How to create Concurrency custom operation block

ConcurrencyCustomOperationBlock.swift

enum OperationState: Int {
    case ready
    case executing
    case finished
}

class CustomOperationBlock: Operation{
    
   private var state: OperationState!
    
    private let finishedKey = "isFinished"
    private let executingKey = "isExecuting"
    
    override init(){
        self.state = .ready
    }
    
    override func start() {
        guard !self.isCancelled else{
            setIsFinished()
            return
        }
        
        self.willChangeValue(forKey: executingKey)
        Thread.detachNewThreadSelector(#selector(main), toTarget: self, with: nil)
        self.state = .executing
        self.didChangeValue(forKey: executingKey)
    }
    
    override func main() {
        doWork()
        setIsFinished()
    }
    
    private func setIsFinished(){
        self.willChangeValue(forKey: finishedKey)
        self.state = .finished
        self.didChangeValue(forKey: finishedKey)
    }
    
    private func doWork(){
        print("Here doing workin for operation")
    }
    
    override var isConcurrent: Bool{
        return true
    }
    
    override var isFinished: Bool{
        return state == .finished
    }
    
    override var isExecuting: Bool{
        return state == .executing
   }
}

let customOperation = CustomOperationBlock()
customOperation.start()