SettingsTableViewController.swift

// A simplified version of my view controller implementation.
// Assume that the user hasn't purchased anything yet, and that I will persist the transaction properly to user defaults.

import UIKit
import StoreKit

/// A view controller for the settings.
class SettingsTableViewController: UITableViewController {

    /// The widget's IAP product, which will exist after we query the app store.
    var widgetProduct: SKProduct?

    override func viewDidLoad() {
        super.viewDidLoad()
        
        guard SKPaymentQueue.canMakePayments() else {
            return
        }
        let request = SKProductsRequest(productIdentifiers: [MY_PRODUCT_IDENTIFIER])
        request.delegate = self
        request.start()
    }

    override func viewDidDisappear(_ animated: Bool) {
        super.viewDidDisappear(animated)
        SKPaymentQueue.default().remove(self)
    }
    
    // ...
    
    /// Prompts the user to purchase the widget if `widgetProduct` is not nil, i.e., if it is available from the app store.
    /// This is called when the user taps a button in the UI somewhere.
    func buyWidgetIfAvailable() {
        guard let widgetProduct = widgetProduct else { return }
        let payment = SKPayment(product: widgetProduct)
        SKPaymentQueue.default().add(payment)
    }
    
}

extension SettingsTableViewController: SKProductsRequestDelegate {

    func productsRequest(_ request: SKProductsRequest, didReceive response: SKProductsResponse) {
        // Show list of available purchases
        for product in response.products {
            if product.productIdentifier == MY_PRODUCT_IDENTIFIER {
                self.widgetProduct = product
            }
        }
    }

    func request(_ request: SKRequest, didFailWithError error: Error) {
        // Alert the user
    }

    /// What to do once the widget is purchased.
    func didPurchaseWidget() {
        // Persist the transaction to user defaults and alert the user.
    }

}

extension SettingsTableViewController: SKPaymentTransactionObserver {

    func paymentQueue(_ queue: SKPaymentQueue, updatedTransactions transactions: [SKPaymentTransaction]) {
        for transaction in transactions {
            handleTransaction(transaction, withQueue: queue)
        }
    }

    func paymentQueueRestoreCompletedTransactionsFinished(_ queue: SKPaymentQueue) {
        for transaction in queue.transactions {
            if transaction.transactionState == .restored {
                didPurchaseWidget()
                queue.finishTransaction(transaction)
            }
        }

        if /* check in user defaults if widget is not purchased */ {
            // Alert the user that there is no previous transaction to restore
        }
    }

    func paymentQueue(_ queue: SKPaymentQueue, restoreCompletedTransactionsFailedWithError error: Error) {
        // Alert user
    }

    /// What to do once we receive a transaction.
    /// - Parameter transaction: The StoreKit transaction.
    /// - Parameter queue: The StoreKit payment queue.
    func handleTransaction(_ transaction: SKPaymentTransaction, withQueue queue: SKPaymentQueue) {
        switch transaction.transactionState {
        case .purchased:
            if transaction.payment.productIdentifier == MY_PRODUCT_IDENTIFIER {
                didPurchaseWidget()
                queue.finishTransaction(transaction)
            }

        case .failed:
            // Alert the user
            // Should I be running finishTransaction?
            // queue.finishTransaction(transaction)

        // Is this case redundant?
        case .restored:
            if transaction.payment.productIdentifier == MY_PRODUCT_IDENTIFIER {
                didPurchaseWidget()
                queue.finishTransaction(transaction)
            }

        default:
            break
        }
    }

}