ArticlesActivity.kt

// ----UI Layer----

// -UI Layer(UI element)-

@AndroidEntryPoint
class ArticlesActivity : AppCompatActivity() {
  // UI 1. ✅ You shouldn't store or keep in memory any application
  // data or state in your app components
  lateinit var binding: ActivityArticlesBinding
  private val articlesViewModel by viewModels<ArticlesViewModel>()
  override fun onCreate(savedInstanceState: Bundle?) {
    super.onCreate(savedInstanceState)
    binding = ActivityArticlesBinding.inflate(layoutInflater)
    setContentView(binding.root)

    // UI 2. ✅ Make sure you take the lifecycle of the UI into consideration
    lifecycleScope.launch {
      articlesViewModel
        .uiState
        .flowWithLifecycle(lifecycle)
        .collect { uiState ->
          applyUiState(uiState)
        }
    }

    binding.refreshButton.setOnClickListener {
      articlesViewModel.onRefresh()
    }
  }

  // UI 3. ✅ You should drive your UI from data models
  private fun applyUiState(uiState: ArticlesUiState) {
    binding.articles.text = uiState
      .articles
      .joinToString("\n") { it.author + ":" + it.title }
  }
}

// -UI Layer(state holder (ViewModel))-

// UI 4. ✅ Define UI state
data class ArticlesUiState(
  // UI 5. ✅ Immutability
  val articles: List<Article>
)

@HiltViewModel
class ArticlesViewModel @Inject constructor(
  private val getArticlesUseCase: GetArticlesUseCase
) : ViewModel() {
  // UI 6. ✅ Use unidirectional data flow
  // The pattern where the state flows down and the events flow up is called a unidirectional data flow (UDF).
  // The uiState is state
  // The onRefresh is event

  private val _uiState = MutableStateFlow(ArticlesUiState(listOf()))

  // UI 7. ✅ Expose UI state
  // UI 8. ✅ As business logic gets more complex, having a singular UiState class where all properties are immediately available becomes increasingly important
  // UI 9. ✅ You should expose the UI state in an observable data holder like LiveData or StateFlow
  val uiState get() = _uiState.asStateFlow()

  init {
    viewModelScope.launch {
      val articles = getArticlesUseCase()
      _uiState.update { currentUiState: ArticlesUiState ->
        currentUiState.copy(
          articles = articles
        )
      }
    }
  }

  // UI 10. ✅ Any work performed in a ViewModel should be main-safe—safe to call from the main thread.
  fun onRefresh() {
    viewModelScope.launch {
      val articles = getArticlesUseCase()
      _uiState.update { currentUiState ->
        currentUiState.copy(
          articles = articles
        )
      }
    }
  }
}

private fun <T> MutableStateFlow<T>.update(function: (currentUiState: T) -> T) {
  value = function(value)
}