SimpleRESTClient

A simple client for making REST API calls using Swift's async/await feature.

Usage

First, define an Environment in which to make API calls.

enum MyEnvironment: Environment {
  case sandbox
  case production
  
  var hostName: String {
    switch self {
      case sandbox:
        return "dev.myhost.com"
      case production:
        return "myhost.com"
    }
  }
}

Then define an Endpoint for the API calls you wish to make:

enum MyEndpoint: Endpoint {
  case users(page: Int?)
  case user(id: Int?)
  case login

  func url(environment: Environment) async throws -> URL {

    var baseURLComponents = URLComponents()
    baseURLComponents.scheme = "https"
    baseURLComponents.host = environment.hostname
    baseURLComponents.port = environment.portNumber

    guard let baseURL = baseURLComponents.url else {
      throw URLError(.unsupportedURL)
    }

    var endpointURLComponents = URLComponents()

    switch self {
    case .users(page: let page):
      endpointURLComponents.path = "/api/users/"
      if let page {
        endpointURLComponents.queryItems = [URLQueryItem(name: "page", value: String(page))]
      }
    case .user(id: let id):
      if let id {
        endpointURLComponents.path = "/api/users/\(id)"
      } else {
        endpointURLComponents.path = "/api/users/"
      }
    case .login:
      endpointURLComponents.path = "/api/login"
    }

    guard let endpointURL = endpointURLComponents.url(relativeTo: baseURL) else {
      throw URLError(.unsupportedURL)
    }

    return endpointURL
  }
}

Next, create a Transport. For network operations, use NetworkTransport. For test operations MockTransport provides the ability to create mocked responses in code or from JSON files.

You can then inject this transport as a dependency to an API client:

var transport = NetworkTransport(environment: MyEnvironment.sandbox)
var apiClient = APIClient(transport: transport)

Then, we can implement our DTO entities and make calls with the APIClient.

struct NewUserRequest: Codable {
  var name: String
  var job: String
}

struct NewUserResponse: Codable {
  var name: String
  var job: String
  var id: String
  var createdAt: String
}

func createUser(name: String, job: String) async throws -> String {
  let endpoint = MyEndpoint.user(id: nil)
  let userRequest = NewUserRequest(name: name, job: job)
  let userResponse: NewUserResponse = try await apiClient.post(endpoint: endpoint, value: userRequest)
  return userResponse.id
}

And so on. We can extend the operation of the Transport by injecting RequestProcessor implementations, for instance to add headers for authentication. These will be executed sequentially but each can operate asynchronously.

struct AuthenticationRequestProcessor: RequestProcessor {

  func process(endpoint: Endpoint, request: URLRequest) async throws -> URLRequest {
  
    switch endpoint {
      case MyEndpoint.login:
        // Login endpoint is unauthenticated.
        return request
      default:
        // Perform whatever steps are needed to get an access token.
        let accessToken = try await something() // ...
        let bearer = "Bearer \(accessToken)"
        return request.updating(headerFields: [HTTPHeader.authorization: bearer])
    }
  }
}

var transport = NetworkTransport(environment: MyEnvironment.sandbox, 
                           requestProcessors: [AuthenticationRequestProcessor()])