A playground to go along with the article http://frankcourville.com/2016/12/05/getting-started-with-operations.html

OperationsPart1.playground

import UIKit
import PlaygroundSupport

PlaygroundPage.current.needsIndefiniteExecution = true

class LoggingOperation : Operation {
    override func main() {
        if isCancelled {
            return;
        }
        
        print("Logging operation")
    }
}

class FetchPokemonOperation : ConcurrentOperation {
    var task: URLSessionDataTask?
    var data: AnyObject?
    
    override func main() {
        if isCancelled {
            completeOperation()
            return
        }
        
        let session = URLSession(configuration: .ephemeral)
        let request = URLRequest(url: URL.init(string: "http://pokeapi.co/api/v2/pokemon/151/")!)
        
        task = session.dataTask(with: request, completionHandler: { (data, response, error) in
            //Be a good citizen and remove the bangs in production, ok? :)
            
            if self.isCancelled {
                self.completeOperation()
                return
            }
            
            let parsedResponse = try! JSONSerialization.jsonObject(with: data!)
            self.data = parsedResponse as AnyObject
            print(self.data!)
            self.completeOperation()
        })
        
        task?.resume()
    }
}

class ConcurrentOperation: Operation {
    private var backing_executing : Bool
    override var isExecuting : Bool {
        get { return backing_executing }
        set {
            willChangeValue(forKey: "isExecuting")
            backing_executing = newValue
            didChangeValue(forKey: "isExecuting")
        }
    }
    
    private var backing_finished : Bool
    override var isFinished : Bool {
        get { return backing_finished }
        set {
            willChangeValue(forKey: "isFinished")
            backing_finished = newValue
            didChangeValue(forKey: "isFinished")
        }
    }
    
    override init() {
        backing_executing = false
        backing_finished = false
        
        super.init()
    }
    
    func completeOperation() {
        isExecuting = false
        isFinished = true
    }
}

let loggingOp = LoggingOperation()
let fetchPokemonOp = FetchPokemonOperation()
let operationQueue = OperationQueue()

operationQueue.addOperation(loggingOp)
operationQueue.addOperation(fetchPokemonOp)