Using async/await concurrency on iOS 14 and before

main.swift

// Created by Helge Heß 2021-06-17

import Foundation

// They use obfuscated names to hide it from us!
import JavaScriptCore

/// Setup our async/await runtime.
let runtime = JSContext()!

/// Ask our async/await runtime to tell us about errors.
runtime.exceptionHandler = { _, error in
  print("Caught exception:", error as Any)
}

/// Add a global `print` function to our async/await runtime
runtime.setObject(
  {()->@convention(block) (JSValue)->Void in { print($0) }}(),
  forKeyedSubscript: "print" as NSString
)


/// We need this protoocol to tell our async/await runtime about URLSession.
@objc protocol AsyncURLSession: JSExport {
  
  /// Give access to `URLSession.shared`
  @objc(shared) // overlap w/ `sharedSession` on the ObjC side
  static var sharedSwift : URLSession { get }
  
  /// Our async/await enabled URL fetcher,
  /// returns an async error or a [ data, response ] tuple.
  func data(_ url: String) -> JSValue
}

/// Here we implement our async aware function.
@objc extension URLSession: AsyncURLSession {
  
  dynamic class var sharedSwift : URLSession { shared }

  func data(_ url: String) -> JSValue {
    guard let ctx = JSContext.current() else { fatalError("No runtime") }
    
    /// Create our continuation
    return JSValue(newPromiseIn: ctx) { resolve, reject in
      guard let url = URL(string: url) else {
        reject?.call(withArguments: [ "invalidURL" ]); return
      }
      
      self.dataTask(with: URLRequest(url: url)) { data, response, error in
        RunLoop.main.perform {
          if let error = error {
            reject?.call(withArguments: [ error.localizedDescription ])
          }
          else if let data = data, let response = response {
            resolve?.call(withArguments: [ [ data, response ] ])
          }
          else {
            reject?.call(withArguments: [ "missingResponse" ])
          }
        }
      }
      .resume()
    }
  }
}
runtime.setObject(URLSession.self, forKeyedSubscript: "URLSession" as NSString)
runtime.evaluateScript("URLSession.shared = URLSession.shared();")


/// Finally, execute our fetch using async/await.
runtime.evaluateScript(#"""

  async function mainActor() {
    let [ data, res ] =
      await URLSession.shared.data("https://zeezide.com/#dontTrackMe")
    
    print(data)
    print(res)
  }
  
  mainActor()
"""#)


/// Keep it alive
RunLoop.main.run()

More information is available in the Async/Await for iOS 14 and before blog post.