AsyncOperation.swift

class AsyncOperation: Operation {
  private(set) var taskHandle: Task.Handle<Void, Error>?

  fileprivate enum State: Equatable {
    case ready
    case executing
    case finished
  }

  fileprivate var state: State = .ready {
    willSet {
      willChangeValue(forKey: "state")
    }
    didSet {
      didChangeValue(forKey: "state")
    }
  }


  /// To be overridden by subclasses to perform their work
  func perform() async throws {
    fatalError("perform() must be overridden in subclass \(type(of: self))")
  }

  /// Main entry point, not to be overridden by subclasses
  override func start() {
    guard !self.isCancelled else {
      return
    }

    self.state = .executing

    self.taskHandle = async {
      do {
        try Task.checkCancellation()
        _ = try await self.perform()
      } catch {}
      self.state = .finished
      self.taskHandle = nil
    }
  }

  override func cancel() {
    super.cancel()
    self.taskHandle?.cancel()
    self.taskHandle = nil
    self.state = .finished
  }

  override var isAsynchronous: Bool {
    return true
  }

  override var isReady: Bool {
    return super.isReady && self.state == .ready
  }

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

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

  @objc class func keyPathsForValuesAffectingIsReady() -> Set<String> {
    return ["state"]
  }

  @objc class func keyPathsForValuesAffectingIsExecuting() -> Set<String> {
    return ["state"]
  }

  @objc class func keyPathsForValuesAffectingIsFinished() -> Set<String> {
    return ["state"]
  }
}