Challenge 3: A Full ViewModel with Loading State — Solution

data class PostsUiState(
    val isLoading: Boolean = false,
    val posts: List<Post> = emptyList(),
    val errorMessage: String? = null
)

class PostsViewModel(private val api: PostApiService) : ViewModel() {
    private val _uiState = MutableStateFlow(PostsUiState())
    val uiState: StateFlow<PostsUiState> = _uiState.asStateFlow()

    init {
        loadPosts()
    }

    fun loadPosts() {
        viewModelScope.launch {
            _uiState.value = _uiState.value.copy(isLoading = true, errorMessage = null)
            when (val result = fetchPosts(api)) {
                is Success -> _uiState.value = _uiState.value.copy(
                    isLoading = false,
                    posts = result.data
                )
                is Error -> _uiState.value = _uiState.value.copy(
                    isLoading = false,
                    errorMessage = result.message
                )
                Loading -> Unit
            }
        }
    }
}

Notes:
- init { loadPosts() } triggers the network call as soon as the
  ViewModel is created — the same construction-time-setup idea as an
  Activity's onCreate (Course 1, Chapter 2), applied to a ViewModel.
- Setting errorMessage = null at the start of loadPosts() clears any
  previous error before retrying — without this, a failed load followed
  by a successful retry could otherwise leave a stale error message
  displayed alongside fresh data.
- The "when (val result = fetchPosts(api))" pattern combines Kotlin
  Fundamentals Chapter 5's exhaustive when over a sealed class with
  capturing the matched value as "result" — every branch updates
  _uiState.value via copy(), keeping isLoading/posts/errorMessage always
  internally consistent, per Chapter 2's UiState pattern.
- The Loading branch is required for the when to be exhaustive even
  though fetchPosts() as written never actually returns
  NetworkResult.Loading — sealed classes force every possible case to be
  handled, not just the ones a particular function happens to produce.