Some questions about async await threading in Swift's new concurrency model.

swift-async-await-example.swift

class ViewController: UIViewController {
    override func viewDidLoad() {
        super.viewDidLoad()
        
        Task {
            // 1️⃣❓ UIViewController is in a MainActor context, so this Task
            // will inherit that, so the following pretend expensive call will
            // be on the main thread and likely block?
            ExpensiveOperationPerformer.doExpensiveLoopAndPrint()
        }

        Task.detached {
            // 2️⃣❓ Is this guaranteed to be off the main thread, so perhaps a
            // better way to do a one-off, expensive operation? If it's not
            // guaranteed, how would I ensure that? Wrap it in an actor 
            // instead of a class? What if it's not my class/I can't 
            // change the code?
            ExpensiveOperationPerformer.doExpensiveLoopAndPrint()
        }

        Task {
            ExpensiveOperationPerformer.printSomeNetworkData()
        }
    }
}

class ExpensiveOperationPerformer {
    @MainActor
    static func printSomeNetworkData() async throws {
        let url = URL(string: "https://example.com/example.json")!
        
        // 3️⃣❓ Will this time consuming network call be guaranteed to NOT
        // execute on main, despite the MainActor context? Or will it block?
        let (data, response) = try await URLSession.data(for: url)
        print(data)
    }

    static func doExpensiveLoopAndPrint() async {
        let upperEnd = 9_999_999_999
        var sum = 0

        for i in 0 ..< upperEnd {
            sum += 1
        }

        print(sum)
    }
}

In that case the downloading (expensive) is done on a background thread though, right, and the "updating the delegate" (cheap) is handled on the main thread? Or is everything truly done on the main thread and it basically does something akin to a Task.yield() while it waits for the next bytes to arrive? Or perhaps I don't understand how modern networking is done on iOS and it's a separate system entirely?

In that case the downloading (expensive) is done on a background thread though, right, and the "updating the delegate" (cheap) is handled on the main thread?

I'm quite sure no other thread in your app is involved. The actual download is being performed by the networking subsystem of the OS (however that works) and is being managed by the kernel (I think). When new data becomes available, the kernel informs your app that new data on the socket is available. No multithreading required.

Or perhaps I don't understand how modern networking is done on iOS and it's a separate system entirely?

NSURLConnection(request:delegate:) isn't modern networking, but I'd imagine the general principle would be the same for URLSession. The difference might be (I don't know) that URLSession could be using GCD dispatch sources or similar (instead of the run loop) to handle notifications from the socket and then calls your callbacks on a background queue (instead of the thread that started the connection).

Thank you so much for taking the time to explain all this! I've learned a lot! ❤️

❤️

Correction: I tried to prove my claims with some sample code and it turns out NSURLConnection does use a separate thread for something. See the screenshot: when I set a breakpoint in one of the delegate calls, there is a com.apple.NSURLConnectionLoader thread:

It's possible this thread does things like setting up the connection, HTTP parsing, etc. This doesn't invalidate everything I said above because it's definitely possible to write asynchronous networking code without any sort of multithreading (e.g. by using the socket APIs directly), but apparently that's not what NSURLConnection does. Sorry for leading you on the wrong track.

You can also see in the screenshot in the main thread's stack trace that it's using the run loop to invoke the delegate. __CFRunLoopDoSource sounds like it's invoking a callback for a run loop source that has new events (the socket).

Here's my code (macOS Command Line Project in Xcode):

import Foundation

final class Loader: NSObject, NSURLConnectionDataDelegate {
  var connection: NSURLConnection? = nil

  func start() {
    let url = URL(string: "https://google.com")!
    connection = NSURLConnection(request: URLRequest(url: url), delegate: self)
  }

  func connection(_ connection: NSURLConnection, didReceive data: Data) {
    print(#function, data)
  }

  func connection(_ connection: NSURLConnection, didFailWithError error: Error) {
    print(#function, error)
  }
}

let loader = Loader()
loader.start()

RunLoop.current.run()