Create a page for threading

[AvdLee]

Using subscribeOn and receiveOn from the receiveOn documentation https://developer.apple.com/documentation/combine/anysubject/3204343-receive

let jsonPublisher = MyJSONLoaderPublisher() // Some publisher.
let labelUpdater = MyLabelUpdateSubscriber() // Some subscriber that updates the UI.

jsonPublisher
    .subscribe(on: backgroundQueue)
    .receiveOn(on: RunLoop.main)
    .subscribe(labelUpdater)