Example Source Code for "A Simple Approach to Thread-Safe Networking in iOS Apps"

SimpleNetworkingExample.swift

import Foundation
import PlaygroundSupport

enum URLResult {
  case response(Data, URLResponse)
  case error(Error, Data?, URLResponse?)
}

extension URLSession {
  @discardableResult
  func get(_ url: URL, completionHandler: @escaping (URLResult) -> Void) -> URLSessionDataTask {
    let task = dataTask(with: url) { data, response, error in
      switch (data, response, error) {
      case let (data, response, error?):
        completionHandler(.error(error, data, response))
      case let (data?, response?, nil):
        completionHandler(.response(data, response))
      default:
        preconditionFailure("expected either Data and URLResponse, or Error")
      }
    }
    task.resume()
    return task
  }

  @discardableResult
  func get(_ left: URL, _ right: URL, completionHandler: @escaping (URLResult, URLResult) -> Void) -> (URLSessionDataTask, URLSessionDataTask) {
    precondition(delegateQueue.maxConcurrentOperationCount == 1,
      "URLSession's delegateQueue must be configured with a maxConcurrentOperationCount of 1.")

    var results: (left: URLResult?, right: URLResult?) = (nil, nil)

    func continuation() {
      guard case let (left?, right?) = results else { return }
      completionHandler(left, right)
    }

    let left = get(left) { result in
      results.left = result
      continuation()
    }

    let right = get(right) { result in
      results.right = result
      continuation()
    }

    return (left, right)
  }
}

extension URLResult {
  var string: String? {
    guard case let .response(data, _) = self,
      let string = String(data: data, encoding: .utf8)
      else { return nil }
    return string
  }
}

let zen = URL(string: "https://api.github.com/zen")!

URLSession.shared.get(zen, zen) { left, right in
  guard case let (quote1?, quote2?) = (left.string, right.string)
    else { return }

  print(quote1, quote2, separator: "\n")
  // Approachable is better than simple.
  // Practicality beats purity.
}

PlaygroundPage.current.needsIndefiniteExecution = true