Hi folks, a question about `mutableStateListOf()`...

# compose

[faisalahmed]

Hi folks, a question about mutableStateListOf() .
add and remove on the list triggers succesful recompositions of the specific item, but changing the value at any index of this list does not trigger any recomposition. Been scratching my head at this. My LazyColumn is keyed to a unique id coming in the data as well. So I am wondering what might be going wrong. Following is an example. Inside ViewModel

val list = mutableStateListOf<Messages>()

fun onMessageChanged(newMessage: Message) {
  val index = list.indexOf(newMessage) // returns correct index
  list[index] = newMessage // does not trigger recomposition
}

[ephemient]

like mutableStateOf(), it compares with ==. assigning the same state doesn't recompose

[faisalahmed]

how can I trigger recomposition for a change in any item of the list?

[Vojtěch Hořánek]

Firstly, you should wrap your mutable state with a remember . Secondly, I would suggest you to use an immutable list and when updating a certain item, create a copy of the original list with the item updated. And the of course, set that list state to that copy

[faisalahmed]

@Vojtěch Hořánek Thanks for the response.
I dont think remember is needed since this logic is sitting inside the ViewModel. About your suggestion, I think that will definitely work, but I just wanted to know why MutableStateList<> is not working the way it clearly mentions it willl?

[Vojtěch Hořánek]

Oh, didnt notice it's inside a viewmodel

[ephemient]

what you are doing is basically

for (i in list.indices) {
    if (list[i] == newMessage) {
        list[i] = newMessage
        break

you're setting it to an equal value, why do you expect anything to happen?

[faisalahmed]

oh no no @ephemient
If you see my code,
the method onMessageChanged is lets supposed a method being triggered with new item coming from a data source.
My code finds that specific item in the list, and then re-writes to that index.

[Stylianos Gakis]

You are searching for the index of newMessage and then assign newMessage to it. You are assigning the same value to where it already was as ephemient suggests.
If you were doing something like val index = list.indexOf(newMessage.uniqueIdentifier) then maybe it’d make sense, but now it does not.
How does Message look internally? It may be that you have inner mutability in the message itself, meaning that compose won’t be able to see it change.

[faisalahmed]

aaah, I found the issue guys. Thanks a lot @Stylianos Gakis and @ephemient .
My message data class overrides equalsTo and uses a unique identifier to compare messages .
Thats where my comparision is failing and no recompositions are triggered.