AsyncOperation

AsyncOperation.swift

import Foundation

open class AsyncOperation: Operation {

    private enum State: String {
        case ready = "isReady"
        case executing = "isExecuting"
        case finished = "isFinished"
    }

    private var _state = Atomic(State.ready)
    private let queue = DispatchQueue(label: "AsyncOperation.state")

    private var state: State {
        get {
            return _state.value
        }
        set {
            queue.sync {
                let oldValue = _state.value
                guard newValue != oldValue else { return }
                willChangeValue(forKey: newValue.rawValue)
                willChangeValue(forKey: oldValue.rawValue)
                _state.mutate { $0 = newValue }
                didChangeValue(forKey: oldValue.rawValue)
                didChangeValue(forKey: newValue.rawValue)
            }
        }
    }

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

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

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

    final override public var isAsynchronous: Bool {
        return true
    }

    final override public func start() {
        if isCancelled {
            state = .finished
        } else {
            main()
        }
    }

    final override public func main() {
        if isCancelled {
            state = .finished
        } else {
            state = .executing
            execute()
        }
    }

    // override this method in subclass to do actual async work and call finish() in completion handler
    open func execute() {
        finish()
    }

    final public func finish() {
        if isExecuting {
            state = .finished
        }
    }
}

https://github.com/radianttap/Swift-Essentials/blob/master/Sources/Operation/AsyncOperation.swift
https://gist.github.com/calebd/93fa347397cec5f88233
https://gist.github.com/parrots/f1a6ca9c9924905fd1bd12cfb640337a