Modelowanie stanu interfejsu za pomocą klas sealed

// Fragile: isLoading=true AND hasError=true is invalid
var isLoading = false
var hasError = false
var data: List<Item>? = null
// 8 combinations, most of which are invalid!

sealed class UiState<out T>
object Loading : UiState<Nothing>()
data class Success<T>(val data: T) : UiState<T>()
data class Error(val message: String, val retryable: Boolean = true) : UiState<Nothing>()
object Empty : UiState<Nothing>()

fun render(state: UiState<List<Item>>) = when (state) {
    Loading -> showProgressBar()
    is Success -> showItems(state.data)
    is Error -> showError(state.message, state.retryable)
    Empty -> showEmptyView()
}

class ItemViewModel : ViewModel() {
    private val _state = MutableStateFlow<UiState<List<Item>>>(Loading)
    val state: StateFlow<UiState<List<Item>>> = _state

    fun loadItems() = viewModelScope.launch {
        _state.value = Loading
        _state.value = try {
            Success(repository.getItems())
        } catch (e: Exception) {
            Error(e.message ?: "Unknown error")
        }
    }
}

fun <T> loading(): UiState<T> = Loading
fun <T> success(data: T): UiState<T> = Success(data)
fun <T> error(msg: String): UiState<T> = Error(msg)

sealed class PageState
object Idle : PageState()
object FirstLoad : PageState()
data class Content(val items: List<Item>, val hasMore: Boolean) : PageState()
data class LoadingMore(val items: List<Item>) : PageState()
data class Error(val items: List<Item>?, val msg: String) : PageState()

fun <T, R> UiState<T>.mapData(transform: (T) -> R): UiState<R> = when (this) {
    is Success -> Success(transform(data))
    is Error -> this
    Loading -> Loading
    Empty -> Empty
}

@Test
fun testLoadingState() {
    viewModel.loadItems()
    assertEquals(Loading, viewModel.state.value)
}
@Test
fun testSuccessState() {
    viewModel.setData(listOf(item))
    assert(viewModel.state.value is Success)
}

// Fragile: two flows must stay in sync
val loading = MutableStateFlow(false)
val error = MutableStateFlow<String?>(null)

// Clean: one sealed state
val state = MutableStateFlow<UiState<List<Item>>>(Loading)

data class Error(
    val message: String,
    val code: Int = 0,
    val retryable: Boolean = true,
    val timestamp: Long = System.currentTimeMillis()
) : UiState<Nothing>()