gistfile1.m

@interface CBImageStore () <NSURLSessionTaskDelegate>

@property (nonatomic, strong) NSMapTable *imageTaskCompletionsByTaskID;
@property (nonatomic, strong) NSMapTable *imageURLTaskCompletionsByTaskID;
@property (nonatomic, strong) NSURLSession *nsurlsession;

@end

@implementation CBImageStore

+ (CBImageStore *)sharedStore
{
    static CBImageStore *_sharedClient = nil;
    static dispatch_once_t onceToken;
    dispatch_once(&onceToken, ^{
        _sharedClient = [[self alloc] init];
    });
    
    return _sharedClient;
}

- (NSURLSession *) nsurlsession
{
  if (!_nsurlsession) {
    _nsurlsession = // create NSURLSession with self as delegate
  }
  return _nsurlsession;
}


- (NSMapTable *) imageTaskCompletionsByTaskID
{
  if (!_imageTaskCompletionsByTaskID) {
    _imageTaskCompletionsByTaskID = [NSMapTable mapTableWithKeyOptions:NSMapTableCopyIn
                                                      valueOptions:NSMapTableCopyIn];
  }
  return _imageTaskCompletionsByTaskID;
}

- (NSMapTable *) imageURLTaskCompletionsByTaskID
{
  if (!_imageURLTaskCompletionsByTaskID) {
    _imageURLTaskCompletionsByTaskID = [NSMapTable mapTableWithKeyOptions:NSMapTableCopyIn
                                                      valueOptions:NSMapTableCopyIn];
  }
  return _imageURLTaskCompletionsByTaskID;
}

- (void) imageForImageName:(NSString *)imageName completion:(void (^)(UIImage *))completionWithImage
{
  [self imageNameToImageURLMapping:^(NSDictionary *mapping) {
    NSURL *imageURL = mapping[imageName];
    NSURLRequest *request = [NSURLRequest requestWithURL:imageURL];
    NSURLSessionDownloadTask *task = [self.nsurlsession downloadTaskWithRequest:request];
    self.imageURLTaskCompletionsByTaskID[@(task.taskIdentifier)] = completionWithMapping;
    
    [task resume];
  }];
}

- (void) imageNameToImageURLMapping:(void (^)(NSDicationary *))completionWithMapping
{
  NSDictionary *mapping = [[NSUserDefaults standardUserDefaults] objectForKey:@"imageURLMapping"];
  if (mapping) {
    completionWithMapping(mapping);
  }
  else {
    NSURLRequest *request = [NSURLRequest requestWithURL:[NSURL URLWithString:@"connors.api/imageURLs"]];
    NSURLSessionDataTask *task = [self.nsurlsession dataTaskWithRequest:request];
    self.imageURLTaskCompletionsByTaskID[@(task.taskIdentifier)] = completionWithMapping;
    
    [task resume];
  }
}

- (void)URLSession:(__unused NSURLSession *)session
              task:(NSURLSessionTask *)task
didCompleteWithError:(NSError *)error
{
    void(^completionWithMapping)(NSDictionary *) = [self.imageURLTaskCompletionsByTaskID objectForKey:@(task.taskIdentifier)];
    if (completionWithMapping) {
      NSDictionary *mapping = [self makeMappingFromResponse:task.response]; // You probably already have this method 
      [[NSUserDefaults standardUserDefaults] setObject:mapping forKey:@"imageURLMapping"];
      completionWithMapping(mapping);
      [self.imageURLTaskCompletionsByTaskID removeObjectForKey:@(task.taskIdentifier)];
    }
}

- (void)URLSession:(NSURLSession *)session
      downloadTask:(NSURLSessionDownloadTask *)downloadTask
didFinishDownloadingToURL:(NSURL *)location
{
  void(^completionWithImage)(UIImage *) = [self.imageTaskCompletionsByTaskID objectForKey:@(downloadTask.taskIdentifier)];
  if (completionWithImage) {
    UIImage *image = [UIImage imageWithData:[NSData dataWithContentsOfURL:location]];
    // You'd want to save the location of the image somewhere so you don't have to do a network request each time.
    completionWithImage(image);
    [self.imageTaskCompletionsByTaskID removeObjectForKey:@(downloadTask.taskIdentifier)];
  }
}


@end