How do I execute onSubscribe action of do operator in Main thread - rx-swift

Question

I execute UI related operation in the onSubscribe action of do operator. I obtain an
Main Thread Checker: UI API called on a background thread
console error in Xcode. I tried to use observe(on: MainScheduler.instance) but without success.
This my snippet:
checkCurrentNetworkAndConnect().observe(on: MainScheduler.instance).do(onSubscribe: {
// UI related stuff
})
I also tried with subscribe(on: MainScheduler.instance) but doesn't work.
How can I achieve this?

Answer 1

This will happen if you are calling subscribe on a background thread. If you call subscribe on a background thread, then the Observable will be subscribed to on that background thread. To solve this, you will need to use subscribe(on:) after the do operator...
Something like this:
func example() {
checkCurrentNetworkAndConnect()
.do(onSubscribe: {
print("UI related stuff")
})
.subscribe(on: MainScheduler.instance)
.subscribe()
}
With the above, even if you call example() on a background thread, the onSubscribe: closure will be executed on the main thread.
If for some reason you also want the checkCurrentNetworkAndConnect() on a background thread, then you can do that with another subscribe(on:) above the do.
All this said, you should not be performing significant side effects inside a do(onSubscribe:) in the first place. It's one thing to put debug code in there but not anything more than that. I've been using RxSwift for 6 years and I've never needed to use a do(onSubscribe:) at all, much less for significant code. There is likely a better way to accomplish the ultimate goal you are trying to accomplish.
In response to your comment. How you go about doing what you want depends very much on specifics not provided. Below I have made some simplifying assumptions. If your problem is different, I suggest you post a new question with the details...
func example(operations: [Observable<Void>], messages: [String], label: UILabel, disposeBag: DisposeBag) {
Observable.concat(operations.enumerated().map { index, op in
op.map { messages[index + 1] }
})
.startWith(messages[0])
.observe(on: MainScheduler.instance)
.bind(to: label.rx.text)
.disposed(by: disposeBag)
}
The code above assumes:
that you have an array of operations expressed as observables
that none of them need data provided by a previous operation
that none of them emit data you care to use or keep
that every operation has a message that needs to be displayed before the operation starts
that there is an additional message to be displayed when the last operation completes