Combine with URLSession.dataTaskPublisher

combine-urlsession.swift

// https://twitter.com/BelleBCooper/status/1192173933983715328

import Combine
import Foundation
import PlaygroundSupport

PlaygroundPage.current.needsIndefiniteExecution = true

/// JSON format:
///
///     {
///       "userId": 1,
///       "id": 1,
///       "title": "delectus aut autem",
///       "completed": false
///     }
struct Todo: Decodable {
  var userId: Int
  var id: Int
  var title: String
  var completed: Bool
}

let url = URL(string: "https://jsonplaceholder.typicode.com/todos/1")!

// Variant 1
// This works by not using the .decode publisher at all.
let subscription1 = URLSession.shared
  .dataTaskPublisher(for: url)
  .print("1-dataTask")
  .tryMap { result -> (model: Todo, response: URLResponse) in
    let model = try JSONDecoder().decode(Todo.self, from: result.data)
    return (model, result.response)
  }
  .print("1-decoded")
  .sink(receiveCompletion: { _ in }, receiveValue: { _ in })

// Variant 2
let dataTaskPublisher = URLSession.shared
  .dataTaskPublisher(for: url)
  .print("2-dataTask")
  // Prevent the network request from executing twice
  // See http://www.cocoawithlove.com/blog/twenty-two-short-tests-of-combine-part-2.html
  .multicast { PassthroughSubject() }
  .autoconnect()
let responsePublisher = dataTaskPublisher.map { $0.response }.mapError { $0 as Error }
let modelPublisher = dataTaskPublisher
  .map { $0.data }
  .decode(type: Todo.self, decoder: JSONDecoder())
  .mapError { $0 as Error }
let subscription2: AnyCancellable = responsePublisher.zip(modelPublisher)
  .print("2-zipped")
  .sink(receiveCompletion: { _ in }, receiveValue: { _ in })