Concurrent NSOperation in Swift

AsynchronousOperation.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 AsynchronousOperation: 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")
            stateQueue.sync(
                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
    }
    
    public final override var isAsynchronous: Bool {
        return true
    }


    // 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() {
        super.start()

        if isCancelled {
            finish()
            return
        }

        state = .executing
        execute()
    }


    // 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
}

Do not call super.start() from your override of start. As the documentation for start says (emphasis added):

If you are implementing a concurrent operation, you must override this method and use it to initiate your operation. Your custom implementation must not call super at any time.

@diwu This pattern is known as "reader-writer" synchronization, allowing for concurrent reads, but all writes are synchronized. This is like the serial dispatch queue pattern, but is conceptually a little more efficient. The difference is only material in high-contention environments, but it doesn't hurt.

BTW, the reader-writer pattern is discussed in the latter part of WWDC 2012 video Asynchronous Design Patterns with Blocks, GCD, and XPC. Note, that video is using the old Swift 2 and Objective-C GCD syntax, but the idea is identical to what you see here.

By the way, I notice that this sample is doing the manual KVN of state. Because it’s a dynamic property, that is not needed. It does the KVN for you.

@calebd What's the licence of the this gist?

Thank you for sharing this.

I am learning to use the Operation class. Can I ask what the following is used for?

// 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"]
}

Here is Registering Dependent Keys