In app purchase helper class

IAPHelper.swift

//  Created by Soulchild on 21/07/2020.
//  Copyright © 2020 fluffy. All rights reserved.
//

import Foundation
import StoreKit
// Need to install the TPInAppReceipt library first
import TPInAppReceipt


open class IAPStore : NSObject {
    public struct Products {
        public static let premium = "your.iap.product.identifier"
        public static let subscription = "your.subscription.product.identifier"
        
        fileprivate static let identifiers : Set<ProductIdentifier> = [Products.premium, Products.subscription]
    }
    
    fileprivate let productIdentifiers: Set<String>
    
    fileprivate var productsRequest: SKProductsRequest?
    fileprivate var productsRequestCompletionHandler: ((_ success: Bool, _ products: [SKProduct]?) -> ())?
    
    // a shared instance with preset product identifiers, we will be using this mostly
    static let shared = IAPStore(productIds: Products.identifiers)
    
    // name of the notification that will be sent after purchase success
    static let IAPStorePurchaseNotification = NSNotification.Name("IAPStorePurchaseNotification")
    
    // name of the notification that will be sent if the purchase failed
    static let IAPStoreFailedNotification = NSNotification.Name("IAPStoreFailedNotification")
    
    // name of the notification that will be sent after restore success
    static let IAPStoreRestoreNotification = NSNotification.Name("IAPStoreRestoreNotification")
    
    // name of the notification that will be sent when there is nothing to restore
    // user tap 'restore purchase' without having any prior purchase
    static let IAPStoreEmptyRestoreNotification = NSNotification.Name("IAPStoreEmptyRestoreNotification")
    
    public init(productIds: Set<ProductIdentifier>) {
        productIdentifiers = productIds
        super.init()
        SKPaymentQueue.default().add(self)
    }
}

// MARK: - Utility functions
extension IAPStore {
    
    /// Request IAP products from App Store server
    /// - Parameter completionHandler: function that will be executed after retrieving the products
    public func requestProducts(completionHandler: @escaping (_ success: Bool, _ products: [SKProduct]?) -> ()){
        
        productsRequest?.cancel()
        productsRequestCompletionHandler = completionHandler
        
        productsRequest = SKProductsRequest(productIdentifiers: productIdentifiers)
        productsRequest!.delegate = self
        productsRequest!.start()
    }
    
    /// Buy the specific IAP Product
    /// - Parameter product: the IAP Product to purchase
    public func purchaseProduct(_ product: SKProduct) {
        print("Purchasing \(product.productIdentifier)...")
        let payment = SKPayment(product: product)
        SKPaymentQueue.default().add(payment)
    }
    
    
    /// Check if user has already purchased the specific IAP product
    /// - Parameter productIdentifier: the IAP Product Identifier
    /// - Returns: boolean
    public func isProductPurchased(_ productIdentifier: String) -> Bool {
        if let receipt = try? InAppReceipt.localReceipt(){
            if receipt.containsPurchase(ofProductIdentifier: productIdentifier){
                return true
            }
        }
        
        return false
    }
    
    
    /// Check if user has active subscription. If product identifier is nil, then it will check
    /// if user has any active subscriptions regardless of product identifier
    /// - Parameter productIdentifier: the product identifier of the subscription
    /// - Returns: boolean
    public func isSubscriptionActive(_ productIdentifier: String?) -> Bool {
        if let receipt = try? InAppReceipt.localReceipt(){
            if let productIdentifier = productIdentifier {
                return receipt.hasActiveAutoRenewableSubscription(ofProductIdentifier: productIdentifier, forDate: Date())
            }
            
            return receipt.hasActiveAutoRenewablePurchases
        }
        
        return false
    }
    
    /// Restore previous purchases, the receipt will be refreshed after restoration
    public func restorePurchases() {
        SKPaymentQueue.default().restoreCompletedTransactions()
    }
    
    /// In case parental control disable kids to buy app, this will return false
    public class func canMakePayments() -> Bool {
        // in case parental control disable kids to buy app, this will return false
        return SKPaymentQueue.canMakePayments()
    }
}

// MARK: - SKProductsRequestDelegate

extension IAPStore: SKProductsRequestDelegate {
    public func productsRequest(_ request: SKProductsRequest, didReceive response: SKProductsResponse) {
        
        let products = response.products
        productsRequestCompletionHandler?(true, products)
        
        // reset the request and completion handler
        productsRequest = nil
        productsRequestCompletionHandler = nil
    }
    
    public func request(_ request: SKRequest, didFailWithError error: Error) {
        productsRequestCompletionHandler?(false, nil)
        
        // reset the request and completion handler
        productsRequest = nil
        productsRequestCompletionHandler = nil
    }
}

// MARK: - SKPaymentTransactionObserver

extension IAPStore: SKPaymentTransactionObserver {
    public func paymentQueue(_ queue: SKPaymentQueue, updatedTransactions transactions: [SKPaymentTransaction]) {
        
        for transaction in transactions {
            switch (transaction.transactionState) {
            case .purchased:
                complete(transaction: transaction)
                break
            case .failed:
                fail(transaction: transaction)
                break
            case .restored:
                restore(transaction: transaction)
                break
            case .deferred:
                break
            case .purchasing:
                break
            @unknown default:
                break
            }
        }
    }
    
    public func paymentQueueRestoreCompletedTransactionsFinished(_ queue: SKPaymentQueue) {
        if(queue.transactions.isEmpty){
            // post notification that there is nothing to restore
            NotificationCenter.default.post(name: IAPStore.IAPStoreEmptyRestoreNotification, object: nil)
            
            return
        }
        
        for transaction in queue.transactions {
            switch (transaction.transactionState) {
            case .purchased:
                complete(transaction: transaction)
                break
            case .failed:
                fail(transaction: transaction)
                break
            case .restored:
                restore(transaction: transaction)
                break
            case .deferred:
                break
            case .purchasing:
                break
            @unknown default:
                break
            }
        }
    }
    
    private func complete(transaction: SKPaymentTransaction) {
        NotificationCenter.default.post(name: IAPStore.IAPStorePurchaseNotification, object: transaction.payment.productIdentifier)
        
        SKPaymentQueue.default().finishTransaction(transaction)
    }
    
    private func restore(transaction: SKPaymentTransaction) {
        NotificationCenter.default.post(name: IAPStore.IAPStoreRestoreNotification, object: transaction.original?.payment.productIdentifier)
        
        SKPaymentQueue.default().finishTransaction(transaction)
    }
    
    private func fail(transaction: SKPaymentTransaction) {
        if let transactionError = transaction.error as NSError?,
            let localizedDescription = transaction.error?.localizedDescription,
            transactionError.code != SKError.paymentCancelled.rawValue {
            
            // only post the fail notification if the error isn't user cancel (ie. user tap 'Cancel' on the payment prompt dialog)
            NotificationCenter.default.post(name: IAPStore.IAPStoreFailedNotification, object: localizedDescription)
        }
        
        SKPaymentQueue.default().finishTransaction(transaction)
    }
}

What is ProductIdentifier in line number 16

What is ProductIdentifier in line number 16

public typealias ProductIdentifier = String works for me