.swift

struct JsonDownloader
{
    public func downloadContentsOfUrl(url: URL, onComplete: @escaping (_ url: URL) -> UIImage) -> Void {
        // This async call will be thrown onto a thread and then this function will return immediately
        DispatchQueue.global(qos: .background).async {
            let json = try! Data(contentsOf: url)
            
            let jsonData = try! JSONSerialization.jsonObject(with: json, options: []) as! [String: Any]

            onComplete(URL(string: jsonData["image_url"] as! String)!)
        }
    }
}