第 01 章 现代 Android 开发框架概述¶

// 声明式UI示例
@Composable
fun UserProfileScreen(user: User, onEditClick: () -> Unit) {
    Column(
        modifier = Modifier
            .fillMaxSize()
            .padding(16.dp),
        horizontalAlignment = Alignment.CenterHorizontally
    ) {
        // 头像
        AsyncImage(
            model = user.avatarUrl,
            contentDescription = "User Avatar",
            modifier = Modifier
                .size(120.dp)
                .clip(CircleShape)
        )

        Spacer(modifier = Modifier.height(16.dp))

        // 用户名
        Text(
            text = user.name,
            style = MaterialTheme.typography.headlineMedium
        )

        // 编辑按钮
        Button(onClick = onEditClick) {
            Text("编辑资料")
        }
    }
}

// ViewModel示例
class UserViewModel @Inject constructor(
    private val getUserUseCase: GetUserUseCase
) : ViewModel() {

    private val _uiState = MutableStateFlow(UserUiState())
    val uiState: StateFlow<UserUiState> = _uiState.asStateFlow()

    fun loadUser(userId: String) {
        viewModelScope.launch {
            _uiState.update { it.copy(isLoading = true) }

            getUserUseCase(userId)
                .onSuccess { user ->
                    _uiState.update {
                        it.copy(user = user, isLoading = false)
                    }
                }
                .onFailure { error ->
                    _uiState.update {
                        it.copy(error = error.message, isLoading = false)
                    }
                }
        }
    }
}

// Repository接口
interface UserRepository {
    suspend fun getUser(userId: String): Result<User>
    suspend fun updateUser(user: User): Result<Unit>
    fun getUsersFlow(): Flow<List<User>>
}

// Repository实现
class UserRepositoryImpl @Inject constructor(
    private val userDao: UserDao,
    private val userApi: UserApi
) : UserRepository {

    override suspend fun getUser(userId: String): Result<User> {
        return try {  // try/catch捕获异常
            // 先尝试从网络获取
            val remoteUser = userApi.getUser(userId)
            userDao.insertUser(remoteUser.toEntity())
            Result.success(remoteUser.toDomain())
        } catch (e: Exception) {
            // 网络失败则从本地获取
            userDao.getUserById(userId)?.let {
                Result.success(it.toDomain())
            } ?: Result.failure(e)
        }
    }
}

// Application类
@HiltAndroidApp
class MyApplication : Application()

// 模块定义
@Module
@InstallIn(SingletonComponent::class)
object NetworkModule {

    @Provides
    @Singleton
    fun provideRetrofit(): Retrofit {
        return Retrofit.Builder()
            .baseUrl(BuildConfig.BASE_URL)
            .addConverterFactory(Json.asConverterFactory())
            .build()
    }
}

// 注入使用
@HiltViewModel
class MyViewModel @Inject constructor(
    private val repository: UserRepository
) : ViewModel()

// UI状态定义
data class NewsUiState(
    val isLoading: Boolean = false,
    val news: List<NewsItem> = emptyList(),
    val error: String? = null,
    val selectedCategory: NewsCategory = NewsCategory.ALL
)

// ViewModel实现
class NewsViewModel @Inject constructor(
    private val getNewsUseCase: GetNewsUseCase
) : ViewModel() {

    private val _uiState = MutableStateFlow(NewsUiState())
    val uiState: StateFlow<NewsUiState> = _uiState.asStateFlow()

    fun selectCategory(category: NewsCategory) {
        _uiState.update { it.copy(selectedCategory = category) }
        loadNews()
    }

    private fun loadNews() {
        viewModelScope.launch {
            _uiState.update { it.copy(isLoading = true, error = null) }

            getNewsUseCase(_uiState.value.selectedCategory)
                .collect { result ->
                    result.onSuccess { news ->
                        _uiState.update {
                            it.copy(news = news, isLoading = false)
                        }
                    }.onFailure { error ->
                        _uiState.update {
                            it.copy(error = error.message, isLoading = false)
                        }
                    }
                }
        }
    }
}

// 生命周期感知的数据收集
@Composable
fun NewsScreen(viewModel: NewsViewModel = hiltViewModel()) {
    val uiState by viewModel.uiState.collectAsStateWithLifecycle()

    when {
        uiState.isLoading -> LoadingIndicator()
        uiState.error != null -> ErrorMessage(uiState.error!!)
        uiState.news.isEmpty() -> EmptyState()
        else -> NewsList(uiState.news)
    }
}

// 在Activity中处理系统栏内边距
class MainActivity : ComponentActivity() {
    override fun onCreate(savedInstanceState: Bundle?) {
        super.onCreate(savedInstanceState)

        // 启用边到边显示
        enableEdgeToEdge()

        setContent {
            MyAppTheme {
                // 使用WindowInsets处理系统栏内边距
                Scaffold(
                    modifier = Modifier.fillMaxSize(),
                    contentWindowInsets = WindowInsets(0, 0, 0, 0)
                ) { innerPadding ->
                    // 内容区域自动处理内边距
                    Content(
                        modifier = Modifier.padding(innerPadding)
                    )
                }
            }
        }
    }
}

// 在Activity中启用预测性返回
class MainActivity : ComponentActivity() {
    override fun onCreate(savedInstanceState: Bundle?) {
        // 启用预测性返回动画
        if (Build.VERSION.SDK_INT >= 35) {
            onBackInvokedDispatcher.registerOnBackInvokedCallback(
                OnBackInvokedDispatcher.PRIORITY_DEFAULT
            ) {
                // 处理返回逻辑
                handleBackNavigation()
            }
        }
    }
}

// 数据模型
data class Task(
    val id: String,
    val title: String,
    val description: String,
    val isCompleted: Boolean,
    val category: Category,
    val dueDate: LocalDateTime?
)

// UI状态
data class TodoUiState(
    val tasks: List<Task> = emptyList(),
    val filter: TaskFilter = TaskFilter.ALL,
    val isLoading: Boolean = false,
    val error: String? = null
)

// Repository接口
interface TaskRepository {  // interface定义类型契约
    fun getTasksFlow(): Flow<List<Task>>
    suspend fun addTask(task: Task): Result<Unit>
    suspend fun updateTask(task: Task): Result<Unit>
    suspend fun deleteTask(taskId: String): Result<Unit>
}