Use same merchant ID in both checkout flows in sample app

cleanup error handling for Apple Pay
Use dispatch_groups to ensure the shop and checkout are completed before continuing with an Apple Pay checkout

Mobile Buy SDK Sample Apps/Sample App Advanced/Mobile Buy SDK Advanced Sample/CheckoutViewController.m

@@ -27,12 +27,13 @@
 #import "CheckoutViewController.h"
 #import "GetCompletionStatusOperation.h"
 #import "SummaryItemsTableViewCell.h"
+#import "ProductListViewController.h"
+
 @import Buy;
 @import PassKit;
 @import SafariServices;
 
 NSString * const CheckoutCallbackNotification = @"CheckoutCallbackNotification";
-NSString * const MerchantId = @"";
 
 @interface CheckoutViewController () <GetCompletionStatusOperationDelegate, SFSafariViewControllerDelegate, PKPaymentAuthorizationViewControllerDelegate>
 
@@ -291,7 +292,7 @@ NSString * const MerchantId = @"";
 {
     PKPaymentRequest *paymentRequest = [[PKPaymentRequest alloc] init];
     
-    [paymentRequest setMerchantIdentifier:MerchantId];
+    [paymentRequest setMerchantIdentifier:MERCHANT_ID];
     [paymentRequest setRequiredBillingAddressFields:PKAddressFieldAll];
     [paymentRequest setRequiredShippingAddressFields:self.checkout.requiresShipping ? PKAddressFieldAll : PKAddressFieldEmail|PKAddressFieldPhone];
     [paymentRequest setSupportedNetworks:@[PKPaymentNetworkVisa, PKPaymentNetworkMasterCard]];

Mobile Buy SDK Sample Apps/Sample App Advanced/Mobile Buy SDK Advanced Sample/ProductListViewController.h

@@ -27,6 +27,10 @@
 @import UIKit;
 @import Buy;
 
+#warning - Enter your merchant ID
+// Adding a merchant ID will show Apple Pay in the BUYProductViewController (on supported devices)
+#define MERCHANT_ID @""
+
 @interface ProductListViewController : UITableViewController
 
 - (instancetype)initWithClient:(BUYClient *)client collection:(BUYCollection*)collection;

Mobile Buy SDK Sample Apps/Sample App Advanced/Mobile Buy SDK Advanced Sample/ProductListViewController.m

@@ -30,10 +30,6 @@
 #import "ProductViewControllerThemeStyleTableViewCell.h"
 #import "ProductViewControllerThemeTintColorTableViewCell.h"
 
-#warning - Enter your merchant ID
-// Adding a merchant ID will show Apple Pay in the BUYProductViewController (on supported devices)
-#define MERCHANT_ID @""
-
 @interface ProductListViewController () <UIViewControllerPreviewingDelegate>
 
 @property (nonatomic, strong) BUYClient *client;

Mobile Buy SDK/Mobile Buy SDK/Data/BUYClient.m

@@ -90,8 +90,6 @@ static NSString *const kBUYClientPathCollectionPublications = @"collection_publi
 		self.queue = dispatch_get_main_queue();
 		
 		NSURLSessionConfiguration *config = [NSURLSessionConfiguration defaultSessionConfiguration];
-		// ensure requests are sent serially
-		config.HTTPMaximumConnectionsPerHost = 1;
 		config.HTTPAdditionalHeaders = @{@"User-Agent": [NSString stringWithFormat:@"Mobile Buy SDK iOS/%@", BUYVersionString]};
 		
 		self.session = [NSURLSession sessionWithConfiguration:config delegate:self delegateQueue:nil];

Mobile Buy SDK/Mobile Buy SDK/View Controllers/BUYViewController.m

@@ -89,12 +89,6 @@ NSString * BUYURLKey = @"url";
 		if (error == nil) {
 			self.shop = shop;
 		}
-		else {
-			
-			if ([self.delegate respondsToSelector:@selector(controllerFailedToStartApplePayProcess:)]) {
-				[self.delegate controllerFailedToStartApplePayProcess:self];
-			}
-		}
 		
 		self.isLoadingShop = NO;
 		
@@ -145,22 +139,66 @@ NSString * BUYURLKey = @"url";
 	// Default to the failure state, since cancelling a payment would not update the state and thus appear as a success
 	self.paymentAuthorizationStatus = PKPaymentAuthorizationStatusFailure;
 	
-	if (self.shop == nil && self.isLoadingShop == NO) {
-		// since requests are sent serially, this will return before the checkout is created
-		[self loadShopWithCallback:nil];
+	/**
+	 *  To perform an Apple Pay checkout, we need both the BUYShop object, and a BUYCheckout
+	 *  We will download both in parallel, and continue with the checkout when they both succeed
+	 */
+	dispatch_group_t group = dispatch_group_create();
+	__block NSError *checkoutError = nil;
+	
+	// download the shop
+	if (self.shop != nil) {
+		dispatch_group_enter(group);
+		[self loadShopWithCallback:^(BOOL success, NSError *error) {
+			
+			if (error) {
+				checkoutError = error;
+				
+				if ([self.delegate respondsToSelector:@selector(controllerFailedToStartApplePayProcess:)]) {
+					[self.delegate controllerFailedToStartApplePayProcess:self];
+				}
+			}
+			NSLog(@"SHOP");
+			dispatch_group_leave(group);
+		}];
 	}
 	
+	// create the checkout on Shopify
+	dispatch_group_enter(group);
 	[self handleCheckout:checkout completion:^(BUYCheckout *checkout, NSError *error) {
-		self.checkout = checkout;
 		
-		if (error == nil) {
+		if (error) {
+			checkoutError = error;
+			
+			if ([self.delegate respondsToSelector:@selector(controller:failedToCreateCheckout:)]) {
+				[self.delegate controller:self failedToCreateCheckout:error];
+			}
+		}
+		else {
+			self.checkout = checkout;
+		}
+
+		dispatch_group_leave(group);
+	}];
+	
+	// When we have both the shop and checkout, we can request the payment with Apple Pay
+	dispatch_group_notify(group, dispatch_get_main_queue(), ^{
+		
+		if (self.checkout && self.shop) {
+			
 			if ([self.delegate respondsToSelector:@selector(controllerWillCheckoutViaApplePay:)]) {
 				[self.delegate controllerWillCheckoutViaApplePay:self];
 			}
-			self.applePayHelper = [[BUYApplePayHelpers alloc] initWithClient:self.client checkout:checkout shop:self.shop];
+			
+			self.applePayHelper = [[BUYApplePayHelpers alloc] initWithClient:self.client checkout:self.checkout shop:self.shop];
+			[self requestPayment];
 		}
-		[self handleCheckoutCompletion:checkout error:error];
-	}];
+		else {
+			if ([self.delegate respondsToSelector:@selector(controller:failedToCreateCheckout:)]) {
+				[self.delegate controller:self failedToCreateCheckout:checkoutError];
+			}
+		}
+	});
 }
 
 - (void)startWebCheckout:(BUYCheckout *)checkout