I would like to define a class such as `ScreenStat...

# getting-started

[Daniele B]

I would like to define a class such as ScreenState and a function like emptyScreen()

data class AppState (
    val masterState : ScreenState<MasterData> = ScreenState<MasterData>(),
    val detailState : ScreenState<DetailData> = emptyScreen(),
)

data class MasterData (
    val countriesList : List<CountriesListItem> = emptyList(),
    ...
)
data class DetailData (
    val countryInfo : CountryInfo = CountryInfo(),
    ...
)

I was trying to follow the Kotlin definitions of List and emptyList()
but I got stuck
it should be something similar to this?

public interface ScreenState<screenData T> {
    val isLoading: Boolean = false,
    val screenData: T? = null,
}

public fun <T> emptyScreen(): ScreenState <T> = EmptyScreen

internal object EmptyScreen : ScreenState <Nothing> {
}

[Youssef Shoaib [MOD]]

You're almost there. First generics are defined just as <T> so no need for that "screenData", second you need out variance on T so that the Nothing thing actually works. Your code should end up being this:

public interface ScreenState<out T> {
    val isLoading: Boolean = false,
    val screenData: T? = null,
}
public fun <T> emptyScreen(): ScreenState <T> = EmptyScreen
internal object EmptyScreen : ScreenState <Nothing> {
}

[Daniele B]

@Youssef Shoaib [MOD] thanks so much!
however I found that it’s not possible to initialize properties in an interface
it looks like with abstract classes is instead possible:

abstract class ScreenState<T>(
    val isLoading: Boolean = false,
    val screenData: T? = null,
)

fun <T> emptyScreen(): ScreenState<T> {
    return object : ScreenState<T>() {}
}

[Youssef Shoaib [MOD]]

Well you can do this instead:

public interface ScreenState<out T> {
    val isLoading: Boolean get() = false,
    val screenData: T? get() = null,
}
public fun <T> emptyScreen(): ScreenState <T> = EmptyScreen
internal object EmptyScreen : ScreenState <Nothing> {
}

It's basically just default getters which can then be overridden depending on what the subclass wants

[Daniele B]

great! thank you!