Consuming Web Services with Swift and Rx @gonzalezreal 

Forget Alamofire 

(at least for 1 hour) 

Let's build a Web API Client from scratch 

→ Model requests using enum → Map JSON without any 3rd party library → Use RxSwift to compose our API calls 

Borders github.com/gonzalezreal/Borders 

https://restcountries.eu/rest/v1/name/Germany? fullText=true 

[ { "name": "Germany", "borders": [ "AUT", "BEL", "CZE", ... ], "nativeName": "Deutschland", ... } ] 

https://restcountries.eu/rest/v1/alpha? codes=AUT;BEL;CZE 

[ { "name": "Austria", "nativeName": "Österreich", ... }, { "name": "Belgium", "nativeName": "België", ... }, { "name": "Czech Republic", "nativeName": "Česká republika", ... } ] 

Modeling the API 

https://restcountries.eu/rest/v1/name/Germany?fullText=true → GET → https://restcountries.eu/rest/v1 → name/Germany → fullText=true 

enum Method: String { case GET = "GET" ... } protocol Resource { var method: Method { get } var path: String { get } var parameters: [String: String] { get } } 

Resource → NSURLRequest 

extension Resource { func requestWithBaseURL(baseURL: NSURL) -> NSURLRequest { let URL = baseURL.URLByAppendingPathComponent(path) guard let components = NSURLComponents(URL: URL, resolvingAgainstBaseURL: false) else { fatalError("...") } components.queryItems = parameters.map { NSURLQueryItem(name: String($0), value: String($1)) } guard let finalURL = components.URL else { fatalError("...") } let request = NSMutableURLRequest(URL: finalURL) request.HTTPMethod = method.rawValue return request } } 

enum CountriesAPI { case Name(name: String) case AlphaCodes(codes: [String]) } 

extension CountriesAPI: Resource { var path: String { switch self { case let .Name(name): return "name/\(name)" case .AlphaCodes: return "alpha" } } var parameters: [String: String] { switch self { case .Name: return ["fullText": "true"] case let .AlphaCodes(codes): return ["codes": codes.joinWithSeparator(";")] } } } 

Demo 

Simple JSON decoding 

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typealias JSONDictionary = [String: AnyObject] protocol JSONDecodable { init?(dictionary: JSONDictionary) } 

func decode(dictionaries: [JSONDictionary]) -> [T] { return dictionaries.flatMap { T(dictionary: $0) } } func decode(data: NSData) -> [T]? { guard let JSONObject = try? NSJSONSerialization.JSONObjectWithData(data, options: []), dictionaries = JSONObject as? [JSONDictionary], objects: [T] = decode(dictionaries) else { return nil } return objects } 

struct Country { let name: String let nativeName: String let borders: [String] } 

extension Country: JSONDecodable { init?(dictionary: JSONDictionary) { guard let name = dictionary["name"] as? String, nativeName = dictionary["nativeName"] as? String else { return nil } self.name = name self.nativeName = nativeName self.borders = dictionary["borders"] as? [String] ?? [] } } 

Demo 

5 min intro to RxSwift 

An API for asynchronous programming with observable streams 

Observable streams → Taps, keyboard events, timers → GPS events → Video frames, audio samples → Web service responses 

Observable 

--1--2--3--4--5--6--| --a--b--a--a--a---d---X --------JSON-| ---tap-tap-------tap---> 

Next* (Error | Completed)? 

[1, 2, 3, 4, 5, 6].filter { $0 % 2 == 0 } [1, 2, 3, 4, 5, 6].map { $0 * 2 } [1, 2, 3, 5, 5, 6].reduce(0, +) 

Array ↓ Observable 

The API Client 

enum APIClientError: ErrorType { case CouldNotDecodeJSON case BadStatus(status: Int) case Other(NSError) } 

NSURLSession let configuration = NSURLSessionConfiguration.defaultSessionConfiguration() let session = NSURLSession(configuration: configuration) let task = self.session.dataTaskWithRequest(request) { data, response, error in // Handle response } task.resume() 

NSURLSession & RxSwift 

final class APIClient { private let baseURL: NSURL private let session: NSURLSession init(baseURL: NSURL, configuration: NSURLSessionConfiguration) { self.baseURL = baseURL self.session = NSURLSession(configuration: configuration) } ... } 

private func data(resource: Resource) -> Observable { let request = resource.requestWithBaseURL(baseURL) return Observable.create { observer in let task = self.session.dataTaskWithRequest(request) { data, response, error in if let error = error { observer.onError(APIClientError.Other(error)) } else { guard let HTTPResponse = response as? NSHTTPURLResponse else { fatalError("Couldn't get HTTP response") } if 200 ..< 300 ~= HTTPResponse.statusCode { observer.onNext(data ?? NSData()) observer.onCompleted() } else { observer.onError(APIClientError.BadStatus(status: HTTPResponse.statusCode)) } } } task.resume() return AnonymousDisposable { task.cancel() } } } 

Demo 

Let's add JSONDecodable to the mix 

func objects(resource: Resource) -> Observable<[T]> { return data(resource).map { data in guard let objects: [T] = decode(data) else { throw APIClientError.CouldNotDecodeJSON } return objects } } 

extension APIClient { func countryWithName(name: String) -> Observable { return objects(CountriesAPI.Name(name: name)).map { $0[0] } } func countriesWithCodes(codes: [String]) -> Observable<[Country]> { return objects(CountriesAPI.AlphaCodes(codes: codes)) } } 

Chaining requests 

flatMap 

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The ViewModel 

typealias Border = (name: String, nativeName: String) class BordersViewModel { let borders: Observable<[Border]> ... } 

self.borders = client.countryWithName(countryName) // Get the countries corresponding to the alpha codes // specified in the `borders` property .flatMap { country in client.countriesWithCodes(country.borders) } // Catch any error and print it in the console .catchError { error in print("Error: \(error)") return Observable.just([]) } // Transform the resulting countries into [Border] .map { countries in countries.map { (name: $0.name, nativeName: $0.nativeName) } } // Make sure events are delivered in the main thread .observeOn(MainScheduler.instance) // Make sure multiple subscriptions share the side effects .shareReplay(1) 

The View(Controller) 

private func setupBindings() { ... viewModel.borders .bindTo(tableView.rx_itemsWithCellFactory) { tableView, index, border in let cell: BorderCell = tableView.dequeueReusableCell() cell.border = border return cell } .addDisposableTo(disposeBag) } 

Questions? Comments? @gonzalezreal https://github.com/gonzalezreal/Borders http://tinyurl.com/consuming-web-services