How to write and use an asynchronous operation

AsyncOperation.swift

import Foundation

class AsyncOperation: Operation {
    enum State: String {
        case isReady, isExecuting, isFinished
    }
    
    override var isAsynchronous: Bool {
        return true
    }
    
    var state = State.isReady {
        willSet {
            willChangeValue(forKey: state.rawValue)
            willChangeValue(forKey: newValue.rawValue)
        }
        didSet {
            didChangeValue(forKey: oldValue.rawValue)
            didChangeValue(forKey: state.rawValue)
        }
    }
    
    override var isExecuting: Bool {
        return state == .isExecuting
    }
    
    override var isFinished: Bool {
        return state == .isFinished
    }
    
    override func start() {
        guard !self.isCancelled else {
            state = .isFinished
            return
        }
        
        state = .isExecuting
        main()
    }
}

FetchOperation.swift

final class FetchOperation: AsyncOperation {

    private(set) var dataFetched: Data?

    private let session = URLSession(configuration: .default)
    private var dataTask: URLSessionDataTask?

    override func main() {

        let url = URL(string: "https://jsonplaceholder.typicode.com/users")!
        dataTask = session.dataTask(with: url) { [weak self] (data, _, _) in
            guard let `self` = self else { return }
            // We could improve this fallback with a Result enum
            guard 
                !self.isCancelled,
                let data = data else {
                self.state = .isFinished
                return
            }

            self.dataFetched = data
            self.state = .isFinished
        }
        dataTask?.resume()
    }
}

Is this thread safe? I'm reading lots about thread safety of writing to state. I'm bit confused.

yea facing same issue.

@robwithhair/@badrinathvm so far I didn't encounter thread problems. I should investigate about this issue.

Looking around, I think we could use an internal GCD queue to update the state with a sync work. I will update this code asap.