附录C Android面试题库¶

val name = "Alice"  // 不可变
var age = 25        // 可变
age = 26            // ✅ 可以
name = "Bob"        // ❌ 编译错误

var nullable: String? = null      // 可为null
var nonNull: String = "value"     // 不可为null

// 安全调用
val length = nullable?.length     // null时返回null

// Elvis操作符
val len = nullable?.length ?: 0   // null时使用默认值

// 非空断言（谨慎使用）
val risky = nullable!!.length     // null时抛出NPE

data class User(val id: String, val name: String, val age: Int)

val user = User("1", "Alice", 25)
val copy = user.copy(name = "Bob")
val (id, name, age) = user  // 解构

// 启动协程
val job = viewModelScope.launch { /* 后台任务 */ }

// 异步返回结果
val deferred = viewModelScope.async { fetchData() }
val result = deferred.await()

// Flow响应式流
flow {
    emit(1)
    emit(2)
}.collect { value -> println(value) }

// 创建Handler
val handler = Handler(Looper.getMainLooper()) {
    // 处理消息
    true
}

// 发送消息
handler.sendMessage(Message.obtain())
handler.post { /* 任务 */ }

class MyViewModel : ViewModel() {
    private val _data = MutableLiveData<String>()
    val data: LiveData<String> = _data

    fun loadData() {
        viewModelScope.launch {
            _data.value = fetchData()
        }
    }
}

@Entity
data class User(@PrimaryKey val id: String, val name: String)

@Dao
interface UserDao {  // interface定义类型契约
    @Query("SELECT * FROM user")
    fun getAll(): Flow<List<User>>

    @Insert
    suspend fun insert(user: User)
}

@Database(entities = [User::class], version = 1)
abstract class AppDatabase : RoomDatabase() {
    abstract fun userDao(): UserDao
}

val workRequest = OneTimeWorkRequestBuilder<MyWorker>()
    .setConstraints(
        Constraints.Builder()
            .setRequiredNetworkType(NetworkType.CONNECTED)
            .build()
    )
    .build()

WorkManager.getInstance(context).enqueue(workRequest)

@Composable
fun Counter() {
    var count by remember { mutableStateOf(0) }

    Button(onClick = { count++ }) {
        Text("Count: $count")  // count变化时重组
    }
}

// 本地状态
var text by remember { mutableStateOf("") }

// 状态提升
@Composable
fun Parent() {
    var text by remember { mutableStateOf("") }
    Child(text = text, onTextChange = { text = it })
}

// ViewModel状态
@HiltViewModel
class MyViewModel : ViewModel() {
    private val _uiState = MutableStateFlow(UiState())
    val uiState: StateFlow<UiState> = _uiState.asStateFlow()
}

// LaunchedEffect：在协程中执行副作用
LaunchedEffect(key) {
    // 在key变化时执行
}

// SideEffect：每次重组成功时执行
SideEffect {
    // 同步到外部系统
}

// DisposableEffect：需要清理的副作用
DisposableEffect(key) {
    // 初始化
    onDispose {
        // 清理
    }
}

// Hilt示例
@HiltViewModel
class MyViewModel @Inject constructor(
    private val repository: MyRepository
) : ViewModel()

// Kotlin object（推荐）
object DatabaseHelper {
    fun query() { }
}

// 伴生对象
class Manager private constructor() {
    companion object {
        @Volatile
        private var instance: Manager? = null

        fun getInstance() = instance ?: synchronized(this) {  // synchronized同步锁，保证线程安全
            instance ?: Manager().also { instance = it }
        }
    }
}

// Hilt单例
@Singleton
class Repository @Inject constructor()

// 1. 使用ViewHolder模式
// 2. 设置固定大小
recyclerView.setHasFixedSize(true)

// 3. 使用合适的LayoutManager
// 4. 分页加载
// 5. 图片懒加载
// 6. 缓存复用

// Android端
MethodChannel(flutterEngine.dartExecutor.binaryMessenger, "com.example/channel")
    .setMethodCallHandler { call, result ->
        when (call.method) {
            "methodName" -> {
                val value = call.argument<String>("key")
                result.success("Response")
            }
        }
    }