 
 extension ViewController: SKProductsRequestDelegate, SKPaymentTransactionObserver {
     
+    // 1. Request products
     func requestProducts() {
         let IDs: Set<String> = ["com.gethomeworktutor.TheGallery.tigerart",  "com.gethomeworktutor.TheGallery.natureart"]
         let productsRequest = SKProductsRequest(productIdentifiers: IDs)

        productsRequest.start()
    }
    
    func productsRequest(_ request: SKProductsRequest, didReceive response: SKProductsResponse) {
        print("Products ready: \(response.products.count)")
        print("Products notready: \(response.invalidProductIdentifiers.count)")

        
        for product in products {
            if product.productIdentifier == art.productIdentifier {
                
                //4. Add product to the queue to purchase it
                SKPaymentQueue.default().add(self)
                let payment = SKPayment(product: product)
                SKPaymentQueue.default().add(payment)
