A boiler plate operation to be used as a subclass

BaseOperation.swift

//  A boiler plate operation to be used as a subclass
//
//  Created by Simon Westerlund on 2016-11-07.
//  Copyright © 2016 Simon Westerlund. All rights reserved.
//
import Foundation

final class BaseOperation: Operation {
    private var _isFinished = false {
        willSet { willChangeValue(forKey: "isFinished") }
        didSet { didChangeValue(forKey: "isFinished") }
    }
    private var _isExecuting = false {
        willSet { willChangeValue(forKey: "isExecuting") }
        didSet { didChangeValue(forKey: "isExecuting") }
    }
    private var _isCancelled = false {
        willSet { willChangeValue(forKey: "isCancelled") }
        didSet { didChangeValue(forKey: "isCancelled") }
    }

    init() {
    }

    private func performOperation() {
        // do stuff
		done()
    }
	
	private func done() {
        _isExecuting = false
        _isFinished = true
    }

    override func start() {
       	_isExecuting = true
        performOperation()
    }

    override func cancel() {
        _isCancelled = true
        done()
	}

    override var isExecuting: Bool {
    	return _isExecuting
    }

    override var isFinished: Bool {
        return _isFinished
    }
    
    override var isCancelled: Bool {
        return _isCancelled
    }
}

Now you can do :
willSet { willChangeValue(for: \.isExecuting) }