Providing the queue to call back on, default to main.

resultqueue.swift

// Taken from https://www.swiftbysundell.com/clips/2/

extension Data {
    func decoded<T: Decodable>(
        as type: T.Type = T.self,
        handledOn resultQueue: DispatchQueue = .main,
        handler: @escaping (Result<T, Error>) -> Void
    ) {
        let queue = DispatchQueue(label: "com.myapp.decoding")
        let decoder = JSONDecoder()

        queue.async {
            let result = Result {
                try decoder.decode(T.self, from: self)
            }

            resultQueue.async { handler(result) }
        }
    }
}