ThingsService.swift

class ThingsService {
    func getThings(onCompletion callback: @escaping (Result<[Thing]>) -> Void) {
        sendHttpRequestAsynchronously(url) { response in
            DispatchQueue.main.async {
                do {
                    let data = try getData(from: response)
                    let things = try doParsing(data)
                    callback(.success(things)))
                } catch error {
                    callback(.failure(error))
                }
            }
        }
    }
}