RAC_SignalForImage.m

// creates a signal that fetches an image in the background, delivering
// it on the UI thread. This signal 'cancels' itself if the cell is re-used before the
// image is downloaded.
-(RACSignal *)signalForImage:(NSURL *)imageUrl {
  
  RACScheduler *scheduler = [RACScheduler schedulerWithPriority:RACSchedulerPriorityBackground];
  
  RACSignal *imageDownloadSignal = [[RACSignal createSignal:^RACDisposable *(id<RACSubscriber> subscriber) {
      NSData *data = [NSData dataWithContentsOfURL:imageUrl];
      UIImage *image = [UIImage imageWithData:data];
      
      //now we can tell the SUBSCRIBER to do something!! This is the key with the signals
      
      [subscriber sendNext:image];
      [subscriber sendCompleted];
      return nil;
    }] subscribeOn:scheduler];
  
  return [[imageDownloadSignal
          takeUntil:self.rac_prepareForReuseSignal]
          deliverOn:[RACScheduler mainThreadScheduler]];
  
}