Flow/Coroutines question: I'm trying to migrate from RxJava to Flow for Android apps. I often write ...

# coroutines

[alexsullivan114]

Flow/Coroutines question: I'm trying to migrate from RxJava to Flow for Android apps. I often write apps where in some starting point of the app (onCreate or the ViewModel init function for example) I subscribe to a bunch of observables. So, a typical init of a view model of mine may look like this:

init {
    myObservable
        .filter { whatever() }
        .map { whatever() }
        .subscribe { pushToMySubject() }

    myOtherObservable
        .filter { whatever() }
        .map { whatever() }
        .subscribe { pushToMyOtherSubject() }

    myThirdObservable
        .filter { whatever() }
        .map { whatever() }
        .subscribe { pushToMyThirdSubject() }
}

But when using flow, we can only call collect (which if I'm not mistaken is analagous to subscribe in RxJava) from within a coroutine context, which makes sense. But, as far as I can tell, calling collect blocks that coroutine context (my terminology might be garbage here) so you can't really have the above setup because we wouldn't execute the last two subscribes until the first observable completes.

So two questions:

1. Is my understanding correct?
2. What's the preferred path to handle this sort of scenario?

[octylFractal]

are these intended to live beyond init?

[alexsullivan114]

yes

[alexsullivan114]

Again this is getting into Android specifics, but there's a coroutine context scoped to the viewmodels lifecycle that the Android APIs expose

[octylFractal]

I think the correct method here is viewModelScope.launch

[alexsullivan114]

yeah that's what I'm using

[octylFractal]

you would launch for each thing you want to collect

[alexsullivan114]

Got it - so for the above flow, we'd want three different calls to launch

[octylFractal]

yea

[alexsullivan114]

word. Thank you!

[Dominaezzz]

You can also call .launchIn(viewModelScope) on the flow itself.

[alexsullivan114]

Awesome tip, thanks Dominic!

[elizarov]

https://kotlinlang.org/docs/reference/coroutines/flow.html#launching-flow

[alexsullivan114]

So if you use launchIn you really need to use onEach or some other side effect operator to actually process your items. Interesting.

[elizarov]

Yes. It is just a DSL. Instead of

compositeDisposable.add(..... .subscribe { ... })

you do

.... .onEach { ... }.launchIn(scope)

You use scope instead of compositeDisposable to track subscriptions and make sure they are closed.