If I have an expensive calculation that I need to ...

# compose

[Colton Idle]

If I have an expensive calculation that I need to run in a composable, then I almost always instinctively reach for remember.

But wouldn't a launched effect also serve a similar purpose if I only want something to recompute if x changes?

i.e.

val someValue = remember(x) { someHeavyCalculation() }

vs something like

val someValue = null

LaunchedEffect(x){
  someValue = someHeavyCalculation()
}

[myanmarking]

Does launched effect runs right away in the same frame, or just in the next? Not sure

[myanmarking]

I think the default dispatcher is .immediate so it might work. But relying on a specification like that is not good i think

[Filip Wiesner]

someValue would have to be mutableStateOf for this to work (apart from it being val instead of var)

[Filip Wiesner]

Does launched effect runs right away in the same frame

Correct me if I'm wrong but I think that LaunchedEffect and all other effects run after successful composition so all changes made in any effect would be only visible in next frame.

[myanmarking]

Thats my doubt as well. You might be right

[Filip Wiesner]

But maybe the "_effects run after successful composition_" part applies only to SideEffect. I am actually not sure now 😅

[vide]

A compose dev will have better insight but I think it applies exclusively to SideEffect. I think LaunchedEffect runs interleaved with the composition 🤔

[vide]

At least trying to call a focus requester from a LaunchedEffect will probably fail as its not in the tree yet

[Stylianos Gakis]

No I am fairly certain that the LaunchedEffect lambda runs after the first successful composition. Which is why you sometimes see people using DisposableEffect with an empty onDispose because they want this to run instantly and not with 1 frame delay.
Relevant thread here
https://kotlinlang.slack.com/archives/CJLTWPH7S/p1622560036078600?thread_ts=1622556885.065600&cid=CJLTWPH7S
and here https://kotlinlang.slack.com/archives/CJLTWPH7S/p1674253277230959?thread_ts=1673947731.751509&cid=CJLTWPH7S
and here https://kotlinlang.slack.com/archives/CJLTWPH7S/p1650034947332099?thread_ts=1650031631.129979&cid=CJLTWPH7S

[vide]

TIL! Thanks for the clarification

[vide]

As a tangential discussion, any idea why calling focusrequesters fails from a LaunchedEffect ? I would imagine the nodes are registered in the tree by the time the previous frame is done