Asynchronous Operation

AsyncOperation.swift

class AsyncOperation: Operation {
  // Only required when you want to manually start an operation
  // Ignored when an operation is added to a queue.
  override var isAsynchronous: Bool { return true }
  
  // This is not the OperationQueue!
  // This is the queue we use to read and write the operation state  in a safe thread way
  private let queue = DispatchQueue(label: "async_operation_private_queue", attributes: .concurrent)
  
  // State is accessed and modified in a thread safe and KVO   compliant way.
  private var _isExecuting: Bool = false
  override private(set) var isExecuting: Bool {
    get {
      return queue.sync { () -> Bool in return _isExecuting }
    }
    set {
      willChangeValue(forKey: "isExecuting")
      queue.sync(flags: [.barrier]) { _isExecuting = newValue }
      didChangeValue(forKey: "isExecuting")
    }
  }
  
  private var _isFinished: Bool = false
  override private(set) var isFinished: Bool {
    get {
      return queue.sync { () -> Bool in return _isFinished }
    }
    set {
      willChangeValue(forKey: "isFinished")
      queue.sync(flags: [.barrier]) { _isFinished = newValue }
      didChangeValue(forKey: "isFinished")
    }
  } 
  
  override func start() {
    guard !isCancelled else {
      finish()
      return
    }
    isFinished = false
    isExecuting = true
    main()
 }
  
  override func main() {
    fatalError("Subclasses must override main without calling super.")
  }
  
  func finish() {
    isExecuting = false
    isFinished = true
  }
}