }
            }
        }
        .currentEntitlementTask(for: ProductID.premium) { state in
            self.state = state
        }
    }
}

SubscriptionStoreView(groupID: "YOUR_GROUP_ID")
    .subscriptionStoreControlStyle(.prominentPicker)
    .subscriptionStoreButtonLabel(.multiline)
    .storeButton(.visible, for: .restorePurchases)
    .storeButton(.visible, for: .redeemCode)
    .subscriptionStorePolicyDestination(url: termsURL, for: .termsOfService)
    .subscriptionStorePolicyDestination(url: privacyURL, for: .privacyPolicy)
    .onInAppPurchaseCompletion { product, result in
        if case .success(.verified(let transaction)) = result {
            await transaction.finish()
        }
    }

SubscriptionStoreView(groupID: "YOUR_GROUP_ID") {
    VStack {
        Image(systemName: "crown.fill").font(.system(size: 60)).foregroundStyle(.yellow)
        Text("Unlock Premium").font(.largeTitle.bold())
        Text("Access all features").foregroundStyle(.secondary)
    }
}
.containerBackground(.blue.gradient, for: .subscriptionStore)

SubscriptionStoreView(groupID: "YOUR_GROUP_ID") {
    SubscriptionPeriodGroupSet()
}
.subscriptionStoreControlStyle(.picker)

StoreView(ids: [ProductID.gems100, ProductID.premium], prefersPromotionalIcon: true)
    .productViewStyle(.large)
    .storeButton(.visible, for: .restorePurchases)
    .onInAppPurchaseCompletion { product, result in
        if case .success(.verified(let transaction)) = result {
            await transaction.finish()
        }
    }

ProductView(id: ProductID.premium) { iconPhase in
    switch iconPhase {
    case .success(let image): image.resizable().scaledToFit()
    case .loading: ProgressView()
    default: Image(systemName: "star.fill")
    }
}
.productViewStyle(.large)

func checkSubscriptionActive(groupID: String) async throws -> Bool {
    let statuses = try await Product.SubscriptionInfo.Status.status(for: groupID)
    for status in statuses {
        guard case .verified = status.renewalInfo,
              case .verified = status.transaction else { continue }
        if status.state == .subscribed || status.state == .inGracePeriod {
            return true
        }
    }
    return false
}

func restorePurchases() async throws {
    try await AppStore.sync()
    await StoreManager.shared.updateEntitlements()
}

func verifyAppPurchase() async {
    do {
        let result = try await AppTransaction.shared
        switch result {
        case .verified(let appTransaction):
            let originalVersion = appTransaction.originalAppVersion
            let purchaseDate = appTransaction.originalPurchaseDate
            // Migration logic for users who paid before subscription model
        case .unverified:
            // Potentially tampered -- restrict features as appropriate
            break
        }
    } catch { /* Could not retrieve app transaction */ }
}

// App account token for server-side reconciliation
try await product.purchase(options: [.appAccountToken(UUID())])

// Consumable quantity
try await product.purchase(options: [.quantity(5)])

// Simulate Ask to Buy in sandbox
try await product.purchase(options: [.simulatesAskToBuyInSandbox(true)])

.onInAppPurchaseStart { product in
    return true  // Return false to cancel
}
.onInAppPurchaseCompletion { product, result in
    if case .success(.verified(let transaction)) = result {
        await transaction.finish()
    }
}
.inAppPurchaseOptions { product in
    [.appAccountToken(userAccountToken)]
}

// WRONG: No listener -- misses renewals, refunds, Ask to Buy approvals
@main struct MyApp: App {
    var body: some Scene { WindowGroup { ContentView() } }
}
// CORRECT: Start listener in App init (see Transaction.updates section above)

// WRONG: Never finished -- reappears in unfinished queue forever
let transaction = try checkVerified(verification)
unlockFeature(transaction.productID)

// CORRECT: Always finish after delivering content
let transaction = try checkVerified(verification)
unlockFeature(transaction.productID)
await transaction.finish()

// WRONG: Using unverified transaction -- security risk
let transaction = verification.unsafePayloadValue

// CORRECT: Verify before using
let transaction = try checkVerified(verification)

// AVOID: Original StoreKit (legacy)
let request = SKProductsRequest(productIdentifiers: ["com.app.premium"])
SKPaymentQueue.default().add(payment)
SKStoreReviewController.requestReview()

// PREFERRED: StoreKit 2
let products = try await Product.products(for: ["com.app.premium"])
let result = try await product.purchase()
try await AppStore.requestReview(in: windowScene)

// WRONG: Grants access to refunded purchases
if case .verified(let transaction) = result {
    purchased.insert(transaction.productID)
}

// CORRECT: Skip revoked transactions
if case .verified(let transaction) = result, transaction.revocationDate == nil {
    purchased.insert(transaction.productID)
}

// WRONG: Wrong for other currencies and regions
Text("Buy Premium for $4.99")

// CORRECT: Localized price from Product
Text("Buy \(product.displayName) for \(product.displayPrice)")

// WRONG: Silently drops pending Ask to Buy
default: break

// CORRECT: Inform user purchase is awaiting approval
case .pending:
    showPendingApprovalMessage()

// WRONG: Check once, never update
func appDidFinish() { Task { await updateEntitlements() } }

// CORRECT: Re-check on Transaction.updates AND on foreground return
// Transaction.updates listener handles mid-session changes.
// Also use .task { await storeManager.updateEntitlements() } on content views.

// WRONG: No restore option -- App Store rejection risk
SubscriptionStoreView(groupID: "group_id")

// CORRECT
SubscriptionStoreView(groupID: "group_id")
    .storeButton(.visible, for: .restorePurchases)

// WRONG: No terms or privacy policy
SubscriptionStoreView(groupID: "group_id")

// CORRECT
SubscriptionStoreView(groupID: "group_id")
    .subscriptionStorePolicyDestination(url: termsURL, for: .termsOfService)
    .subscriptionStorePolicyDestination(url: privacyURL, for: .privacyPolicy)