Swift4 Operation Sample

import UIKit
import PlaygroundSupport

PlaygroundPage.current.needsIndefiniteExecution = true

class MyOperation: Operation {
    
    enum State {
        case ready, executing, finished
        var keyPath: String {
            switch self {
            case .ready:
                return "isReady"
            case .executing:
                return "isExecuting"
            case .finished:
                return "isFinished"
            }
        }
    }
    
    var state = State.ready {
        willSet {
            willChangeValue(forKey: newValue.keyPath)
            willChangeValue(forKey: state.keyPath)
        }
        didSet {
            didChangeValue(forKey: oldValue.keyPath)
            didChangeValue(forKey: state.keyPath)
        }
    }
    // MARK: - NSOperation
    
    override var isReady: Bool {
        return super.isReady && state == .ready
    }
    override var isExecuting: Bool {
        return state == .executing
    }
    override var isFinished: Bool {
        return state == .finished
    }
    override var isAsynchronous: Bool {
        return true
    }
    
    var url: URL
    
    init(url: URL) {
        self.url = url
    }
    
    override func start() {
        guard !isCancelled else {
            state = .finished
            return
        }
        state = .executing
        main()
    }
    
    override func main() {
        URLSession.shared.dataTask(with: url, completionHandler: {(data, res, error) in
            guard !self.isCancelled else {
                self.state = .finished
                return
            }
            if let res = res as? HTTPURLResponse {
                print("\(self.url.absoluteString) : \(res.statusCode)")
            }
            self.state = .finished
        } ).resume()
    }
}


class MyObject: NSObject {
    override init() {
        super.init()
        let o1 = MyOperation(url: URL(string:"https://www.google.com")!)
        let o2 = MyOperation(url: URL(string:"https://www.gamer.com.tw")!)
        let o3 = MyOperation(url: URL(string:"https://twitter.com/paste_app")!)
        let o4 = MyOperation(url: URL(string:"https://tw.yahoo.com")!)
        let o5 = MyOperation(url: URL(string:"https://www.yahoo.co.jp")!)
        let queue = OperationQueue()
        queue.maxConcurrentOperationCount = 3
        

        let finishedOpration = BlockOperation {
            print("Block Complete")
        }
        for o in [o1,o2,o3,o4,o5] {
            queue.addOperation(o)
        }
        queue.addOperation(finishedOpration)
        queue.addObserver(self, forKeyPath: "operationCount", options: [.new], context: nil)
        
    }
    
    override func observeValue(forKeyPath keyPath: String?, of object: Any?, change: [NSKeyValueChangeKey : Any]?, context: UnsafeMutableRawPointer?) {
        if let q = object as? OperationQueue {
            print("\(q.operationCount)")
        }
    }
}


let a = MyObject()