An Operation subclass that does work asynchronously

AsyncOperation.swift

import Foundation

class AsyncOperation: Operation {
    
    private var _isFinished: Bool = false
    private var _isExecuting: Bool = false
    
    open func execute() { assertionFailure("Overide execute() in subclass") }

    override var isAsynchronous: Bool { return true }
    override var isConcurrent: Bool { return true }
    
    override var isFinished: Bool { return _isFinished }
    override var isExecuting: Bool { return _isExecuting }

    final override func start() {
        guard !isCancelled else {
            willChangeValue(forKey: "isFinished")
            _isFinished = true
            didChangeValue(forKey: "isFinished")
            return
        }
        
        willChangeValue(forKey: "isExecuting")
        Thread.detachNewThreadSelector(#selector(main), toTarget: self, with: nil)
        _isExecuting = true
        didChangeValue(forKey: "isExecuting")
    }
    
    final override func main() { execute() }

    //Call completeOperation() within your overriden execute method when work is completed      
    final func completeOperation() {
        willChangeValue(forKey: "isFinished")
        willChangeValue(forKey: "isExecuting")
        _isExecuting = false
        _isFinished = true
        didChangeValue(forKey: "isExecuting")
        didChangeValue(forKey: "isFinished")
    }
}