Hello, Im working on a MVI implementation and I wa...

# android-architecture

[Emiliano Schiavone]

Hello, Im working on a MVI implementation and I want to know how I can structure my State. I though on a simple data class like (val isLoading, val content, val error, etc) and also for a sealed class like State { Loading, Error, Content, Idle }. I like the sealed class approach but i do not know how I can access to previous state. any idea or a better approach for sealed class for State

[Orhan Tozan]

What do you mean with "previous state"?

[Marko Novakovic]

data class and sealed class are totally different. data class is product type and sealed class is sum type all that means is that you have different number of cases to handle

[Marko Novakovic]

for example: take a look at Pair<Boolean, Boolean>. this is 4 cases to handle. 2 * 2, 2 for first Boolean (true and false) and 2 for second Boolean

[Marko Novakovic]

now imagine that with your data class that has 5 parameters and every parameter had different parameters. it can get out of hand pretty quickly

[Marko Novakovic]

data class makes you handle more cases

[Emiliano Schiavone]

Im working with sealed class for MviState (State.Loading, State.Content, State.Error), but my guess if how to manage this scenario.
1. Action GetUsers -> State.Loading and then State.Content
2. Action.RefreshUsers -> ??? if I set State.Loading, I will loss the State.Content (prev state), so if there is any error I will loss the Content

[Marko Novakovic]

sealed class on the other hand has a lot less, potentially, cases to handle

[Marko Novakovic]

I personally always use sealed class to model my states

[Emiliano Schiavone]

Do you have any reducer or something like that?

[Marko Novakovic]

if you have sealed class with Loading, Error and Success than you have only 3 states to handle. Success probably has some data so you need to handle those too but it reduces the number of cases because it only has 1 for Error and 1 for 1 Loading

[Marko Novakovic]

I mutate state with a reducer function

[Emiliano Schiavone]

So for example, you loaded data and then you refresh and get an error, how do you manage that to get the prev state, bc you are in Error State there

[Marko Novakovic]

depends. usually I go one of two directions:
1. I do event sourcing -> keep record of all states
2. I save success state in local variable
approach varies based on requirements

[Orhan Tozan]

@Emiliano Schiavone why do you want to get to the previous state when you have an error state?

[Emiliano Schiavone]

@Orhan Tozan bc in some case you display an error state but you have data, for example if you are refreshing data and you get an error, you had data before refreshing

[Orhan Tozan]

Ok, in that case, your error state needs to have that data. But in your case, I would just give the Data state an error property

[Marko Novakovic]

and you can have parts of the UI react to specific state

[Marko Novakovic]

for example your RecyclerView -> LazyList part of the UI react just to Success and on Error you have other part of UI that reacts to it

[Marko Novakovic]

what makes sense for your usecase