How do I make `Flow.transformLatest()` complete when upstream completes? ```upstream .transformLa...

# getting-started

[Daniele Segato]

How do I make Flow.transformLatest() complete when upstream completes?

upstream
  .transformLatest { state ->
    emitAll(dataForState(state))
  }

say upstream is

currentState
  .takeWhile { state -> isCloseNowState(state) }

This would be easy if I had control of it, I could use transformWhile , what if I don't?

How do I cancel dataForState when upstream completes?

Actually, this is challenging even when I use transformWhile() cause it is not cancelling the previous one I'd need a transformLatestWhile

[Dominaezzz]

I believe there is a transformWhile.

[Daniele Segato]

Sure, what if the

observeState
  .takeWhile { state -> isStopState(state) }

comes from upstream / a function I've no control in?

[Dominaezzz]

Oh nvm, I think just understood the problem.

[Daniele Segato]

In other words, how do I tell transformLatest to stop when upstream completes?

[Daniele Segato]

You are right, transformWhile works perfectly if you control all the chain. I should have asked the question better

[Dominaezzz]

You can use onCompletion and emit a cancellation token.

[Daniele Segato]

@Dominaezzz Actually, even with transformWhile is not possible
what I would need is a transformLatestWhile
(see my modified question)

[Dominaezzz]

Yeah I realized.

[Dominaezzz]

Cancellation token could just be null I guess.

[Daniele Segato]

I did something like this

upstream
  .map { it as MyState? }
  .onComplete { emit(null) }
  .transformLatest { state ->
    if (state != null) {
      emitAll(dataForState(state)
    }
  }

if I had a nullable emission I could have build a wrapper and did the same

[Dominaezzz]

Nice