How to update just only one variable of an UiState...

# compose

[Pablo]

How to update just only one variable of an UiState sealed class?

[Pablo]

This one:

sealed interface MainScreenUiState {
    object Loading : MainScreenUiState
    data class Error(val throwable: Throwable) : MainScreenUiState
    data class Success(
        val busStopsServerReady: Boolean,
        val busStopsDataAmount: Int,
    ) : MainScreenUiState
}

Which is initialized this way:

init {
    viewModelScope.launch(Dispatchers.IO) {
        try{
            // sources status
            val busStopsServerReady = networkBusRepository.getStops().isNotEmpty()
            val busStopsDataAmount = defaultMainBusRepository.getBusStops().size

            // update ui state
            _uiState.value = MainScreenUiState.Success(
                busStopsServerReady = busStopsServerReady,
                busStopsDataAmount = busStopsDataAmount
            )
        } catch (e: Throwable) {
            _uiState.value = MainScreenUiState.Error(e)
        }
    }
}

[Pablo]

What happens if I only want to update one of the Sucess variables?

fun updateUiState(value: Int) {
    //todo _uiState.busStopsDataAmount = value
}

[Seri]

Your sealed interface MainScreenUiState is immutable, meaning instances of it cannot be changed after they are created

[Seri]

instead, you issue a new instance with the important properties modified. See the .copy(...) method here https://kotlinlang.org/docs/data-classes.html#copying

[Pablo]

what? I'm using the sealed interface pattern showed in google app architecture samples

[Pablo]

they are using this, so it must be possible to change values

[Seri]

What line is your _uiState defined?

[Pablo]

private val _uiState = MutableStateFlow<MainScreenUiState>(MainScreenUiState.Loading)
val uiState: StateFlow<MainScreenUiState> = _uiState

[Michael Krussel]

The state objects themselves are Immutable not the _uiState.value. So the normal way to update a single property in the state object is to use the copy function. But since you have a sealed interface for the state, not every class in the sealed hierarchy has the property you want to update. My preferred approach to this is to add state updating functions in the interface. Basically have a function for each event that can change the state, and then each implementor of the interface, it returns a new state object based on the logic for what should happen for that event when you are in the given state. If the event is not valid for the current state, then that state could make it a no op and return itself, throw an exception, or go to an error state, based on your need

[Pablo]

it seems to be very complex, I don't understand you... maybe can you give a sample for modifying the value of my sample?

[Michael Krussel]

I don't know what your rules are.

What should updateUiState do if the current state is Loading. Should it create a Success state with a default busStopsServerReady?

[Michael Krussel]

sealed interface MainScreenUiState {
    fun updateState(value: Int): MainScreenUiState
    object Loading : MainScreenUiState {
        fun updateState(value: Int): MainScreenUiState = Success(true, value)
    }
    data class Error(val throwable: Throwable) : MainScreenUiState {
        fun updateState(value: Int): MainScreenUiState = this
    }
    data class Success(
        val busStopsServerReady: Boolean,
        val busStopsDataAmount: Int,
    ) : MainScreenUiState {
       fun updateState(value: Int): MainScreenUiState = copy(busStopDataAmount = value)
    }
}

fun updateUiState(value: Int) {
    //todo _uiState.value = _uiState.value.updateState(value)
}

[Pablo]

well, I don't feel confortable with that approach

[Pablo]

I don't need a function for updating content on error and loading

[Pablo]

I just need to update a single value on my success state

[Pablo]

does not exist a cleaner and simpler way?

[Michael Krussel]

Then you need to do an instance of check in the updateUiState function, but polymorphism is generally considered better and simpler then doing a type check.

[Pablo]

can you show me that other way? with the check

[Michael Krussel]

fun updateUiState(value: Int) {
    val currentValue = _uiState.value as? Success ?: return
    ui_State.value = currentValue.copy(busStopDataAmount = value)
}

[Seri]

MutableStateFlow has its own update function for convenience
https://kotlinlang.org/api/kotlinx.coroutines/kotlinx-coroutines-core/kotlinx.coroutines.flow/-mutable-state-flow/#1996476265%2FFu[…]ns%2F1975948010

e.g.

class CounterModel {
    private val _counter = MutableStateFlow(0) // private mutable state flow
    val counter = _counter.asStateFlow() // publicly exposed as read-only state flow

    fun inc() {
        _counter.update { count -> count + 1 } // atomic, safe for concurrent use
    }
}

[Pablo]

Seri can you apply that to my sample?

[Seri]

I won't write it for you, no. But I encourage you to think about how StateFlow is being used here, and how it's a container for the underlying data

[Seri]

This may help https://developer.android.com/kotlin/flow/stateflow-and-sharedflow#stateflow

[Pablo]

I definitively still have a problem with flows understanding

[Pablo]

I checked the site but didn't found any .update sample there applyed to a Success sealed class like in my case

[Pablo]

can't figure out how to migrate your proposal to my sample

[Pablo]

I finally get it working doing this:

if (_uiState.value is MainScreenUiState.Success)
    _uiState.value = (_uiState.value as MainScreenUiState.Success).copy(busStopsDataAmount = 1)

I don't like... but seems to be the simpler way, its very similar to the one propossed by Michael I think