Combine_subscribeOn.swift

let backgroundQueue = DispatchQueue.global(qos: .background)

Publishers.Just(1)
    .map { _ in
        print(Thread.isMainThread) // true
    }
    .subscribe(on: backgroundQueue) 
    .map { _ in
        print(Thread.isMainThread) // false
    }.sink {
        print(Thread.isMainThread) // false