Challenge 2: A UiState Data Class — Solution

data class TodoUiState(
    val items: List<String> = emptyList(),
    val isLoading: Boolean = true
)

class TodoViewModel : ViewModel() {
    private val _uiState = MutableStateFlow(TodoUiState())
    val uiState: StateFlow<TodoUiState> = _uiState.asStateFlow()

    fun addItem(text: String) {
        _uiState.value = _uiState.value.copy(
            items = _uiState.value.items + text,
            isLoading = false
        )
    }
}

Notes:
- _uiState.value.copy(items = _uiState.value.items + text, isLoading = false)
  creates a brand new TodoUiState with the updated items list and
  isLoading flag, leaving the original _uiState.value's TodoUiState
  instance itself completely untouched (data classes built from val
  properties are immutable — Kotlin Fundamentals Chapter 4).
- items + text (list + single element) produces a NEW list containing
  every existing item plus the new one — this relies on Kotlin
  Fundamentals Chapter 6's collection operators, not mutation of the
  existing list in place.
- Both fields are updated together in one copy() call, which is exactly
  what keeps the state internally consistent — there's no possible
  intermediate moment where isLoading is stale relative to items,
  because the whole TodoUiState is replaced atomically.