AsyncOperationExtension.swift

extension AsyncOperation {

    override var isAsynchronous: Bool {
        return true
    }

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

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

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