concurrent.swift

func concurrent_setResizedImage(withURL url: NSURL) {
    let backgroundQueue = dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0)
    let mainQueue = dispatch_get_main_queue()

    dispatch_async(backgroundQueue) { [frame] in
        let resizedURL = photonize(url: url, forSize: frame.size)
        let request = requestForImageUrl(resizedURL)
        fetchData(request) { data in
            dispatch_async(backgroundQueue, { 
                guard let image = imageFromResponseData(data) else {
                    return
                }
                dispatch_async(mainQueue, {
                    applyImage(image, to: self)
                })
            })
        }
    }
}