Where should I read lazy column item offset?

Barrufet 0
Viewed 31

Basically I have a launched effect reading the item offset of a lazy column. and based on the offset I change the height/alpha etc..

AS is warning me that it should not be read inside a compose function because it will change a lot, so my question is where should I be reading and what is the best practice here? Should I make a lambda updating the offset outside of the function and read in the highest one? Or the parent one?

Open to suggestions

AS Warning

android-jetpack-compose android
1 Answers

You can read it inside a derivedStateOf.

  val someData = remember {
      derivedStateOf { 
          val offset = lazyListState.firstVisibleItemScrollOffset
          val firstVisibleItem = lazyListState.firstVisibleItemIndex
          // Convert to some data here and read this data
      }
  }

I did similar thing in this library for animating color, scale and alpha of items with

val animationData by remember {
    derivedStateOf {
        val animationData = getAnimationProgress(
            lazyListState = lazyListState,
            initialFirstVisibleIndex = initialFirstVisibleIndex,
            indexOfSelector = indexOfSelector,
            itemScaleRange = itemScaleRange,
            showPartialItem = showPartialItem,
            globalIndex = globalIndex,
            selectedIndex = selectedIndex,
            availableSpace = availableSpace,
            itemSize = itemSizePx,
            spaceBetweenItems = spaceBetweenItems,
            visibleItemCount = visibleItemCount,
            totalItemCount = totalItemCount,
            inactiveScale = inactiveItemScale,
            inactiveColor = inactiveColor,
            activeColor = activeColor
        )

        selectedIndex = animationData.globalItemIndex
        animationData
    }
}

If you read a change you can read LaunchedEffect with snapshotFlow either but as i checked same lazyListState is returned so it's not possible to read any change but you can read lazyListState.firstVisibleItemScrollOffset or any changing value as alternative to derivedStateOf.

For instance

LaunchedEffect(Unit){
    snapshotFlow { lazyListState.firstVisibleItemScrollOffset }
        .onEach { 
            // Here we get the change in offset of first visible item
            
            // You might read another value here and do calculation
            lazyListState.firstVisibleItemIndex
        }.launchIn(this)
}

I think reading lazyListState wouldn't pose any problems either. I use this when i only read one value like firstVisibleItemScrollOffset, or layoutInfo inside snapshotFlow

Thracian0
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