Hi, I’m experimenting with state flow to see if i can get some sort of reactive stack implementation...

# coroutines

[samuel]

Hi, I’m experimenting with state flow to see if i can get some sort of reactive stack implementation. My idea is that i have a  class that holds the state, which only one subscriber observes, but multiple classes can update the state. This is what i have so far (in 🧵), but i have some strange behaviour. Calling replaceState works as expected with the subscriber getting the new value, however, calling updateState doesn’t. Does any one have a clue why this happens?

[samuel]

sealed class State {
    object FirstState: State()
    object SecondState: State()
}

class StateHolder {
    private val _state: MutableStateFlow<State> =
            MutableStateFlow(LinkedList(listOf(State.FirstState)))

    val state: StateFlow<State> =
            _state.mapLatest{ it.peekFirst() }
                    .stateIn(GlobalScope, SharingStarted.Eagerly, State.FirstState)

    // works
    fun replaceState(newState: State) {
        _state.value = LinkedList(listOf(newState))
    }

    // Doesn't work
    fun updateState(newState: State) {
        _state.value = _state.value.apply { push(newState) }
    }
}

// Called from another class
myScope.launch {
    state.collectLatest { state ->
        Timber.e("state value $state")
    }   
}
sealed class State {
    object FirstState: State()
    object SecondState: State()
}

[Dominaezzz]

StateFlow doesn't update subscribers if the value is set to the same object.

[Dominaezzz]

You'll have to create a new object

[samuel]

ahh! i didn’t expect the two objects to be the same given the contents have been changed, but it makes sense now, thank you 🙏

[Michael Ferguson]

To be more specific, StateFlow uses .equals to compare and conflate values. So attempting to force an emission by simply copying/clone an object into a new object will fail.

[Michael Ferguson]

It’s not like LiveData in that respect.

[samuel]

Thats a pretty important detail to know, thank you. Feels rather unfortunate that in this case i would have to convert the old linked list to a list and create new linked list out of that. thats incorrect, can create a new object directly from the old collection