AsyncOperation.swift

import Foundation

class AsyncOperation: Operation {
    public enum State: String {
        case ready, executing, finished

        fileprivate var keyPath: String {
            return "is" + rawValue.capitalized
        }
    }
    public var state = State.ready {
        willSet {
            willChangeValue(forKey: newValue.keyPath)
            willChangeValue(forKey: state.keyPath)
        }
        didSet {
            didChangeValue(forKey: oldValue.keyPath)
            didChangeValue(forKey: state.keyPath)
        }
    }

    override var isReady: Bool {
        return super.isReady && state == .ready
    }
    override var isFinished: Bool { return state == .finished }
    override var isExecuting: Bool { return state == .executing }
    override var isAsynchronous: Bool { return true }

    override func start() {
        if isCancelled {
            state = .finished
            return
        }

        main()
        state = .executing
    }

    override func cancel() {
        super.cancel()
        state = .finished
    }
}