UICollectionView + AFNetworking + AutoScroll/Pagination?

I use UICollectionView + AFNetworking 2 for loading images asynchronously from a webservice. It's working great. However, how do I implement automatic scrolling (when user reaches bottom of page, or close to it, the API is called to fetch more cells).

I've seen lots of examples but there doesn't seem to be a best practice. For example, one tutorial suggested scrollViewDidScroll, but that's called every time the view is scrolled. Is that the correct implementation?

PS. I'm coding in Swift.

Here's my cell creation code:

func collectionView(collectionView: UICollectionView!, cellForItemAtIndexPath indexPath: NSIndexPath!) -> UICollectionViewCell! {

    let cell = collectionView.dequeueReusableCellWithReuseIdentifier(cellIdentifier, forIndexPath: indexPath) as UICollectionViewCell

    let offer = self.offers[indexPath.row]
    var cellImageView = cell.viewWithTag(100) as UIImageView
    let requestURL = NSURLRequest(URL: offer.largeImageURL)
    cellImageView.setImageWithURLRequest(
        requestURL,
        placeholderImage:placeholderImage,
        success: {
            (request:NSURLRequest!, response:NSHTTPURLResponse!, image:UIImage!) in
            cellImageView.image = image
        },
        failure: {
            (request:NSURLRequest!, response:NSHTTPURLResponse!, error:NSError!) in
            NSLog("GET Image Error: " + error.localizedDescription)
        }
    )
    return cell
}

You could do it in a scrollViewDidScroll delegate method or collectionView:cellForItemAtIndexPath: (here you could detect that last cell is loading). What you would need to do is to create a wrapper for AFNetworking call to prevent it from loading the data few times (some kind of flag that indicates that it's already loading).

After that you could insert additional cell with some kind of activity indicator at the bottom and remove it after AFNetworking call is finished.