Download Asynchronously using block objects - Objective-C

DownloadAsyncUsingBlock.objc

@implementation ViewController

- (void)loadDataAsync{
    // Download the Objects from a JSON file on the server
    Downloader *dl = [[Downloader alloc] init];
    [dl downloadObjects:self.aID andCompletionHandler:^(NSArray *objectsArray){
        // Set the data to your model for instance
        // self.mArray = objectsArray;
        
        // Perform refresh on main thread.
        dispatch_async(dispatch_get_main_queue(), ^{
            // Refresh a tableView for instance
            // [self.tableView reloadData];
        });
    }];
}

@end


@implementation Downloader

/**
  Implement this function to return the default values for the download.
*/
- (NSArray *)defaultObjects{

    return nil;
}

/**
 Download Asynchronously using block objects from a JSON file.
 */
- (void)downloadObjects:(void(^)(NSArray *aArray))completionHandler{
    
    NSURL *aURL = [NSURL URLWithString:@"http://the/url/file.json"];
    // This NSMUtableArray is the one that we are going to pass to the completion handler in order to do some work asynchronously.
    __block NSMutableArray *objectsArray = [[NSMutableArray alloc] init];
    
    NSURLSessionDownloadTask *downloadQuizzIDsTask = [[NSURLSession sharedSession]
                                                   downloadTaskWithURL:aURL completionHandler:^(NSURL *location, NSURLResponse *response, NSError *error) {
                                                       // 2
                                                       if(!downloadError){
                                                           
                                                           NSData *jsonData = [NSData dataWithContentsOfURL:location];

                                                           NSError *jsonError = nil;
                                                           NSDictionary *jsonAsADict = [NSJSONSerialization JSONObjectWithData:jsonData
                                                                                                                        options:0
                                                                                                                          error:&jsonError];
                                                           if(! jsonError) {
                                                               for (NSDictionary *aDict in [jsonAsADict objectForKey:@"theKey"]){
                                                                   // Do the work on the object here

                                                               }
                                                           } else {
                                                              // Handle the parsing error if necessary
                                                               NSLog(@"Error in parsing JSON");
                                                           }

                                                           completionHandler(objectsArray);
                                                       }else{
                                                          // Handle the download error
                                                          /* Download a default set of objects and send them for completion.
                                                          NSArray *defaultObjects = [self defaultObjects];
                                                           
                                                          completionHandler(defaultObjects);*/
                                                       }
                                                   }];
    
    
    
    // 1
    [downloadQuizzIDsTask resume];
}

@end