NetworkManager.swift

final class NetworkManager {

    let baseURL: String

    private var session = URLSession(configuration: URLSessionConfiguration.default,
                                     delegate: nil,
                                     delegateQueue: nil)

    private let queue = DispatchQueue(label: "com.organization.network-manager", attributes: .concurrent)

    init(baseURL: String) {
        self.baseURL = baseURL
    }

    deinit {
        self.session.finishTasksAndInvalidate()
    }

    func sendRequest<E: Encodable, D: Decodable>(route: Route, params: E, decodeTo: D.Type, completion: @escaping (D?, Error?) -> Void) {

        queue.async { [weak self] in

            guard let self = self else {
                completion(nil, NetworkManagerError.managerIsNil)
                return
            }

            guard let url = URL(string: self.baseURL + route.stringValue) else {
                completion(nil, NetworkManagerError.invalidURL)
                return
            }

            var request = URLRequest(url: url)
            request.httpMethod = route.method
            request.addValue("application/json", forHTTPHeaderField: "Content-Type")
            request.addValue("application/json", forHTTPHeaderField: "Accept")

            guard let jsonData = try? JSONEncoder().encode(params) else {
                completion(nil, NetworkManagerError.errorEncodingJson)
                return
            }

            request.httpBody = jsonData

            let task = self.session.dataTask(with: request) { data, response, error in

                guard let data = data, error == nil else {
                    completion(nil, NetworkManagerError.nilResponse)
                    return
                }

                guard let result = try? JSONDecoder().decode(D.self, from: data) else {
                    completion(nil, NetworkManagerError.errorDecodingJson)
                    return
                }

                completion(result, nil)
            }

            task.resume()
        }

    }

    func sendRequest<D: Decodable>(route: Route, decodeTo: D.Type, completion: @escaping (D?, Error?) -> Void) {

        queue.async { [weak self] in

            guard let self = self else {
                completion(nil, NetworkManagerError.managerIsNil)
                return
            }
            
            guard let url = URL(string: self.baseURL + route.stringValue) else {
                completion(nil, NetworkManagerError.invalidURL)
                return
            }

            var request = URLRequest(url: url)
            request.httpMethod = route.method
            request.addValue("application/json", forHTTPHeaderField: "Content-Type")
            request.addValue("application/json", forHTTPHeaderField: "Accept")

            let task = self.session.dataTask(with: request) {  data, response, error in

                guard let data = data, error == nil else {
                    completion(nil, NetworkManagerError.nilResponse)
                    return
                }

                guard let result = try? JSONDecoder().decode(D.self, from: data) else {
                    completion(nil, NetworkManagerError.errorDecodingJson)
                    return
                }

                completion(result, nil)
            }

            task.resume()
        }

    }
}