Asynchronous Operation for synchronous Queue

AsynchronousOperation.swift

class AsynchronousOperation: Operation {
    @objc private enum OperationState: Int {
        case ready
        case executing
        case finished
    }

    private let stateQueue = DispatchQueue(label: Bundle.main.bundleIdentifier! + ".rw.state", attributes: .concurrent)
    private var _state: OperationState = .ready

    @objc private dynamic var state: OperationState {
        get { return stateQueue.sync { _state } }
        set { stateQueue.sync(flags: .barrier) { _state = newValue } }
    }

    // MARK: - Various `Operation` properties

    open override var isReady: Bool { return state == .ready && super.isReady }
    public final override var isExecuting: Bool { return state == .executing }
    public final override var isFinished: Bool { return state == .finished }

    // KVN for dependent properties

    open override class func keyPathsForValuesAffectingValue(forKey key: String) -> Set<String> {
        if ["isReady", "isFinished", "isExecuting"].contains(key) {
            return [#keyPath(state)]
        }

        return super.keyPathsForValuesAffectingValue(forKey: key)
    }

    // Start

    public final override func start() {
        if isCancelled {
            finish()
            return
        }

        state = .executing

        main()
    }

    open override func main() {
        fatalError("Subclasses must implement `main`.")
    }

    public final func finish() {
        if isExecuting { state = .finished }
    }
}

QueueExample.swift

let queue = OperationQueue()
queue.name = name
queue.maxConcurrentOperationCount = 1

queue.addOperation(SomeAsyncOperation())
queue.addOperation(SomeAsyncOperation())
queue.addOperation(SomeAsyncOperation())

queue.observe(\.operationCount, options: [.new]) { [weak self] _, change in
    if change.newValue! == 0 {
        // finish queue
    }
}

SomeAsyncOperation.swift

class SomeAsyncOperation: AsynchronousOperation {
    override func main() {
        finish() // call whe u finish task
    }
}