MainViewController.swift

override func didReceiveMemoryWarning() {
    super.didReceiveMemoryWarning()
    // Dispose of any resources that can be recreated.
    self.imageCache.removeAllObjects()
}


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

    //In storyboard, my collection view cell has the identifier "PostCell"    
    let cell = collectionView.dequeueReusableCellWithReuseIdentifier("PostCell", forIndexPath: indexPath) as! CollectionViewCell
    
    //I have an array called "posts" that's filled with PFObjects that have UIImage data
    let postings: PFObject = posts[indexPath.row]
    
    //setting my cell's string property called "objectId" to the PFObject's ID for the purpose of caching shown below
    cell.objectId = postings.objectId! as String
    
    let theImage: AnyObject = postings["imageFile"] as! PFFile
    
    if let image = self.imageCache.objectForKey(postings.objectId!) as? UIImage {
        
        cell.imageView.image = image
        println("had this photo in cache")
        
    } else {
        
        theImage.getDataInBackgroundWithBlock { (imageData: NSData?, error: NSError?) -> Void in
            
            if error != nil {
                
                println("error caching or downloading image")
                return
            }
            
            let cellImage = UIImage(data:imageData!, scale: 1.0)
            self.imageCache.setObject(cellImage!, forKey: postings.objectId!)
            cell.imageView.image = cellImage
            println("just added another photo to cache")
        }
    }
    
    return cell
}

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class CollectionViewCell: UICollectionViewCell {
    
    var objectId: String!

}