Hi everyone, I have a doubt regarding the flatMapL...

# flow

[Andrea]

Hi everyone, I have a doubt regarding the flatMapLatest and the UDF. I have pasted a simplified code version with more details. I would like to create a UIState in my viewmodel, with several properties (example like HomeUiState in JetNews). One of these properties is a list of users that might be filtered by the user.
1 - The first approach would be exposing the list, which would be filtered every time the userId changes. In this case I would not be able to add the list into my UIState.
2 - The second one would be collecting the flow into the init and update the UIState in the onEach. In this case though, I would need to trigger the userId from the UI in order to start it the first time.
Any suggestions for situations like this? Am I doing anything wrong?
Thanks!

data class HomeUiState(
    val users: MutableList<User>
)

class HomeViewModel(
) : ViewModel(){

    private val repository: Repository = Repository()

    private val viewModelState = MutableStateFlow(HomeUiState(mutableListOf()))
    val uiState: StateFlow<HomeUiState> = viewModelState
        .stateIn(
            viewModelScope,
            SharingStarted.Eagerly,
            viewModelState.value
        )

    val userId = MutableStateFlow(1)

    //  1 solutions: Keep the list out the UIState and collect it in compose with collectAsState 
    val userList = userId.flatMapLatest {
        repository.getUsersList()
    }.stateIn(
        viewModelScope,
        SharingStarted.WhileSubscribed(5000),
        emptySet()
    )

    // 2 solution, flatMapLatest for the filtered list
    init {
        userId.flatMapLatest {
            repository.getUsersList(it)
        }.onEach { users ->
            viewModelState.update {
                it.copy(users = users) }
        }.launchIn(viewModelScope)
    }
}

[Nick Allen]

1. I suggest List instead of MutableList when passing data around.
2. You don't need to call stateIn on a MutableStateFlow, it is a StateFlow . If you are worried about callers casting it, you could call .asStateFlow().
3. Seems like you want the HomeUiState so just go with that
4. stateIn implementation starts a coroutine that collects and updates a MutableStateFlow so there's really no difference conceptually. Just use stateIn since it has the code you need already.
You could do this with just one expression:

val uiState: StateFlow<HomeUiState> = userId
    .flatMapLatest { repository.getusersList(it) }
    .map { HomeUiState(it) }
    .stateIn(
        viewModelScope,
        SharingStarted.WhileSubscribed(5000),
        HomeUiState(emptyList())
    )

If there's more to HomeUiState then you can use combine to build it from multiple `Flow`s.

[Andrea]

Thanks for the advices 1 and 2, I have removed and clear the code to paste it here and I didn’t notice them. You are correct regarding the List and the stateIn, thanks. I like the solution of the map, but what if I have another property in my HomeUiState? I would need
for example,

data class HomeUiState(
    val users: List<User>,
    val messages: List<Message>
)

and messages value should be updated in another fun inside the viewmodel using, ad example

fun loadMessages() {
    viewModelScope.launch { 
        // call repository and load someeMessages
        viewModelState.update {
            it.copy(messages = someeMessages)
        }
    }
}

I need to keep the private mutableStateFlow to update other properties. I have tried this solution, but maybe there is a better one

val uiState: StateFlow<HomeUiState> = userId.flatMapLatest {
        repository.getUsersList(it)
    }
    .map {
        HomeUiState(it, listOf())
    }
    .combine(viewModelState) { f1, f2 ->
        viewModelState.update { it.copy(users = f1.users) }
        viewModelState.value
    }
    .stateIn(
        viewModelScope,
        SharingStarted.WhileSubscribed(5000),
        HomeUiState(listOf(), listOf())
    )

[Nick Allen]

combine(usersFlowexpression, messagesFlowExpression, ::HomeUiState)
   .stateIn(…)

When you combine, it gives you the latest from each flow. You can then build the HomeUiState from these latest values.

[Nick Allen]

If the flows don't match your data constructor exactly, you can use a lambda instead of constructor reference.

[Andrea]

Thanks for your advices, it works and I didn’t think about using the combine in this case, thank you!