Operation example for the article https://medium.com/@raulriera/nsoperations-nsoperationqueue-oh-my-88b707f9ba2e

URLSessionOperation.swift

public typealias URLSessionOperationCompletion = (data: NSData?, response: NSHTTPURLResponse?, error: NSError?) -> Void

public class URLSessionOperation: Operation {
    
    private var task: NSURLSessionDataTask?
    private var completionHandler: URLSessionOperationCompletion?
    
    /**
     Returns an instance of `URLSessionOperation`
     
     - parameter session: The session that will be used for this request. The default value is `.sharedSession()`.
     - parameter url: The URL with the location of the resource to request against.
     - parameter completion: The block executed iff the request completes.
     */
    public init(session: NSURLSession = .sharedSession(), url: NSURL, completion: URLSessionOperationCompletion) {
        super.init()
        
        name = url.absoluteString
        
        task = session.dataTaskWithURL(url) { [weak self] (data: NSData?, response: NSURLResponse?, error: NSError?) in
            self?.completeOperationWithBlock(completion, data: data, response: response as? NSHTTPURLResponse, error: error)
        }
    }
    
    public override func cancel() {
        super.cancel()
        task?.cancel()
    }
    
    public override func start() {
        if cancelled {
            _finished = true
        } else {
            _executing = true
            task?.resume()
        }
    }
    
    // MARK: Private
    
    private func completeOperationWithBlock(completion: URLSessionOperationCompletion, data: NSData?, response: NSHTTPURLResponse?, error: NSError?) {
        if cancelled == false || error?.code != NSURLErrorCancelled {
            dispatch_async(dispatch_get_main_queue()) {
                completion(data: data, response: response, error: error)
            }
        }
        
        completeOperation()
    }
    
}