Check iOS auto-renewable subscriptions if they are still valid

ValidateRenewablePurchases.m

- (void)validateRenewablePurchases:(NSArray<NSString *> *)products callback:(void (^)(NSDictionary *values, NSString *errorMessage))callback
{
    // Sending into background
    dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_BACKGROUND, 0), ^{ @autoreleasepool{
    
        void (^callbackOnMainThread)(NSDictionary *values, NSString *errorMessage) = ^(NSDictionary *values, NSString *errorMessage) {
            dispatch_async(dispatch_get_main_queue(), ^{
                callback(values, errorMessage);
            });
        };
        
        // Load the receipt from the app bundle.
        NSURL *receiptURL = [[NSBundle mainBundle] appStoreReceiptURL];
        NSData *receiptData = [NSData dataWithContentsOfURL:receiptURL];
    
        //
        if (receiptData)
        {
            BOOL isSandbox = [[receiptURL lastPathComponent] isEqualToString:@"sandboxReceipt"];
            
            // Make JSON object that describes the request
            NSDictionary *requestContents = @{
                @"receipt-data" : [receiptData base64EncodedStringWithOptions:0],
                @"password" : kStoreKitReceiptValidationSharedSecred
            };
            
            //
            NSError *error = nil;
            NSData *requestData = [NSJSONSerialization dataWithJSONObject:requestContents options:0 error:nil];
            if (requestData)
            {
                // Create a POST request with the receipt data.
                NSURL *storeURL = isSandbox ?
                    [NSURL URLWithString:@"https://sandbox.itunes.apple.com/verifyReceipt"] :
                    [NSURL URLWithString:@"https://buy.itunes.apple.com/verifyReceipt"];
                
                //
                NSMutableURLRequest *storeRequest = [NSMutableURLRequest requestWithURL:storeURL];
                [storeRequest setHTTPMethod:@"POST"];
                [storeRequest setHTTPBody:requestData];
                
                // Sending synchronous request in place
                error = nil;
                NSURLResponse *response = nil;
                NSData *resData = [NSURLConnection sendSynchronousRequest:storeRequest returningResponse:&response error:&error];
                if (error)
                {
                    callbackOnMainThread(nil, error.localizedDescription);
                }
                else
                {
                    NSDictionary *jsonResponse = [NSJSONSerialization JSONObjectWithData:resData options:0 error:&error];
                    if (jsonResponse == nil)
                    {
                        callbackOnMainThread(nil, error.localizedDescription);
                    }
                    else
                    {
                        NSLog(@"jsonResponse:%@", jsonResponse);
                        
                        //
                        NSArray *latestReceipts = jsonResponse[@"latest_receipt_info"];
                        if ([latestReceipts isKindOfClass:[NSArray class]])
                        {
                            //
                            NSMutableDictionary *result = [NSMutableDictionary dictionary];
                            
                            // get every requested product
                            for (NSString *product in products)
                            {
                                // Filter out items with requested product
                                NSPredicate *predicate = [NSPredicate predicateWithBlock:^BOOL(id  _Nullable evaluatedObject, NSDictionary<NSString *,id> * _Nullable bindings) {
                                    return [evaluatedObject[@"product_id"] isEqual:product];
                                }];
                                NSArray *filteredReceipts = [latestReceipts filteredArrayUsingPredicate:predicate];
                                NSArray *sortedFiltered = [filteredReceipts sortedArrayUsingComparator:^NSComparisonResult(id  _Nonnull obj1, id  _Nonnull obj2) {
                                    return [obj1[@"expires_date_ms"] compare:obj2[@"expires_date_ms"]];
                                }];
                                
                                // Get maximum value of expiration date
                                NSString *maxTimeStringMs = sortedFiltered.lastObject[@"expires_date_ms"];
                                if ([maxTimeStringMs isKindOfClass:[NSString class]])
                                {
                                    // crap starts here
                                    // Was "expires_date_ms": "1479951889000"
                                    NSString *maxTimeStringSeconds = [maxTimeStringMs substringToIndex:maxTimeStringMs.length - 3];
                                    // Now "expires_date_ms": "1479951889"
                                    // Now we can fit double
                                    NSTimeInterval doubleValue = [maxTimeStringSeconds doubleValue];
                                    NSDate *expirationDate = [NSDate dateWithTimeIntervalSince1970:doubleValue];
                                    NSDate *now = [NSDate date];
                                    
                                    BOOL isValid = [expirationDate compare:now] == NSOrderedDescending;
                                    result[product] = [NSNumber numberWithBool:isValid];
                                }
                            }
                            
                            callbackOnMainThread(result, nil);
                        }
                        else
                        {
                            callbackOnMainThread(nil, nil);
                        }
                    }
                }
            }
            else
            {
                // smth went wrong with JSON encoding
                callbackOnMainThread(nil, error.localizedDescription);
            }
        }
        else
        {
            // no receipt, no message, be silent
            callbackOnMainThread(nil, nil);
        }
        
    }});
}