Using Combine for making asynchronous network requests

APIClient

Here in Combine we have access to dataTaskPublisher and decode() wrappers which makes our networking code much more functional than using closures.

We use .map(\.data) keypath to access the data property from the data task arguments, namely response, data and error.

Since our publisher returns an array of stations we use map again .map { $0.data.stations } to get the stations array.

We handle dispatching back to the main thread in the APIClient so the view controller does not have to do this work.

.eraseToAnyPublisher hides the implementation detail from the client code.

class APIClient {
  func fetchData() throws -> AnyPublisher<[Station], Error> {
    let endpoint = "https://gbfs.citibikenyc.com/gbfs/en/station_information.json"
    guard let url = URL(string: endpoint) else {
      throw AppError.badURL(endpoint)
    }
    return URLSession.shared.dataTaskPublisher(for: url)
      .map(\.data)
      .decode(type: ResultsWrapper.self, decoder: JSONDecoder())
      .map { $0.data.stations }
      .receive(on: DispatchQueue.main)
      .eraseToAnyPublisher()
  }
}

View Controller subscribing to the fetchData() publisher

We subscribe to the publisher and use .sink to receive the emitted values from the publisher.

We store our subscribers in the case of the need to cancel a subscription .store(in: &subscriptions.

private var subscriptions: Set<AnyCancellable> = []

private func fetchData() {
  do {
    let publisher = try apiClient.fetchData()
    publisher
      .sink(receiveCompletion: { (completion) in
        print(completion)
      }, receiveValue: { [weak self]  (stations) in
        self?.updateSnapshot(with: stations)
      })
    .store(in: &subscriptions
    )
  } catch {
    print(error)
  }
}

More resources on Combine

  1. Introducing Combine – WWDC 2019
  2. Combine in Practice – WWDC 2019
  3. Using Combine
  4. Apple docs – Combine
  5. Apple docs – Processing URL Session Data Task Results with Combine

Complete project on Github

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