class AsyncOperation: Operation {

    internal enum State {
        case pending, executing, finished
    }

    private var managedKeyPaths: [KeyPath<AsyncOperation, Bool>] {
        return [\AsyncOperation.isExecuting, \AsyncOperation.isFinished]
    }

    internal var state = State.pending {
        willSet { managedKeyPaths.forEach { willChangeValue(for: $0) } }
        didSet { managedKeyPaths.forEach { didChangeValue(for: $0) } }
    }

    override var isAsynchronous: Bool {
        return true
    }

    override var isExecuting: Bool {
        return state == .executing
    }

    override var isFinished: Bool {
        return state == .finished
    }
}

extension Operation {
    func cancelRecursively() {
        cancel()
        dependencies.forEach { $0.cancelRecursively() }
    }
}

extension Array where Element: Operation {
    func cancel() {
        self.forEach { $0.cancel() }
    }
}

extension ArraySlice where Element: Operation {
    func cancel() {
        self.forEach { $0.cancel() }
    }
}