Does a ViewModel member have to be defined to be of a State

Question

Does a ViewModel member have to be defined to be of a State lt gt type in order to invoke recomposion on change

nitrog42 · Answer

a state will trigger recomposition on change but it doesn t have to be on the viewmodel part ie collectAsState observeAsState produceState which will transform anything in a state that will trigger recomposition

nitrog42 · Answer

your viewmodel member should be a flow or livedata or anything observable though if it s just a var String for example you should probably use a State mutableStateOf as you said or it won t trigger any recomposition on the UI part

Mehdi Haghgoo · Answer

I have a list stored in my ViewModel it is LiveData lt String gt and using observeAsState does not give me the updated list in the Compose code

nitrog42 · Answer

nitrog42 · Answer

Without the code it can be hard to understand what happens but if you use a MutableList it can cause this issue even though with a LiveData I think it should work thinking face

Mehdi Haghgoo · Answer

Hmm I am seeing some red warnings in the doc smile Maybe that includes what I have done Pasting the code in a bit

Mehdi Haghgoo · Answer

```class MainViewModel ViewModel TODO Implement the ViewModel private val currentMessage MutableLiveData lt String gt = MutableLiveData private val messages MutableLiveData lt MutableList lt String gt gt = MutableLiveData val messages LiveData lt MutableList lt String gt gt = messages val currentMessage LiveData lt String gt = currentMessage fun setCurrentMessage message String currentMessage value = message fun onAddMessage message String val temp = mutableListOf message apply messages value let addAll it messages value = temp messages value add message Log d MainViewModel onAddMessage New message added $message Total $ messages value size ``` Code that uses it ```Column Log d MainFragment onCreateView number of messages $ messages size messages forEach msg gt Text msg Modifier padding 8 dp ```

nitrog42 · Answer

I think your issue is messages contains no value slightly smiling face

nitrog42 · Answer

```private val messages MutableLiveData lt MutableList lt String gt gt = MutableLiveData ``` gt messages is empty you need to either use MutableLiveData mutableList or pass the list as in your commented code

nitrog42 · Answer

but honnestly if you are doing a 100 compose new project you should probably use `val message = mutableStateListOf `

Mehdi Haghgoo · Answer

I assume messages should be updated with ViewModel onAddMessage message

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

A mutable list inside a state holder a MutableState StateFlow LiveData etc is always a smell Either use an immutable list or that mutableStateListOf Assigning the same instance of a list to a state holder generally won t re emit because the same list instance is always equal to itself

Colton Idle · Answer

gt A mutable list inside a state holder a MutableState StateFlow LiveData etc is always a smell Makes sense gt Either use an immutable list ok gt or that mutableStateListOf I would assume mutableStateListOf is the thing I should reach for first typically

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

I don t think there s a single right answer Depends on other things there are trade offs as with anything Immutable data structures are often generally easier to reason about in async concurrent environments

Colton Idle · Answer

Yeah typically I ve always chosen immutable first but compose has thrown me in a loop because of snapshot state that basically acts like it s mutable and it feels like its magic to not have to call copy eveywhere lol

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

Not sure if this will help but I wrote something about this yesterday <https dev to zachklipp two mutables dont make a right 2kgp|https dev to zachklipp two mutables dont make a right 2kgp>