NetworkRequest.swift

struct NetworkRequest<T: Decodable> {
        
    public enum FetchResults {
        case success(response: T)
        case error(error: String)
    }
    
    
    static func fetchJson(with url: URL, completion: @escaping (FetchResults) -> Void) {
        
        URLSession.shared.dataTask(with: url) { (data, response, error) in
            
            if let error = error {
                completion(.error(error: error.localizedDescription))
                return
            }
            
            guard let data = data else {
                completion(.error(error: "Data is invalid"))
                return
            }
            
            do {
                completion(.success(response: try JSONDecoder().decode(T.self, from: data)))
            } catch let error {
                completion(.error(error: error.localizedDescription))
            }
            
        }.resume()
        
    }
    
    
}

struct User: Decodable {
    let name: String
    let uid: String
}

if let url = URL(string: "some url") {
    
    NetworkRequest<User>.fetchJson(with: url) { (result) in
        switch result {
        case .success(let user):
            print(user)
        case .error(let msg):
            print(msg)
        }
    }
    
}