Is there a way to report a value change in the exa...

# compose

[dimsuz]

Is there a way to report a value change in the example below assuming I have no way to modify State?

class State(
  val value by mutableStateOf(false)
)

fun Component(state: State, reportValueChange: (Boolean) -> Unit)) {
  val v = state.value 
  // how to call reportValueChange(v) here
  // **only** when "v" changes?
}

I have tried snapshotFlow { state.value }.collect(::reportValueChange) but it emits nothing on value changes

[Adam Powell]

Generally you want to avoid using composition as a change event dispatcher. Why is Component the thing responsible for telling the caller when state changes, when the caller is the one providing state to Component?

[Adam Powell]

How have you tested your snapshotFlow?

[dimsuz]

My component uses the Swipeable modifier.

When swipe finishes and settles I want to notify other parts of code about this. usecases: orchestration of some further ui changes or analytics.

And SwipeableState has isAnimating: Boolean and that's how I plan to detect that swipe has finished: when isAnimating changes from true to false.

There's a confirmStateChange labmbda, but it's invoked too soon, prior to swipe start.

[dimsuz]

As for snapshotFlow I have tried (inside the launched effect)

snapshotFlow { swipeableState }.collect { log(it.isAnimating) }
// and
snapshotFlow { swipeableState.isAnimating }.collect { log(it) }

but the both emit the single value only

[dimsuz]

I'll ask a more specific question in a separate thread

[Zach Klippenstein (he/him) [MOD]]

Can you share more code around where you're calling collect? That looks right to me, and I thought I'd even written similar code in the past and it worked.

[dimsuz]

val swipeableState = rememberSwipeableState(initialValue = findInitialAnchor())
Timber.e("is animating ${swipeableState.isAnimationRunning}")
val flow = snapshotFlow { swipeableState.isAnimationRunning }
LaunchedEffect(Unit) {
  flow.collect {
    Timber.e("animated changed to ${it}, state now is ${swipeableState.currentValue}")
  }
}

It's something like this.

[Adam Powell]

this test passes:

@Test
fun snapshotFlowTest(): Unit = runBlocking {
    class StateClass {
        var value by mutableStateOf(false)
    }

    suspend fun <T> Channel<T>.receiveWithTimeout(): T = withTimeout(1000) { receive() }

    val instance = StateClass()

    val ch = Channel<Boolean>(Channel.BUFFERED)
    val job = launch(start = CoroutineStart.UNDISPATCHED) {
        snapshotFlow { instance.value }.collect { ch.send(it) }
    }

    assertFalse("initial value", ch.receiveWithTimeout())

    Snapshot.withMutableSnapshot {
        instance.value = true
    }

    assertTrue("after change 1", ch.receiveWithTimeout())

    Snapshot.withMutableSnapshot {
        instance.value = false
    }

    assertFalse("after change 2", ch.receiveWithTimeout())

    job.cancel()
}

so yes, I'm also curious what's different

[dimsuz]

As I swipe my above example prints a lot of changes from true to false for the first Timber call and only one false for the second

[dimsuz]

hmm. maybe I should try to come up with a more minimal example then

UPD: My code was wrong, Zach helped me in another thread. I didn't provide a key to the effect and was recreating flow on each recompose.

[Chris Johnson]

@dimsuz Hey. Quick question. So you mean if you don't pass a key into `LaunchedEffect(key)`except for Unit or `true`it will call the code inside your launchedEffect on every recomposition? I thought until you changed your key, it would keep the same LaunchedEffect?

[dimsuz]

Oops, I wasn't clear enough above. What I meant was that I did 2 things wrong: 1. provided a wrong key (Unit) 2. created the flow outside of the effect body.

LaunchedEffect body will be called as you've said: on each key change, not on each recomposition. If you provide Unit or (literal) true it will be called only once.

[Chris Johnson]

Awesome. That's what I was thinking. Thanks for confirming!

[Zach Klippenstein (he/him) [MOD]]

I could not repro the behavior you had initially either, i’m getting all the animation state updates from my snapshotFlow even if I create it outside the effect, don’t remember it, and pass Unit as the effect key.

[dimsuz]

Hmmm. I may have had another key mixed into LaunchedEffect at the time I had this problem: I was trying different things all at once. If I'll manage to catch this again, will try a minimal example and post here.
Thanks for looking into this, after you hints everything works fine for me!