Manually ensure thread safety for public singletons in Swift 6?

Using Swift
1

I've written a lot of UserDefaults related stuff in this class, and I'm using authToken as an example here. authToken is referenced in prepare the synchronised nonisolated protocol-method of the web library Moya.
I would like to know if this approach is really secure and reliable. I would appreciate for any help!

public final class AppDefaults: ObservableObject {
    public static var shared: AppDefaults {
        queue.sync {
            singleton
        }
    }
    nonisolated(unsafe) private static let singleton = AppDefaults()
    private init() {}
    private static let queue = DispatchQueue(label: "xxx.yyyy.zzzz.AppDefaults", qos: .userInitiated)

    @AppStorage(UserDefaults.authToken) var authToken: String = ""
}
2

statics are automatically backed by dispatch_once(), therefore you can simply do:

public final class AppDefaults {
    public static let shared = AppDefaults()
}

You also probably want to mark this class as Sendable.

1 Like
3

Thanks for your reply! Actually this class is a Non-Sendable class, which also can't be isolated to MainActor due to usage constraints, so I'd like to manually ensure thread-safety for it.

4

Yes you can manually ensure thread-safety, for example using a Mutex, and still mark the class Sendable. The compiler knows about Mutex and makes this possible.

5

Or if you are using other synchronization primitives such as dispatch queues, you can still mark the class @unchecked Sendable.

6 
    public static var shared: AppDefaults {
        queue.sync {
            let lock = NSLock()
            lock.lock(); defer { lock.unlock() }
            return singleton
        }
    }

Thank you! NSLock can be used in my project env, under iOS 17.x.

7

Reread my first answer, you do not need all this.

8 
public final class AppDefaults: ObservableObject, @unchecked Sendable {
    public static let shared = AppDefaults()
    private init() {}
    private let queue = DispatchQueue(label: "xxx.yyyy.zzzz.AppDefaults", qos: .userInitiated)

    @AppStorage(UserDefaults.wepediasKeyAuthToken) private var dq_authToken: String = ""
    var authToken: String {
        get {
            queue.sync { dq_authToken }
        }
        set {
            queue.async { [self] in
                dq_authToken = newValue
            }
        }
    }
}

I ended up with a lot more lines of code, but the effort of migrating to Swift 6 will all be worth it! Thank you for your help!