Do I have place upload or download tasks onto a background thread or does Swift do it for me? - swift

Question

I am unsure about the thread/queue control in Swift for HTTP methods.
Working on the part of my app that makes GET requests, as well as POST and others, from a 3rd party server.
From my main UIViewController, I initialize the class with:
let uploadService = UploadService()
lazy var uploadSession: URLSession = {
let configuration = URLSessionConfiguration.default
return URLSession(configuration: configuration, delegate: self, delegateQueue: .main)
}()
Then later on I call it from the same UIViewController, let uploadService = UploadService()
Before I start to add more functionality to the class below I wanted to ask about:
return URLSession(configuration: configuration, delegate: self, delegateQueue: .main)
When the UploadService class calls methods to my UIViewController, Xcode warned me to do it from the main thread/queue, which I do. So my questions is, does Swift automatically move anything HTTP related to the non-main thread/queue so I don't have to worry about it? Because if that's the case, great. But then why ask me to place it in the main thread/queue, as I did up above? (which I did from a tutorial).
Just want to be clear, should I be declaring it the background thread/queue, or does Swift handle that for me regardless of how I declared it up above?
class UploadService {
var uploadSession = URLSession.shared
func start(upFile: XFile, script: String, upLoadInvoiceClass: UploadInvoice) {
var request = upFile.makeUrlReq(upFile: upFile, script: script)
uploadSession.uploadTask(with: request, from: request.httpBody )
{ (data, response, error) in
if let response = response {
upLoadoiceClass.upResp(resp: response)
}
if let error = error {
upLoadoiceClass.upErr(error: error)
}
if let data = data {
upLoadoiceClass.upData(data: data)
}
}.resume()
}
}

Answer 1

URLSession ALWAYS does network interactions on a background thread. You don't have to worry about that.
The completion handlers/delegate methods are also run on a background thread by default. That lets you do time-consuming processing on the resulting data without tying up the main thread.
Unless you point give your URL session a different delegate queue when you create it, you should wrap any UI code you put in your completion handlers/delegate methods in a call to the main thread.
Alternately, you can create a URLSession and give it a foreground queue in the delegateQueue parameter you pass in the initializer. If you do that then your completion handlers/delegate methods will be run on that foreground queue (and thus run on the main thread.) If you do that you don't need to explicitly wrap UIKit calls in a call to the main thread, but you will stall the UI if you do time-consuming work in your URLSession completion handlers/delegate methods