[Android] 003. 状態ホルダーとUI状態(UiState)

class HogeViewModel {
    private val _uiState = MutableStateFlow(UiState())
    val uiState = _uiState.asStateFlow()
    
    data class UiState(
        val isLoading: Boolean = false,
        val text: String = "huga"
    )

    init {
        _uiState.update(...)
    }
}

fun HogeView(vm: ViewModel) {
    val uiState by viewModel.uiState.collectAsState()
    ...
}

sealed interface OnboardingUiState {
    /**
     * The onboarding state is loading.
     */
    data object Loading : OnboardingUiState
    /**
     * The onboarding state was unable to load.
     */
    data object LoadFailed : OnboardingUiState

    /**
     * There is no onboarding state.
     */
    data object NotShown : OnboardingUiState

    /**
     * There is a onboarding state, with the given lists of topics.
     */
    data class Shown(
        val topics: List<FollowableTopic>,
    ) : OnboardingUiState {
        /**
         * True if the onboarding can be dismissed.
         */
        val isDismissable: Boolean get() = topics.any { it.isFollowed }
    }
}

class ForYouViewModel(...) {
    val onboardingUiState: StateFlow<OnboardingUiState> =
        combine(
            shouldShowOnboarding,
            getFollowableTopics(),
        ) { shouldShowOnboarding, topics ->
            if (shouldShowOnboarding) {
                OnboardingUiState.Shown(topics = topics)
            } else {
                OnboardingUiState.NotShown
            }
        }
            .stateIn(
                scope = viewModelScope,
                started = SharingStarted.WhileSubscribed(5_000),
                initialValue = OnboardingUiState.Loading,
            )

}

internal fun ForYouScreen(...) {
    val onboardingUiState by viewModel.onboardingUiState.collectAsStateWithLifecycle()
    ForYouScreen(onboardingUiState)
}

internal fun ForYouScreen(
    onboardingUiState: OnboardingUiState
) {
    val isOnboardingLoading = onboardingUiState is OnboardingUiState.Loading

    LazyVerticalStaggeredGrid(...) {
        onboarding(
            onboardingUiState = onboardingUiState,
            ...
    }
}

private fun LazyStaggeredGridScope.onboarding(
    onboardingUiState: OnboardingUiState
) {
    when (onboardingUiState) {
        OnboardingUiState.Loading,
        OnboardingUiState.LoadFailed,
        OnboardingUiState.NotShown,
        -> Unit

        is OnboardingUiState.Shown -> {
        ...
}