Android中高级面试（二）：架构组件与协程实战精要

class UserViewModel : ViewModel() {
    private val _userData = MutableLiveData<User>()
    val userData: LiveData<User> get() = _userData

    // 数据加载逻辑封装在ViewModel中
    fun loadUser(userId: String) {
        viewModelScope.launch {
            _userData.value = repository.fetchUser(userId)
        }
    }
}

// Activity/Fragment中的使用
class ProfileActivity : AppCompatActivity() {
    private val viewModel by viewModels<UserViewModel>()

    override fun onCreate(savedInstanceState: Bundle?) {
        viewModel.userData.observe(this) { user ->
            updateUI(user)
        }
    }
}

class SearchViewModel(private val savedStateHandle: SavedStateHandle) : ViewModel() {
    val query: MutableLiveData<String> = savedStateHandle.getLiveData("query_key")

    fun setQuery(query: String) {
        savedStateHandle["query_key"] = query
    }
}

// 获取实例时注入SavedStateHandle
val viewModel: SearchViewModel by viewModels {
    SavedStateViewModelFactory(application, this)
}

val userIdLiveData = MutableLiveData<String>()
val userLiveData: LiveData<User> = Transformations.switchMap(userIdLiveData) { id ->
    repository.getUserById(id)
}

// 复合数据转换示例
val userProfile = Transformations.map(userLiveData) { user ->
    "${user.name} | ${user.email}"
}

// 错误示例：在Activity中使用匿名Observer
viewModel.data.observe(this) { data ->
    updateView(data)
    // 当Activity销毁时，此匿名Observer无法自动移除
}

// 正确方案：使用LifecycleOwner
class ProfileFragment : Fragment() {
    override fun onViewCreated(view: View, savedInstanceState: Bundle?) {
        viewModel.data.observe(viewLifecycleOwner) { data ->
            updateViews(data)
        }
    }
}

class OrderViewModel : ViewModel() {
    private val _orderStatus = MutableLiveData<OrderStatus>()
    val orderStatus: LiveData<OrderStatus> get() = _orderStatus

    fun fetchOrderDetails(orderId: String) {
        viewModelScope.launch(Dispatchers.IO) {
            try {
                val order = repository.getOrder(orderId)
                withContext(Dispatchers.Main) {
                    _orderStatus.value = OrderStatus.Success(order)
                }
            } catch (e: Exception) {
                _orderStatus.value = OrderStatus.Error(e)
            }
        }
    }
}

// 并行请求优化
suspend fun fetchDashboardData() = coroutineScope {
    val userDeferred = async { getUserData() }
    val notificationsDeferred = async { getNotifications() }
    
    val user = userDeferred.await()
    val notifications = notificationsDeferred.await()
    
    DashboardData(user, notifications)
}

@Dao
interface UserDao {
    @Query("SELECT * FROM users")
    fun getAllUsers(): Flow<List<User>>  // 使用Flow实现实时数据库监听

    @Insert
    suspend fun insertUser(user: User)  // 挂起函数保证线程安全
}

// ViewModel中使用
viewModelScope.launch {
    userDao.getAllUsers().collect { users ->
        _userList.value = users
    }
}

// 使用Android Profiler检测步骤：
// 1. 执行可能泄漏的操作（如旋转设备）
// 2. 触发GC
// 3. 检查Activity/Fragment实例是否仍被持有

// 使用LeakCanary自动检测
dependencies {
    debugImplementation 'com.squareup.leakcanary:leakcanary-android:2.9.1'
}

// LiveData示例
viewModel.data.observe(viewLifecycleOwner) { data ->
    updateUI(data)
}

// StateFlow示例
viewModel.uiState.collectLatest { state ->
    when (state) {
        is UiState.Loading -> showProgress()
        is UiState.Success -> showData(state.data)
        is UiState.Error -> showError(state.exception)
    }
}