When it comes to the `LaunchedEffect` side effect ...

# compose

[Thomas]

When it comes to the LaunchedEffect side effect I usually see two patterns,
Pattern 1:

LaunchedEffect(state.value){
    doSomething()
}

Pattern 2:

LaunchedEffect(state){
    snapshotFlow{ state.value }.collect{ doSomething() }
}

1. When do we prefer using one over the other?
2. Are there difference in the two patterns from the point of view of the doSomething function?
3. Would it be different if doSomething takes in state.value as argument for pattern 1, and it in pattern 2?

[m.c.shin]

I guess that state is defined as val, so Pattern 1 is right.

[Thomas]

It seems that they pretty much behave the same, its just that pattern 2 allows us to transform a compose's State into a Flow ... Is this correct?

[Thomas]

@m.c.shin. https://developer.android.com/develop/ui/compose/side-effects#snapshotFlow

The official guide uses a val State object as the key for LaunchedEffect

I am curious about this too, whats wrong with just passing Unit when the state variable is val?

[Fergus Hewson]

Unit if you only want it to fire once, state.value if you want it fire every time the state changes.

[Thomas]

@Fergus Hewson in my example the body of the LaunchedEffect is followed immediately by a snapshotFlow , such that the collect block is fired every time the state changes

[Thomas]

So if I understand correctly, the snapshotFlow is collected on "enter composition", and is alive until the parent composable exits composition

[Fergus Hewson]

Yeah, you probably only want to collect this once. Unless there is something reason you want to re sub to the flow ... it is a new flow?

[m.c.shin]

@Thomas,
In android official guide case, It will be same between using key to listState and Unit.

[Thomas]

@Fergus Hewson I'm just investigating the behavior of side effect, you could imagine the doSomething function to be something like sending analytics information 🤔

[Thomas]

(slighlty edited question for clarity)

[Thomas]

I am really curious if there is difference in Pattern 1 and Pattern 2 when it comes to the behaviour of doSomething funciton invocations 👀

More specifically I wonder if there are "gotchas" to pattern 2

[m.c.shin]

I think that both are almost same, but there has different that timing at doSomething() function called.

[m.c.shin]

Pattern1 :
state.value changed -> recomposing -> launchedEffect re-called -> doSomething()

Pattern2 :
state.value changed -> flow collected -> doSomething()

[m.c.shin]

Considering timing, pattern2 seems to be better.

[Thomas]

yeah that is how understand it would work too, but in both cases, isn't the doSomething task handled basically the same by the CoroutineDispatcher?

If I am not mistaken, the CoroutineDispatcher of both the LaunchedEffect block and the snapshotFlow block is exactly the same as the one used by the Recomposer, therefore they should behave the same? 🤔

[m.c.shin]

You are right about which CoroutineDispatcher it runs on. The only difference is the timing at doSomething() function is called.

[m.c.shin]

Pattern1 case depends on recomposing, and Pattern2 is not.

[Thomas]

Oh right! In pattern 2 the parent composable did not make any "read" attempts to the state.value , therefore the parent composable was never observing changes to the state. Because of this the parent don't necessarily have to recompose when state.value changes

[Ben Trengrove [G]]

The second pattern is much better from a performance perspective, pattern 1 you are both causing recomposition of the parent scope and reallocating and launching a new coroutine scope every time state changes, very expensive.

Pattern 2 you launch once, and then just continue to observe. Much cheaper

[Ben Trengrove [G]]

As for the key, you should key with state rather than Unit because you need to relaunch if the actual state holder class changes (not the value in it), or else you would still be observing the old one

[Thomas]

In retrospect that was such a stupid question... I simply had to think in terms of recomposition scope and remember retainment lifetime to see the issues 🐵

Thanks again for all the answers!

[Viktor Nyblom]

@Thomas, two things:

1. no such thing as a stupid question.
2. I also relearned the recomposition lesson, so you asking this helped me too. 😊