kotlin-expert

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Kotlin Expert

Kotlin 专家指南

Expert guidance for Kotlin development, Android, coroutines, Kotlin Multiplatform, and modern JVM development.

为Kotlin开发、Android、协程、Kotlin Multiplatform及现代JVM开发提供专业指导。

Core Concepts

核心概念

Kotlin Fundamentals

Kotlin 基础

Null safety
Extension functions
Data classes
Sealed classes
Coroutines and Flow
Higher-order functions

空安全
扩展函数
数据类
密封类
协程与Flow
高阶函数

Android Development

Android 开发

Jetpack Compose
ViewModel and LiveData
Room database
Retrofit networking
Dependency injection (Hilt)
Android lifecycle

Jetpack Compose
ViewModel与LiveData
Room数据库
Retrofit网络请求
依赖注入（Hilt）
Android生命周期

Kotlin Multiplatform

Shared business logic
Platform-specific implementations
iOS and Android targets
Common module architecture

共享业务逻辑
平台特定实现
iOS与Android目标平台
通用模块架构

Modern Kotlin Syntax

现代Kotlin语法

kotlin

// Data classes
data class User(
    val id: String,
    val name: String,
    val email: String,
    val createdAt: LocalDateTime = LocalDateTime.now()
)

// Sealed classes for type-safe states
sealed class Result<out T> {
    data class Success<T>(val data: T) : Result<T>()
    data class Error(val exception: Exception) : Result<Nothing>()
    object Loading : Result<Nothing>()
}

// Extension functions
fun String.isValidEmail(): Boolean {
    return this.contains("@") && this.contains(".")
}

// Scope functions
fun processUser(user: User) {
    user.run {
        println("Processing user: $name")
        // 'this' refers to user
    }

    user.let { u ->
        // 'it' or custom name refers to user
        println(u.email)
    }

    user.apply {
        // Modify properties
        // Returns the object
    }
}

// Null safety
fun findUser(id: String): User? {
    return database.find(id)
}

val user = findUser("123")
val name = user?.name ?: "Unknown" // Elvis operator
user?.let { println(it.name) } // Safe call with let

// When expression
fun getUserStatus(user: User): String = when {
    user.isActive && user.isPremium -> "Premium Active"
    user.isActive -> "Active"
    else -> "Inactive"
}

kotlin

// Data classes
data class User(
    val id: String,
    val name: String,
    val email: String,
    val createdAt: LocalDateTime = LocalDateTime.now()
)

// Sealed classes for type-safe states
sealed class Result<out T> {
    data class Success<T>(val data: T) : Result<T>()
    data class Error(val exception: Exception) : Result<Nothing>()
    object Loading : Result<Nothing>()
}

// Extension functions
fun String.isValidEmail(): Boolean {
    return this.contains("@") && this.contains(".")
}

// Scope functions
fun processUser(user: User) {
    user.run {
        println("Processing user: $name")
        // 'this' refers to user
    }

    user.let { u ->
        // 'it' or custom name refers to user
        println(u.email)
    }

    user.apply {
        // Modify properties
        // Returns the object
    }
}

// Null safety
fun findUser(id: String): User? {
    return database.find(id)
}

val user = findUser("123")
val name = user?.name ?: "Unknown" // Elvis operator
user?.let { println(it.name) } // Safe call with let

// When expression
fun getUserStatus(user: User): String = when {
    user.isActive && user.isPremium -> "Premium Active"
    user.isActive -> "Active"
    else -> "Inactive"
}

Coroutines and Flow

协程与Flow

kotlin

import kotlinx.coroutines.*
import kotlinx.coroutines.flow.*

class UserRepository {
    private val api: UserApi

    // Suspend function
    suspend fun fetchUser(id: String): User {
        return withContext(Dispatchers.IO) {
            api.getUser(id)
        }
    }

    // Flow for reactive streams
    fun observeUsers(): Flow<List<User>> = flow {
        while (true) {
            val users = fetchUsers()
            emit(users)
            delay(5000) // Refresh every 5 seconds
        }
    }.flowOn(Dispatchers.IO)

    // StateFlow for state management
    private val _users = MutableStateFlow<List<User>>(emptyList())
    val users: StateFlow<List<User>> = _users.asStateFlow()

    suspend fun refreshUsers() {
        _users.value = fetchUsers()
    }
}

// Coroutine scopes
class UserViewModel : ViewModel() {
    private val repository = UserRepository()

    fun loadUsers() {
        viewModelScope.launch {
            try {
                val users = repository.fetchUser("123")
                // Update UI
            } catch (e: Exception) {
                // Handle error
            }
        }
    }

    // Parallel execution
    suspend fun loadMultipleUsers(ids: List<String>): List<User> {
        return coroutineScope {
            ids.map { id ->
                async { repository.fetchUser(id) }
            }.awaitAll()
        }
    }

    // Flow transformation
    fun searchUsers(query: String): Flow<List<User>> {
        return repository.observeUsers()
            .map { users -> users.filter { it.name.contains(query, ignoreCase = true) } }
            .distinctUntilChanged()
            .debounce(300)
    }
}

kotlin

import kotlinx.coroutines.*
import kotlinx.coroutines.flow.*

class UserRepository {
    private val api: UserApi

    // Suspend function
    suspend fun fetchUser(id: String): User {
        return withContext(Dispatchers.IO) {
            api.getUser(id)
        }
    }

    // Flow for reactive streams
    fun observeUsers(): Flow<List<User>> = flow {
        while (true) {
            val users = fetchUsers()
            emit(users)
            delay(5000) // Refresh every 5 seconds
        }
    }.flowOn(Dispatchers.IO)

    // StateFlow for state management
    private val _users = MutableStateFlow<List<User>>(emptyList())
    val users: StateFlow<List<User>> = _users.asStateFlow()

    suspend fun refreshUsers() {
        _users.value = fetchUsers()
    }
}

// Coroutine scopes
class UserViewModel : ViewModel() {
    private val repository = UserRepository()

    fun loadUsers() {
        viewModelScope.launch {
            try {
                val users = repository.fetchUser("123")
                // Update UI
            } catch (e: Exception) {
                // Handle error
            }
        }
    }

    // Parallel execution
    suspend fun loadMultipleUsers(ids: List<String>): List<User> {
        return coroutineScope {
            ids.map { id ->
                async { repository.fetchUser(id) }
            }.awaitAll()
        }
    }

    // Flow transformation
    fun searchUsers(query: String): Flow<List<User>> {
        return repository.observeUsers()
            .map { users -> users.filter { it.name.contains(query, ignoreCase = true) } }
            .distinctUntilChanged()
            .debounce(300)
    }
}

Android Jetpack Compose

kotlin

import androidx.compose.foundation.layout.*
import androidx.compose.material3.*
import androidx.compose.runtime.*
import androidx.compose.ui.Modifier

@Composable
fun UserListScreen(viewModel: UserViewModel = viewModel()) {
    val users by viewModel.users.collectAsState()
    val isLoading by viewModel.isLoading.collectAsState()

    Scaffold(
        topBar = { TopAppBar(title = { Text("Users") }) }
    ) { padding ->
        if (isLoading) {
            CircularProgressIndicator(
                modifier = Modifier.fillMaxSize()
            )
        } else {
            LazyColumn(
                modifier = Modifier.padding(padding)
            ) {
                items(users) { user ->
                    UserCard(user = user)
                }
            }
        }
    }
}

@Composable
fun UserCard(user: User) {
    Card(
        modifier = Modifier
            .fillMaxWidth()
            .padding(8.dp)
    ) {
        Column(modifier = Modifier.padding(16.dp)) {
            Text(
                text = user.name,
                style = MaterialTheme.typography.headlineSmall
            )
            Text(
                text = user.email,
                style = MaterialTheme.typography.bodyMedium
            )
        }
    }
}

kotlin

import androidx.compose.foundation.layout.*
import androidx.compose.material3.*
import androidx.compose.runtime.*
import androidx.compose.ui.Modifier

@Composable
fun UserListScreen(viewModel: UserViewModel = viewModel()) {
    val users by viewModel.users.collectAsState()
    val isLoading by viewModel.isLoading.collectAsState()

    Scaffold(
        topBar = { TopAppBar(title = { Text("Users") }) }
    ) { padding ->
        if (isLoading) {
            CircularProgressIndicator(
                modifier = Modifier.fillMaxSize()
            )
        } else {
            LazyColumn(
                modifier = Modifier.padding(padding)
            ) {
                items(users) { user ->
                    UserCard(user = user)
                }
            }
        }
    }
}

@Composable
fun UserCard(user: User) {
    Card(
        modifier = Modifier
            .fillMaxWidth()
            .padding(8.dp)
    ) {
        Column(modifier = Modifier.padding(16.dp)) {
            Text(
                text = user.name,
                style = MaterialTheme.typography.headlineSmall
            )
            Text(
                text = user.email,
                style = MaterialTheme.typography.bodyMedium
            )
        }
    }
}

Room Database

Room 数据库

kotlin

import androidx.room.*
import kotlinx.coroutines.flow.Flow

@Entity(tableName = "users")
data class UserEntity(
    @PrimaryKey val id: String,
    @ColumnInfo(name = "name") val name: String,
    @ColumnInfo(name = "email") val email: String,
    @ColumnInfo(name = "created_at") val createdAt: Long
)

@Dao
interface UserDao {
    @Query("SELECT * FROM users")
    fun getAllUsers(): Flow<List<UserEntity>>

    @Query("SELECT * FROM users WHERE id = :userId")
    suspend fun getUserById(userId: String): UserEntity?

    @Insert(onConflict = OnConflictStrategy.REPLACE)
    suspend fun insertUser(user: UserEntity)

    @Update
    suspend fun updateUser(user: UserEntity)

    @Delete
    suspend fun deleteUser(user: UserEntity)

    @Query("DELETE FROM users WHERE id = :userId")
    suspend fun deleteUserById(userId: String)
}

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

    companion object {
        @Volatile
        private var INSTANCE: AppDatabase? = null

        fun getDatabase(context: Context): AppDatabase {
            return INSTANCE ?: synchronized(this) {
                val instance = Room.databaseBuilder(
                    context.applicationContext,
                    AppDatabase::class.java,
                    "app_database"
                ).build()
                INSTANCE = instance
                instance
            }
        }
    }
}

kotlin

import androidx.room.*
import kotlinx.coroutines.flow.Flow

@Entity(tableName = "users")
data class UserEntity(
    @PrimaryKey val id: String,
    @ColumnInfo(name = "name") val name: String,
    @ColumnInfo(name = "email") val email: String,
    @ColumnInfo(name = "created_at") val createdAt: Long
)

@Dao
interface UserDao {
    @Query("SELECT * FROM users")
    fun getAllUsers(): Flow<List<UserEntity>>

    @Query("SELECT * FROM users WHERE id = :userId")
    suspend fun getUserById(userId: String): UserEntity?

    @Insert(onConflict = OnConflictStrategy.REPLACE)
    suspend fun insertUser(user: UserEntity)

    @Update
    suspend fun updateUser(user: UserEntity)

    @Delete
    suspend fun deleteUser(user: UserEntity)

    @Query("DELETE FROM users WHERE id = :userId")
    suspend fun deleteUserById(userId: String)
}

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

    companion object {
        @Volatile
        private var INSTANCE: AppDatabase? = null

        fun getDatabase(context: Context): AppDatabase {
            return INSTANCE ?: synchronized(this) {
                val instance = Room.databaseBuilder(
                    context.applicationContext,
                    AppDatabase::class.java,
                    "app_database"
                ).build()
                INSTANCE = instance
                instance
            }
        }
    }
}

Dependency Injection with Hilt

使用Hilt实现依赖注入

kotlin

import dagger.Module
import dagger.Provides
import dagger.hilt.InstallIn
import dagger.hilt.components.SingletonComponent
import javax.inject.Singleton

@Module
@InstallIn(SingletonComponent::class)
object NetworkModule {

    @Provides
    @Singleton
    fun provideRetrofit(): Retrofit {
        return Retrofit.Builder()
            .baseUrl("https://api.example.com")
            .addConverterFactory(GsonConverterFactory.create())
            .build()
    }

    @Provides
    @Singleton
    fun provideUserApi(retrofit: Retrofit): UserApi {
        return retrofit.create(UserApi::class.java)
    }
}

@HiltAndroidApp
class MyApplication : Application()

@AndroidEntryPoint
class MainActivity : ComponentActivity() {
    @Inject lateinit var repository: UserRepository
}

kotlin

import dagger.Module
import dagger.Provides
import dagger.hilt.InstallIn
import dagger.hilt.components.SingletonComponent
import javax.inject.Singleton

@Module
@InstallIn(SingletonComponent::class)
object NetworkModule {

    @Provides
    @Singleton
    fun provideRetrofit(): Retrofit {
        return Retrofit.Builder()
            .baseUrl("https://api.example.com")
            .addConverterFactory(GsonConverterFactory.create())
            .build()
    }

    @Provides
    @Singleton
    fun provideUserApi(retrofit: Retrofit): UserApi {
        return retrofit.create(UserApi::class.java)
    }
}

@HiltAndroidApp
class MyApplication : Application()

@AndroidEntryPoint
class MainActivity : ComponentActivity() {
    @Inject lateinit var repository: UserRepository
}

Kotlin Multiplatform

Kotlin Multiplatform 跨平台开发

kotlin

// commonMain
expect class Platform() {
    val name: String
}

expect fun platformSpecificFunction(): String

// androidMain
actual class Platform actual constructor() {
    actual val name: String = "Android ${android.os.Build.VERSION.SDK_INT}"
}

actual fun platformSpecificFunction(): String = "Android implementation"

// iosMain
actual class Platform actual constructor() {
    actual val name: String = UIDevice.currentDevice.systemName()
}

actual fun platformSpecificFunction(): String = "iOS implementation"

// Shared business logic
class UserService {
    suspend fun fetchUser(id: String): User {
        // Shared logic works on all platforms
        return api.getUser(id)
    }
}

kotlin

// commonMain
expect class Platform() {
    val name: String
}

expect fun platformSpecificFunction(): String

// androidMain
actual class Platform actual constructor() {
    actual val name: String = "Android ${android.os.Build.VERSION.SDK_INT}"
}

actual fun platformSpecificFunction(): String = "Android implementation"

// iosMain
actual class Platform actual constructor() {
    actual val name: String = UIDevice.currentDevice.systemName()
}

actual fun platformSpecificFunction(): String = "iOS implementation"

// Shared business logic
class UserService {
    suspend fun fetchUser(id: String): User {
        // Shared logic works on all platforms
        return api.getUser(id)
    }
}

Best Practices

最佳实践

Kotlin Style

Kotlin 编码风格

Use val over var when possible
Leverage null safety features
Use data classes for DTOs
Prefer extension functions
Use sealed classes for type-safe states
Follow naming conventions

尽可能使用val而非var
充分利用空安全特性
使用数据类作为DTO
优先使用扩展函数
使用密封类实现类型安全状态
遵循命名规范

Coroutines

协程

Use appropriate dispatchers (IO, Main, Default)
Handle cancellation properly
Avoid GlobalScope
Use structured concurrency
Prefer Flow over LiveData
Use StateFlow for state

使用合适的调度器（IO、Main、Default）
正确处理取消操作
避免使用GlobalScope
使用结构化并发
优先使用Flow而非LiveData
使用StateFlow管理状态

Android

Android 开发

Follow MVVM architecture
Use Jetpack Compose for UI
Implement proper lifecycle handling
Use dependency injection
Handle configuration changes
Implement proper error handling

遵循MVVM架构
使用Jetpack Compose构建UI
实现正确的生命周期管理
使用依赖注入
处理配置变更
实现完善的错误处理

Anti-Patterns

反模式

❌ Using !! (non-null assertion) ❌ GlobalScope.launch ❌ Blocking main thread ❌ Not handling coroutine cancellation ❌ Tight coupling ❌ God classes ❌ Ignoring memory leaks

❌ 使用!!（非空断言） ❌ GlobalScope.launch ❌ 阻塞主线程 ❌ 未处理协程取消 ❌ 紧耦合 ❌ 上帝类 ❌ 忽略内存泄漏

Resources

参考资源

Kotlin Docs: https://kotlinlang.org/docs/
Android Developers: https://developer.android.com/kotlin
Kotlin Coroutines: https://kotlinlang.org/docs/coroutines-overview.html
Jetpack Compose: https://developer.android.com/jetpack/compose

Kotlin文档：https://kotlinlang.org/docs/
Android开发者官网：https://developer.android.com/kotlin
Kotlin协程：https://kotlinlang.org/docs/coroutines-overview.html
Jetpack Compose：https://developer.android.com/jetpack/compose