EndPoint.swift

enum HTTPMethod: String {
    case get = "GET"
}

class HTTPLayer {
    
    let baseURL = URL(string: "https://hamed-gh.com")!
    
    func request(at endpoint: Endpoint, completion: @escaping (Data?, Error?) -> Void) {
        let url = baseURL.appendingPathComponent(endpoint.path)
        let task = URLSession.shared.dataTask(with: url) { (data, response, error) in
            completion(data, error)
        }
        task.resume()
    }
    
}

enum Endpoint {
    
    case listTransactions
    case transactionDetails(String)
    
    var method: HTTPMethod {
        return .get
    }
    
    var path: String {
        switch self {
        case .listTransactions:
            return "/api/transactions/"
        case .transactionDetails(let id):
            return "/api/transactions/id"
        }
    }
    
}

struct Transaction: Decodable {
    let id: String
    let amount: Double
}

struct TransactionDetails: Decodable {
    let payload: [String: String]
}

enum Result<T> {
    case success(T)
    case failure(NSError)
}

class APIClient {
    
    let httpLayer: HTTPLayer
    init(httpLayer: HTTPLayer) {
        self.httpLayer = httpLayer
    }
    
    func transactions(completion: @escaping (Result<[Transaction]>) -> Void) {
        self.httpLayer.request(at: .listTransactions) { (data, error) in
            if error != nil {
                completion(.failure(error! as NSError))
            } else {
                do {
                    let transactions = try JSONDecoder().decode([Transaction].self, from: data!)
                    completion(.success(transactions))
                } catch {
                    completion(.failure(self.defaultError))
                }
            }
        }
    }
    
    func transactionDetails(for transaction: Transaction, completion: @escaping (Result<TransactionDetails>) -> Void) {
        self.httpLayer.request(at: .transactionDetails(transaction.id)) { (data, error) in
            if error != nil {
                completion(.failure(error! as NSError))
            } else {
                do {
                    let transactions = try JSONDecoder().decode(TransactionDetails.self, from: data!)
                    completion(.success(transactions))
                } catch {
                    completion(.failure(self.defaultError))
                }
            }
        }
    }
    
    var defaultError: NSError = NSError(domain: "", code: 1, userInfo: nil)
    
}