Asynchronous Operation in Swift 3

AsyncOperation.swift

import Foundation
import Dispatch

class AsyncOperation: Foundation.Operation {

    private let queue = DispatchQueue(label: "async.operation.queue")

    private var _executing = false
    private var _finished = false

    override internal(set) var isExecuting: Bool {
        get {
            return _executing
        }
        set {
            if _executing != newValue {
                willChangeValue(forKey: "isExecuting")
                _executing = newValue
                didChangeValue(forKey: "isExecuting")
            }
        }
    }

    override internal(set) var isFinished: Bool {
        get {
            return _finished
        }
        set {
            if _finished != newValue {
                willChangeValue(forKey: "isFinished")
                _finished = newValue
                didChangeValue(forKey: "isFinished")
            }
        }
    }

    override var isAsynchronous: Bool {
        return true
    }

    override func start() {
        if isCancelled {
            isFinished = true
            return
        }

        isExecuting = true

        queue.async {
            print("Start")
            sleep(5)
            print("Done")
        }
    }
    
}