Android架构面试（八）：从Jetpack到模块化设计

// 典型反例：所有逻辑堆砌在Activity中
class MainActivity : AppCompatActivity() {
    override fun onCreate(savedInstanceState: Bundle?) {
        // UI初始化、网络请求、数据处理全部混杂
        val data = loadDataFromNetwork() // 阻塞主线程
        updateUI(data)
        saveToDatabase(data)
    }
}

class UserViewModel(private val repo: UserRepository) : ViewModel() {
    private val _userData = MutableStateFlow<User?>(null)
    val userData: StateFlow<User?> = _userData.asStateFlow()

    init {
        viewModelScope.launch {
            repo.fetchUser().collect { user ->
                _userData.value = user
            }
        }
    }
}
// 关键特性：
// 1. 使用viewModelScope自动取消协程
// 2. StateFlow替代LiveData实现更纯粹的状态管理

@Dao
interface UserDao {
    @Query("SELECT * FROM user WHERE region = :region")
    fun getUsersByRegion(region: String): Flow<List<User>>

    @Transaction  // 支持复杂事务
    @Query("UPDATE user SET status = :status WHERE id = :userId")
    suspend fun updateUserStatus(userId: String, status: Int)
}

// 使用技巧：
// 1. 结合Flow实现数据库变化实时监听
// 2. 使用@Transaction确保多表操作原子性

// 分层结构示例
class LoginViewModel(
    private val authRepo: AuthRepository
) : ViewModel() {
    // UI状态集中管理
    data class LoginState(
        val isLoading: Boolean = false,
        val error: String? = null,
        val isSuccess: Boolean = false
    )

    private val _state = MutableStateFlow(LoginState())
    val state: StateFlow<LoginState> = _state.asStateFlow()

    fun login(username: String, password: String) {
        viewModelScope.llaunch {
            _state.value = LoginState(isLoading = true)
            try {
                authRepo.login(username, password)
                _state.value = LoginState(isSuccess = true)
            } catch (e: Exception) {
                _state.value = LoginState(error = e.message)
            }
        }
    }
}

sealed class MainIntent {
    object LoadData : MainIntent()
    data class UpdateItem(val id: String) : MainIntent()
}

data class MainState(
    val items: List<Item> = emptyList(),
    val isLoading: Boolean = false
)

class MainViewModel : ViewModel() {
    private val _state = MutableStateFlow(MainState())
    val state: StateFlow<MainState> = _state.asStateFlow()

    fun processIntent(intent: MainIntent) {
        when (intent) {
            is MainIntent.LoadData -> loadData()
            is MainIntent.UpdateItem -> updateItem(intent.id)
        }
    }
}

// 使用Hilt依赖注入实现模块解耦
@Module
@InstallIn(SingletonComponent::class)
object NetworkModule {
    @Provides
    fun provideRetrofit(): Retrofit {
        return Retrofit.Builder()
            .baseUrl("https://api.example.com/")
            .addConverterFactory(GsonConverterFactory.create())
            .build()
    }
}

// 在feature模块中使用
@AndroidEntryPoint
class UserRepository @Inject constructor(
    private val apiService: ApiService
) { /* ... */ }

@Test
fun `login should update state on success`() = runTest {
    // 1. 创建测试依赖
    val mockRepo = mockk<AuthRepository>()
    coEvery { mockRepo.login(any(), any()) } returns Unit
    
    // 2. 创建ViewModel
    val vm = LoginViewModel(mockRepo)
    
    // 3. 执行操作
    vm.login("test", "password")
    
    // 4. 验证状态变化
    val state = vm.state.first { !it.isLoading }
    assertTrue(state.isSuccess)
}

@HiltAndroidTest
class LoginScreenTest {
    @get:Rule
    val hiltRule = HiltAndroidRule(this)

    @Test
    fun display_error_when_login_fails() {
        // 1. 启动Compose测试
        composeTestRule.setContent {
            AppTheme { LoginScreen() }
        }
        
        // 2. 模拟错误状态
        val vm = hiltNavGraphViewModel<LoginViewModel>()
        vm.updateState(LoginState(error = "Invalid credentials"))
        
        // 3. 验证UI显示
        composeTestRule.onNodeWithText("Invalid credentials").assertIsDisplayed()
    }
}