Need help migrating mutablestateof uistate to flow...

# compose

[Pablo]

Need help migrating mutablestateof uistate to flows:

sealed interface BooksGridUiState {
    data class Success(
        val booksList: List<BookData>,
        val selectedBookId: String? = null
    ) : BooksGridUiState
    object Error : BooksGridUiState
    object Loading : BooksGridUiState
}
var uiState: BooksGridUiState by mutableStateOf(BooksGridUiState.Loading)
    private set
fun selectBook(id: String?) {
    (uiState as? BooksGridUiState.Success)?.let {
        uiState = it.copy(selectedBookId = id)
    }
}

How can I migrate that to Flow? I'm totally stuck.
I know that this UiState with success, error and loading is the common pattern nowadays for a screen that loads content, but can't find a sample of it working with flows, and I need also to modify a value, and previously it was done with it.copy and now I don't know how to do it with flows

[Caleb Cook]

data class BookData(
    val id: String,
)
    
sealed interface BookState {
    data class Success(
        val bookList: List<BookState>,
        val selectedBookId: String? = null,
    ) : BookState
    data object Loading : BookState
    data object Error : BookState
}

private val _uiBookState = MutableStateFlow<BookState>(BookState.Loading)
val uiBookState = _uiBookState.asStateFlow()

fun selectBook(id: String?) {
    _uiBookState.value = BookState.Success(
        bookList = listOf(),
        selectedBookId = id
    )
}

[Pablo]

is that the best way? there is not a copy option?

[Pablo]

I mean, do I need to create a new instance every time a change on selected book is produced? also you are setting bookList to empty list, but I need to to keep the current bookList

[Pablo]

also, another point, why you used a _ mutable variable and a non mutbale variable? is not possible to simply add a "private set" below the mutable variable like it was done in the sample code I provided? to avoid boilerplate variables

[Jakub Legindi]

Well you can use copy here to change only the properties you want to change, so there would be no need to set the list again. But in the meaning of creating new instance, this is what copy is actually doing. See here: https://kotlinlang.org/docs/data-classes.html#copying

Regarding your second question, I think this is mainly about preferences and project you work on. I am using the same approach as Caleb which is more clear and without any dependencies on compose in the ViewModel what could be necessary in Multiplatform projects.

You are just transforming the private one, while the public one is changing based on it. You are also avoiding human error in case you don't set private set which would allow you to update uiState directly from Composable.

There is approach with copy, instead of creating new BookState.Success data class.

data class BookData(
    val id: String,
)

sealed interface BookState {
    data class Success(
        val bookList: List<BookState>,
        val selectedBookId: String? = null,
    ) : BookState
    data object Loading : BookState
    data object Error : BookState
}

private val _uiBookState = MutableStateFlow<BookState>(BookState.Loading)
val uiBookState = _uiBookState.asStateFlow()

fun selectBook(id: String?) {
    _uiBookState.update { bookState ->
        if (bookState !is BookState.Success) return@update bookState
        bookState.copy(
            selectedBookId = id,
        )
    }
}

[Jakub Legindi]

or this

fun selectBook(id: String?) {
    _uiBookState.update { bookState ->
        val successBookState = bookState as? BookState.Success ?: return@update bookState

        successBookState.copy(
            selectedBookId = id,
        )
    }
}

or using let like you did, but I am not fan of let as it can be abused by developers not using it properly 😄

[Pablo]

having this scenario, which is very similar:

sealed interface UiState {
    object Success : UiState
    object Error : UiState
    data class Loading(
        val progress: Int = 0
    ) : UiState
}
var uiState = MutableStateFlow<UiState>(UiState.Loading())
    private set

What do you think about this approach for updating the progress value in the Loading UiState?

fun setProgress(value: Int) {
    (uiState.value as? UiState.Loading)?.let {
        uiState.value = it.copy(progress = value)
    }
    Log.d("XXXX", "decode progress: $value")
}

[Jakub Legindi]

I would go this way, but it is my personal preference again. I am avoiding having compose code inside of ViewModel .

sealed interface UiState {
        data object Success : UiState
        data object Error : UiState
        data class Loading(
            val progress: Int = 0
        ) : UiState
    }
    private val _uiState: MutableStateFlow<UiState> = MutableStateFlow(UiState.Loading())
    val uiState = _uiState.asStateFlow()

    fun setLoadingProgress(value: Int) {
        _uiState.update { uiState ->
            if (uiState !is UiState.Loading) return@update uiState

            uiState.copy(
                progress = value,
            )
        }
        Log.d("XXXX", "decode progress: $value")
    }
}

[Pablo]

Please can you clarify me which is the compose code? I'm not aware of it

[Jakub Legindi]

Well i thought you were using mutableStateOf here, but not, is it var - nevermind then. Sorry for that.

[Jakub Legindi]

no need of var here at all btw

[Pablo]

You mean this can work exactly the same?

sealed interface UiState {
    object Success : UiState
    object Error : UiState
    data class Loading(
        val progress: Int = 0
    ) : UiState
}
val uiState = MutableStateFlow<UiState>(UiState.Loading())

fun setProgress(value: Int) {
    (uiState.value as? UiState.Loading)?.let {
        uiState.value = it.copy(progress = value)
    }
    Log.d("XXXX", "decode progress: $value")
}

[Pablo]

seems to be more simple and with less boilerplate code, maybe is something in your code that is better and I didn't notice about it?

[Jakub Legindi]

Well you can collect it the same way as StateFlow in your Composable, but you should not expose Mutable properties out of the ViewModel . By that you are exposing MutableStateFlow which could lead to some unwanted side effects. Of course you can do that, but you don't want to do that.

[Jakub Legindi]

That is also why the Mutable one in my solution is private, you cannot set private set here, because the MutableStateFlow is still the same, the value in it isn't

[Pablo]

Now I see why is better your proposal, thank you

[Pablo]

and why you prefeer to do

_uiState.update { uiState ->
            if (uiState !is UiState.Loading) return@update uiState

            uiState.copy(
                progress = value,
            )
        }

Instead of

(uiState.value as? UiState.Loading)?.let {
        uiState.value = it.copy(progress = value)
    }

[Jakub Legindi]

It is just more readable