Hi, I try ti use `Single<T>.doOnAfterSuccess...

# reaktive

[lehakorshun]

Hi, I try ti use Single<T>.doOnAfterSuccess , as doc says

Calls the action with the emitted value when the Single signals onSuccess. The action is called after the observer is called.

but my action is called first, before onSuccess in subscriber. what can be wrong?

[Arkadii Ivanov]

Hello. I think we will need a reproducer. The operator is covered with test, should not be any issues: https://github.com/badoo/Reaktive/blob/d0df73a4bbc3f47343541c645949f6c10e2b2cfb/reaktive/src/commonTest/kotlin/com/badoo/reaktive/single/DoOnAfterSuccessTest.kt#L36

[lehakorshun]

I had next code

singleOf(getState().check(intent.words))
                        .subscribeOn(mainScheduler)
                        .map { acceptedWords -> return@map (getState().acceptedWords + acceptedWords) }
                        .doOnAfterSuccess { if (getState().goalCount <= it.size) publish(TrainingStore.Label.Complete) }
                        .map { acceptedWords -> Result.Accepted(acceptedWords) }
                        .observeOn(mainScheduler)
                        .subscribeScoped(
                            isThreadLocal = true,
                            onSuccess = { result -> dispatch(result) }
                        )

and I receive Complete label before I receive result from onSuccess

[lehakorshun]

but sorry, problem is in the lambda, which is return wrong state

[Arkadii Ivanov]

This is expected. You have observeOn before subscribe and so onSuccess call is scheduled asynchronously. doOnAfterSuccess is called after map below it, but before onSuccess, which is scheduled.

[lehakorshun]

hm… thx, but it is not obviously for me 😞

[lehakorshun]

when I removed observeOn it is help

[lehakorshun]

thx a lot 🙂

[Arkadii Ivanov]

It might not be very obvious indeed. As mentioned in the docs, the operator's callback is called after the observer's onSuccess. In this case the observer is the map operator below it.

[lehakorshun]

is it happing because we have observeOn operator?

[lehakorshun]

because I tried to move doOnAfterSuccess after map operator, it was not help

[Arkadii Ivanov]

By default all operators are synchronous, only those which accept Scheduler can do something asynchronously. So when doOnAfterSuccess 's upstream signals onSuccess , the value is received by doOnAfterSuccess, which firstly calls its downstream Observer (in this case it is the map operator right below), and when the call returns the doOnAfterSuccess callback is called. What happens in the downstream:
• the map operator receives the onSuccess value, maps it with the provided mapper function and calls its downstream Observer - the observerOn operator below the map operator, which is below the doOnAfterSuccess operator
• the observeOn operator schedules onSuccess emission via the provided Scheduler and returns the onSuccess call
• the map operator also returns the onSuccess call
• the doOnAfterSuccess operator now calls the callback
• the subscribe-onSuccess below observeOn is called at some point later.
Since in the provided call everything runs on the main thread, the subscribe/onSuccess will be called only when the wlole chain is finished, and so the main thread unblocked.
In case of different schedulers, the subscribe/onSuccess would be called concurrently.

[lehakorshun]

Thx for describing:)