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Concurrent NSOperation in Swift
import Foundation
/// An abstract class that makes building simple asynchronous operations easy.
/// Subclasses must implement `execute()` to perform any work and call
/// `finish()` when they are done. All `NSOperation` work will be handled
/// automatically.
open class Operation: Foundation.Operation {
// MARK: - Properties
private let stateQueue = DispatchQueue(
label: "com.calebd.operation.state",
attributes: .concurrent)
private var rawState = OperationState.ready
@objc private dynamic var state: OperationState {
get {
return stateQueue.sync(execute: { rawState })
set {
willChangeValue(forKey: "state")
flags: .barrier,
execute: { rawState = newValue })
didChangeValue(forKey: "state")
public final 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
// MARK: - NSObject
@objc private dynamic class func keyPathsForValuesAffectingIsReady() -> Set<String> {
return ["state"]
@objc private dynamic class func keyPathsForValuesAffectingIsExecuting() -> Set<String> {
return ["state"]
@objc private dynamic class func keyPathsForValuesAffectingIsFinished() -> Set<String> {
return ["state"]
// MARK: - Foundation.Operation
public override final func start() {
if isCancelled {
state = .executing
// MARK: - Public
/// Subclasses must implement this to perform their work and they must not
/// call `super`. The default implementation of this function throws an
/// exception.
open func execute() {
fatalError("Subclasses must implement `execute`.")
/// Call this function after any work is done or after a call to `cancel()`
/// to move the operation into a completed state.
public final func finish() {
state = .finished
@objc private enum OperationState: Int {
case ready
case executing
case finished
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