CombinaMainQueueQuestion.swift

import Foundation
import PlaygroundSupport
import Combine

PlaygroundPage.current.needsIndefiniteExecution = true

let queue = DispatchQueue(label: "some")
_ = Future<String, Never> { promise in
    queue.asyncAfter(deadline: .now() + 2, execute: {
        print(Thread.current)
        promise(.success("TADA"))
    })
}
.subscribe(on: DispatchQueue.main)
.eraseToAnyPublisher()
.sink(receiveValue: { action in
    print(action)
})

_ = Future<String, Never> { promise in
    queue.asyncAfter(deadline: .now() + 2, execute: {
        print(Thread.current)
        promise(.success("NE TADA"))
    })
}
.eraseToAnyPublisher()
.sink(receiveValue: { action in
    print(action)
})

print("Wait...")

/*
> Wait...
> <NSThread: 0x600001c011c0>{number = 6, name = (null)}
> TADA
> <NSThread: 0x600001c011c0>{number = 6, name = (null)}
*/