[SWIFT] How to add In-App Purchases in your iOS app.

IAPHandler.swift

//
//  IAPHandler.swift
//
//  Created by Dejan Atanasov on 13/07/2017.
//  Copyright © 2017 Dejan Atanasov. All rights reserved.
//

import UIKit
import StoreKit

enum PurchaseEvent {
    case disabled
    case restored
    case failedToGetProductFromStore
    case failedToRestorePurchases
    case cancelled
    case completed

    func message() -> String{
        switch self {
        case .disabled: return "Purchases are disabled on your device."
        case .restored: return "You have successfully restored your purchase."
        case .failedToGetProductFromStore: return "The product could not be retrieved. Try again by relaunching the app."
        case .failedToRestorePurchases: return "Restoring your purchases was unsuccessful. Please try again."
        case .cancelled, .completed: return ""//don't do anything
        }
    }
}

class IAPHandler: NSObject {
    private override init() {}
    static let shared = IAPHandler()

    fileprivate var productsRequest = SKProductsRequest()
    fileprivate var iapProductsRetrievedFromAppStore = [SKProduct]()

    /// Shows an alert message informing about success or failure in certain cases.
    var onPurchaseProcessingStopped: ((PurchaseEvent) -> Void)?

    /// Enable to update UI when payment processing has started
    var onBeginPaymentProcess: (() -> Void)?

    // MARK: - MAKE PURCHASE OF A PRODUCT
    func purchaseMyProduct(productIdentifier: String){
        if iapProductsRetrievedFromAppStore.count == 0 { return }

        if SKPaymentQueue.canMakePayments() {
            guard let product = productWithIdentifier(productIdentifier) else {
                onPurchaseProcessingStopped?(.failedToGetProductFromStore)
                return
            }
            onBeginPaymentProcess?()
            let payment = SKPayment(product: product)
            SKPaymentQueue.default().add(self)
            SKPaymentQueue.default().add(payment)
            print("PRODUCT TO PURCHASE: \(product.productIdentifier)")
        } else {
            onPurchaseProcessingStopped?(.disabled)
        }
    }

    private func productWithIdentifier(_ productIdentifier: String) -> SKProduct? {
        return iapProductsRetrievedFromAppStore.first(where: { (product) -> Bool in
            return product.productIdentifier == productIdentifier
        })
    }

    // MARK: - RESTORE PURCHASE
    func restorePurchase(){
        SKPaymentQueue.default().add(self)
        SKPaymentQueue.default().restoreCompletedTransactions()
    }

    // MARK: - FETCH AVAILABLE IAP PRODUCTS
    func fetchAvailableProducts(){

        // Put here your IAP Products ID's
        let productIdentifiers: Set = ["iapProductIdentifierExample"]

        productsRequest = SKProductsRequest(productIdentifiers: productIdentifiers)
        productsRequest.delegate = self
        productsRequest.start()
    }
}

extension IAPHandler: SKProductsRequestDelegate, SKPaymentTransactionObserver {
    // MARK: - REQUEST IAP PRODUCTS
    func productsRequest (_ request:SKProductsRequest, didReceive response:SKProductsResponse) {

        if (response.products.count > 0) {
            iapProductsRetrievedFromAppStore = response.products
            for product in iapProductsRetrievedFromAppStore{
                let numberFormatter = NumberFormatter()
                numberFormatter.formatterBehavior = .behavior10_4
                numberFormatter.numberStyle = .currency
                numberFormatter.locale = product.priceLocale
                let price1Str = numberFormatter.string(from: product.price)
                print(product.localizedDescription + "\nfor just \(price1Str!)")
            }
        }
    }

    //MARK: - Restoring Purchases
    func paymentQueue(_ queue: SKPaymentQueue, restoreCompletedTransactionsFailedWithError error: Error) {
        onPurchaseProcessingStopped?(.failedToRestorePurchases)
    }

    func paymentQueueRestoreCompletedTransactionsFinished(_ queue: SKPaymentQueue) {
        onPurchaseProcessingStopped?(.restored)
    }

    //MARK: - IAP PAYMENT QUEUE
    func paymentQueue(_ queue: SKPaymentQueue, updatedTransactions transactions: [SKPaymentTransaction]) {
        for transaction in transactions {
            switch transaction.transactionState {
            case .purchased, .restored:
                handlePurchasedOrRestored(transaction: transaction)
            case .failed: //called also when user taps cancel after starting the purchase.
                SKPaymentQueue.default().finishTransaction(transaction)
                onPurchaseProcessingStopped?(.cancelled)
            case .deferred, .purchasing:
                break
            }
        }
    }

    //TODO receipt, call NSURL *receiptURL = [[NSBundle mainBundle] appStoreReceiptURL];
    //https://developer.apple.com/library/content/releasenotes/General/ValidateAppStoreReceipt/Chapters/ValidateRemotely.html#//apple_ref/doc/uid/TP40010573-CH104-SW2

    func paymentQueue(_ queue: SKPaymentQueue, shouldAddStorePayment payment: SKPayment, for product: SKProduct) -> Bool {
        // check if user can purchase the product
        return true // or false if user shall not purchase the product yet
    }

    private func handlePurchasedOrRestored(transaction: SKPaymentTransaction) {
        SKPaymentQueue.default().finishTransaction(transaction)
        UserDefaults.standard.set(true, forKey: transaction.payment.productIdentifier)
        NotificationCenter.default.post(name: Notification.Name(rawValue: transaction.payment.productIdentifier), object: nil)
        onPurchaseProcessingStopped?(.completed)
    }
}