Example.swift

func purchaseProduct(product : SKProduct, success: @escaping SuccessBlock, failure: @escaping FailureBlock){
        guard SKPaymentQueue.canMakePayments() else {
            return
        }
        guard SKPaymentQueue.default().transactions.last?.transactionState != .purchasing else {
            return
        }
        
        self.successBlock = success
        self.failureBlock = failure
        
        let payment = SKPayment(product: product)
        SKPaymentQueue.default().add(payment)
    }