Hi, the state objects i define end up containing a...

# orbit-mvi

[Rak]

Hi, the state objects i define end up containing a lot of nulls. The examples I’ve seen usually use numerics or lists which are easy to initialise to non-null values. But what about strings or complex data structures? I attempted to make my state object a sealed class hierarchy but I couldnt work out how to reduce it. I guess I am looking for validation that I’m doing it correctly (as I can’t find any complex examples to learn from)

[Mikolaj Leszczynski]

There are no hard and fast rules. Sometimes it will be easier to go with nullables, sometimes a sealed class is better.

The downside of using a sealed class is that often when you reduce you need to break down the possibilities using when. It can get complicated. It might be worth extracting the reducers if that’s the case.

I think someone over here used https://github.com/Tinder/StateMachine with such an approach to make it more manageable. This increases complexity and indirection so it has its downsides too.

TL;DR no perfect solution for this exists :)

[appmattus]

cc @Guilherme Delgado

[Guilherme Delgado]

Hello!
For the state itself I’ve used a data class to control the state changes (reduce) I’ve used a FSM (that one from Tinder) to help “connecting”. Small example:

ViewModel:

//entry point called from View
fun start() {    stateMachine.transition(TimerState.Event.OnStart)    }

//state machine
state<TimerState.State.Idle> {
            ...
            on<TimerState.Event.OnStart> {
                transitionTo(TimerState.State.CountingDown, TimerState.SideEffect.StartCountDown)
            }
        }
...
onTransition {
            ...
            when (val effect = validTransition.sideEffect) {
                ...
                is TimerState.SideEffect.StartCountDown -> play()
            }
        }

//state change
private fun play() {
        intent { ...
                 reduce { ...
                       state.play() // state is a data class and play() it's a helper method to change some val values, ex:  fun play() = copy(isCountingDown = true, ...)
                    }
                }
            }
        }
    }

Hope it helps

[Guilherme Delgado]

it adds more code (the FSM part) but for me it makes the code very clear and it describes itself easily.

[Guilherme Delgado]

I intend to open source this project, but I’m on hold ’cause it’s KMM 😅

[miqbaldc]

In our case working with @Anang Kur workaround using this approach:

// in viewmodel
sealed class States {
    object Initial : States()
    object UserFetched : States()
}

data class VerificationState(
    val agent: Agent? = null,
    val states: States = States.Initial,
)

// for example, we use Nothing for side-effect

class OurViewModel(
    // dependencies
) : ViewModel(), ContainerHost<VerificationState, Nothing> {

    override val container: Container<VerificationState, Nothing> = container(initialState = VerificationState())

    fun getUser(token: String) = intent {
        runCatching {
            agentUseCase(token)
        }.onSuccess {
            it.collect { agent ->
                reduce {
                    state.copy(agent = agent, states = States.UserFetched) // switch the state(s) here
                }
            }
        }.onFailure {
            postSideEffect(...) // do something when failures
        }
    }

the Tinder/StateMachines seems a good alternative in our parts, thanks @Guilherme Delgado. Might need to checks on it for our usecase and interopability later!

// our Activity
    // onCreate
    viewModel.observe(this, state = ::render)

    private fun render(state: VerificationState) {
        when (state.states) {
            States.Initial -> {}
            States.UserFetched -> successGetUser(state.agent)
        }
    }

[miqbaldc]

https://proandroiddev.com/managing-the-ui-state-by-using-a-finite-state-machine-and-mvi-architecture-36d84056c616

found this by @Guilherme Delgado thanks for writing the article! til

[miqbaldc]

Anyone have tried of https://github.com/nsk90/kstatemachine as alternative for state machine?

Recently found the KStateMachine from this article: https://medium.com/@karthik.dusk/state-machine-in-android-kotlin-4f04e4121062

and this discussion about Tinder/StateMachine comparisons: https://github.com/nsk90/kstatemachine/discussions/29#discussioncomment-2447595

[Guilherme Delgado]

Thanks for sharing, gonna take a look when possible 😉