While taking the `Pathway` first code lab `Compose Basics` s

Question

While taking the `Pathway` first code lab `Compose Basics` section `Animating your list` I have noticed that `LazyColumn` doesn t remember the `isSelected` state of the item when recomposing I have copy pasted the code from the `Pathway` and got the same results Just when you scroll until the item is no more visible when you scroll back the `isSelected` state is lost IMO this is wrong cuz the state should be cached anyway and when recomposing the compiler should consider the previous state Should I `Hoist` the state of the `Greeting` widget so I can manage it in the `LazyColumn` scope or this is just a Compose `Bug` thinking face Thanks in advance

jim · Answer

Hoisting state is never a bad idea wink

Issa · Answer

Well that s not efficient in my opinion confused even though it seems like a better idea however a map is needed to sync the state of the items and their `isSelected` state gt added complexity

Issa · Answer

< jim> So here is what I did I hoisted the state to the caller scope `NamesList` in this case Now `LazyColumn` is not recomposing the items if the `state` changes It only does when the item is being visible on the screen again ``` Composable fun NameList names List lt String gt modifier Modifier = Modifier val selectedStates = remember mutableStateOf mutableMapOf lt Int Boolean gt val map = mutableMapOf lt Int Boolean gt names forEachIndexed index gt map index = false selectedStates value = map LazyColumn modifier = modifier itemsIndexed items = names index name gt val i Boolean = selectedStates value index false Greeting name = name isSelected = i onSelected = selectedStates value index = it Divider color = Color Black ```

jim · Answer

Sorry the API not very discoverable You d want to use `mutableStateMapOf` instead of `mutableMapOf` else Compose won t know when the map is mutated

Issa · Answer

< jim> Thanks that did the trick wink As far as I understand `remember ` remembers the state for only one composition invocation however not `recomposition` done by the `LazyFoo` apis cuz these guys they unsubscribe from the previous and init a new invocation which yields a fresh state So far here is the code if you want to update the `Pathway` code lab ``` Composable fun NameList names List lt String gt modifier Modifier = Modifier val selectedStates = remember mutableStateMapOf lt Int Boolean gt apply names mapIndexed index gt index to false toMap also putAll it LazyColumn modifier = modifier itemsIndexed items = names index name gt Greeting name = name isSelected = selectedStates index == true onSelected = selectedStates index = it Divider color = Color Black ``` Thanks again for your help wink

Jorkoh · Answer

It s called the Compose basics codelab because it s about the basics the Using State in Compose delves into the hoisting pattern also with selected stuff in a lazyColumn Anyway you don t even need a map just a list of the selected items or their ids

Andrey Kulikov · Answer

also if you will use `rememberSaveable` instead of `remember` the state would survive the item being scrolled off and on

Issa · Answer

I will give `rememberSaveable` api a try this sounds really interesting < Andrey Kulikov>

Issa · Answer

< Andrey Kulikov> That works like a charm Although reading the docs of `rememberSaveable ` ```It behaves similarly to remember but the stored value will survive the activity or process recreation using the saved instance state mechanism for example it happens when the screen is rotated in the Android application ``` I am wondering if abusing this patter with say 1000 items in a list where each item is using `rememberSaveable` under the hood I am wondering if this has any performance penalties thinking face Other than that I loved the idea and it was exactly what I was looking for Thanks again

Issa · Answer

I also noticed that it doesn t work in Interactive Preview mode but maybe this is something to be fixed in the future versions of compose

Andrey Kulikov · Answer

yes this value will also be restored when the activity would be recreated no performance penalty but a bit of an extra memory to be kept like 1000 booleans in this case

Andrey Kulikov · Answer

but only if you will really scroll over 1000 items

Issa · Answer

That s interesting I wonder what would be the solution of implementing a viewHolder for the list that s something i will look into maybe there is a way to achieve something similar to recyclerView

Andrey Kulikov · Answer

Sorry what do you mean Why do you need a ViewHolder

Issa · Answer

In real world cases the state of an item is not just `isSelected` state it might be more complex depending on the UI So I thought maybe by creating a `ViewHolder` like API it would be easier to hold the state I am not sure if this is the responsibility of the item itself or the `LazyColumn`