When having a sealed class hierarchy for your `uiS...

# compose

[Lukasz Kalnik]

When having a sealed class hierarchy for your `uiState`:

sealed class MyScreenState
data object EmptyState : MyScreenState()
data class ContentState(val title: String, val content: String) : MyScreenState()

How do you handle conditional uiState updates based on the current subclass?

val uiState: MutableStateFlow<MyScreenState> = MutableStateFlow(EmptyState)

fun onContentChanged(newContent: String) {
    uiState.update { (it as? ContentState)?.copy(content = newContent) ?: it }
}

This seems neither elegant nor readable.

[Lukasz Kalnik]

Alternative could be with when, but it still requires the lone it inside of update()

uiState.update {
    when (it) {
        is ContentState -> it.copy(content = newContent)
        else -> it
    }
}

[Lukasz Kalnik]

On the other hand, wrapping the whole update block in an if or ?.let is not thread safe. There could be a race condition, where the uiState gets updated by another thread just after executing the if or ?.let and we overwrite this other update.

(uiState.value as? ContentState)?.let { currentUiState ->
    // potential race condition and overwriting parallel updates
    uiState.value = currentUiState.copy(content = newContent)
}

[ephemient]

you could

run {
    uiState.update {
        if (it !is ContentState) return@run
        it.copy(content = newContent)
    }
}

which could be slightly more efficient since it avoids a CAS in the !is case

[Lukasz Kalnik]

Yes, that's a good suggestion

[Lukasz Kalnik]

I tried modifying the "official" `MutableStateFlow.update()`:

public inline fun <T, reified U : T> MutableStateFlow<T>.updateIfSubtype(function: (U) -> T) {
    while (true) {
        val prevValue = value
        if (prevValue is U) {
            val nextValue = function(prevValue)
            if (compareAndSet(prevValue, nextValue)) {
                return
            }
        } else return
    }
}

Usage:

uiState.updateIfSubtype<MyScreenState, ContentState> { it.copy(content = newContent) }

The only disadvantage is that you have to explicitly specify the parent type.

[ephemient]

uiState.updateIfSubtype<_, ContentState> { ... } or swap the parameters around with

inline fun <reified U, T> MutableStateFlow<T>.updateIfSubtype(function: (U) -> T) where U : T {

to write uiState.updateIfSubtype<ContentState, _> { ... }

[Lukasz Kalnik]

Oh that's nice, I didn't know this 🙂

[Lukasz Kalnik]

Thanks!

[Michael Krussel]

Another option is to add abstract functions to the MyScreenState and let each state return an updated state based on its own logic. If there is no change it just returns itself.

[Lukasz Kalnik]

I want to avoid writing the same state in this case (I'm not sure if compiler optimizes it away)