subscriptionOfferUpsell.swift

@available(iOS 12.2, *)
func showPromoUpsell() {

      Purchases.shared.entitlements { (entitlements, error) in

          // 1: - Get the discount for the product we want to offer
          //
          guard let product = entitlements?["pro_cat"]?.offerings["monthly_cats"]?.activeProduct else {
              print("Error finding monthly_cat product")
              return
          }
          guard let productDiscount = product.discounts.first(where: { $0.identifier == "promo_cat" }) else {
              print("Error finding promo_cat discount")
              return
          }

          // 2: - Fetch the payment discount
          //
          Purchases.shared.paymentDiscount(for: productDiscount, product: product, completion: { (paymentDiscount, err) in

              guard let discount = paymentDiscount else {
                  print("Payment discount doesn't exist. Check error.")
                  return
              }


              // 3: - Show the user the offer to purchase and save a flag
              //      in local storage so we don't show again
              self.presentPromoAlert(productDiscount, shouldPurchase: {

                  Purchases.shared.makePurchase(product, discount: discount, { (transaction, info, error, cancelled) in
                      self.configureCatContentFor(purchaserInfo: info)
                  })
              })
          })
      }

  }

  // Helper function to present promo alert view
  func presentPromoAlert(_ discount: SKProductDiscount, shouldPurchase: @escaping (() -> Void)) {
      let price = discount.price

      let promoAlert = UIAlertController(
          title: "Thanks for being a loyal user!",
          message: "We'd like to offer you an exclusive deal of only \(price) for 3-months of pro access.",
          preferredStyle: .alert)

      promoAlert.addAction(UIAlertAction(
          title: "No thanks",
          style: .cancel,
          handler: nil))

      promoAlert.addAction(UIAlertAction(
          title: "Sure!",
          style: .default,
          handler: { _ in
              shouldPurchase()
      }))
      UserDefaults.standard.set(true, forKey: "has_seen_promo_cat_discount")
      self.present(promoAlert, animated: true, completion: nil)
  }