Encapsulating Network Requests Using Moya

Since the introduction of NSURLSession in iOS7, the question of needing third party networking frameworks such as Alamofire has been hotly contested. While decoupling your application from third party dependencies can be a great thing, reputable libraries can provide a lot of battle-tested functionality that can simplify your code. Moya gives us another reason to choose Alamofire.

Moya is built on top of Alamofire, which brings in new features:

Type-safe network requests using enumerations
Compile-time checking of API endpoint accesses
Simplified unit testing with built-in test stub

We will be pulling forecast information from the Dark Sky API, a free and easy weather API, using Moya.

Forecast Request

Method: GET

https://api.darksky.net/forecast/[key]/[latitude],[longitude]

Comparing NSURLSession to Moya

To appreciate the benefits of Moya, let's first look at how the API can be accessed with plain NSURLSession:

class WeatherService {

let defaultSession = URLSession(configuration: .default)

let darkSkyKey = "[SecretKey]"

var dataTask: URLSessionDataTask?

typealias QueryResult = (Weather, String) -> ()

var weather: Weather = Weather()

var errorMessage = ""

func getCurrentWeather(latitude: Double, longitude: Double, completion: @escaping QueryResult) {

dataTask?.cancel()

if var urlComponents = URLComponents(string: " https://api.darksky.net/") {

urlComponents.query = "forecast/\(self.darkSkyKey)/\(latitude),\(longitude)?exclude=minutely,flags,alerts,hourly"

guard let url = urlComponents.url else { return }

var request = URLRequest(url: url)

request.setValue("application/json", forHTTPHeaderField: "Content-Type")

dataTask = defaultSession.dataTask(with: request) { data, response, error in

defer { self.dataTask = nil }

if let error = error {

self.errorMessage += "Error: " + error.localizedDescription + "\n"

} else if let data = data,

let response = response as? HTTPURLResponse,

response.statusCode == 200 {

self.parseWeatherData(data)

DispatchQueue.main.async {

completion(self.weather, self.errorMessage)

}

dataTask?.resume()

}

fileprivate func parseWeatherData(_ data: Data) {

//Code to parse data object to Weather

}

It's fairly straightforward to do a single GET call using NSURLSession. However, the class and functions can become easily bloated if there are more complex calls with multiple APIs and endpoints. Without producing a custom API manager, each endpoint function would require manual configuration, parsing, and validation that is error-prone. With Moya, code complexity and errors are reduced through the use of enumerations.

There are two main components of Moya:

Target - A Moya target is an API service. In our example, DarkSky is a target. The target describes all the endpoints available, and the information (e.g. header, request body, etc.) required for each endpoint to successfully perform the request.
Provider - The MoyaProvider is the primary object that is instantiated with a given target type to make network calls.

To begin, an enumeration implementing the TargetType protocol is needed.

TargetType requires conformance of the following variables:

baseURL
- Type: URL
- The target's base URL to build endpoint
- Example: https://api.darksky.net
path
- Type: String
- The path of each endpoint relative to the base URL.
- Example: /forecast/
method
- Type: Moya.Method
- The HTTP method for each endpoint.
- Example: .get
sampleData
- Type: Data
- Provide mocked data for each endpoint for testing.
headers
- Type: [String:String]?
- The HTTP headers needed for each endpoint.
- Example: ["Content-Type": "application/json"]
validationType
- Type: ValidationType
- Define what HTTP codes are considered successful.
- Example: HTTP codes between 200-299 using .successCodes