Hi, how to update composable when a list changes by `rember { mutableStateOf(mutableListOf<Int&gt...

# compose

[Lance Gao]

Hi, how to update composable when a list changes by rember { mutableStateOf(mutableListOf<Int>())}?

[Lance Gao]

It does not work when I change the list.

[mattinger]

are you mutating the actual list? state does not update unless you re-assign the state itself. ie, this will work:

val state = remember { mutableStateOf(listOf<Int>()) }
state.value = state.value + 1

This creates a new list with the additional element appended to the end of it and reassigns the value in the state. This allows the observers (ie, the recomposer) to know that the value changed and things need recomposing.

However, if you do this:

state.value.add(1)

You are simply changing the data inside the list, which no one is observing. There's no way for the recomposer to know that anything changed.

[mattinger]

I think a good rule of thumb is keep immutable objects inside of a MutableState. So in your case, you want the type to be: MutableState<List<Int>> and not MutableState<MutableList<Int>>

[Lance Gao]

Thanks for your reply! So I must assign state a new list to notify the recomposer right?

[mattinger]

correct

[Lance Gao]

Okay thanks a lot

[mattinger]

mutableStateList is a special type of object that implements the StateObject interface. That interface is a hook into having mutable objects like this be able to inform observers of their changes. I still personally perfer to use a raw, immutable list than to use this special implementation

[Lance Gao]

Ok thanks for your suggestion

[Colton Idle]

This is also good if you haven't read it https://dev.to/zachklipp/two-mutables-dont-make-a-right-2kgp