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: state.keyPath)
            willChangeValue(forKey: newValue.keyPath)
        }
        didSet {
            didChangeValue(forKey: oldValue.keyPath)
            didChangeValue(forKey: state.keyPath)
        }
    }
}