let productIdentifiers: Set<String> = /* ... get product identifers */
StoreKitClient.getProducts(productIdentifiers: productIdentifiers).then { productsResponse in
    // Handle products...
}.catch { error in
    // Handle exception...
}

let product: SKProduct = /* ... get product from `StoreKitClient.getProducts` */
let payment: SKPayment = SKPayment(product: product)
StoreKitClient.getTransaction(payment: payment).then { transaction in
    // Check `transaction.transactionState`...
}.catch { error in
    switch ((error as NSError).domain, (error as NSError).code) {
    case (SKErrorDomain, SKError.paymentCancelled.rawValue):
        // User cancelled, don't show an error...
    default:
        // Handle exception...
    }
}

StoreKitClient.getTransactions().then { transactions in
    // Handle completed transactions...
}.catch { error in
    // Handle exception...
}

StoreKitClient.getReceipt().then { receipt in
    // Check `Bundle.main.appStoreReceiptURL`...
}.catch { error in
    // Handle exception...
}