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🎛️ Context & Dispatcher

📖 什么是 CoroutineContext？​

CoroutineContext 是决定协程如何执行的配置集合。它指定在哪个线程上执行、名称是什么等！

💡 Dispatcher​

基本 Dispatcher​

fun main() = runBlocking {
    // Main - UI 线程 (Android/Desktop)
    launch(Dispatchers.Main) {
        // UI 更新
    }

    // IO - 网络/文件操作
    launch(Dispatchers.IO) {
        println("IO: ${Thread.currentThread().name}")
    }

    // Default - CPU 密集型任务
    launch(Dispatchers.Default) {
        println("Default: ${Thread.currentThread().name}")
    }

    // Unconfined - 特殊情况
    launch(Dispatchers.Unconfined) {
        println("Unconfined: ${Thread.currentThread().name}")
    }

    delay(100)
}

Dispatchers.IO​

suspend fun readFile(): String = withContext(Dispatchers.IO) {
    // 文件读取、网络请求等
    delay(1000)
    "文件内容"
}

suspend fun writeFile(content: String) = withContext(Dispatchers.IO) {
    // 文件写入
    delay(500)
    println("文件保存: $content")
}

fun main() = runBlocking {
    val content = readFile()
    writeFile(content)
}

Dispatchers.Default​

suspend fun heavyComputation(): Int = withContext(Dispatchers.Default) {
    // CPU 密集型计算
    var result = 0
    repeat(1_000_000) {
        result += it
    }
    result
}

fun main() = runBlocking {
    val result = heavyComputation()
    println("计算结果: $result")
}

🎯 实战示例​

按层级使用 Dispatcher​

// Repository - IO
class UserRepository {
    suspend fun fetchUser(id: String): User = withContext(Dispatchers.IO) {
        delay(1000)  // 网络请求
        User(id, "张三")
    }
}

// UseCase - Default
class ProcessUserUseCase {
    suspend fun process(user: User): ProcessedUser = withContext(Dispatchers.Default) {
        // 数据处理
        delay(500)
        ProcessedUser(user.name.uppercase())
    }
}

data class User(val id: String, val name: String)
data class ProcessedUser(val displayName: String)

fun main() = runBlocking {
    val repo = UserRepository()
    val useCase = ProcessUserUseCase()

    val user = repo.fetchUser("123")
    val processed = useCase.process(user)
    println("结果: ${processed.displayName}")
}

并行 IO 操作​

suspend fun loadAllData(): Triple<String, String, String> = coroutineScope {
    val user = async(Dispatchers.IO) {
        delay(1000)
        "用户数据"
    }

    val posts = async(Dispatchers.IO) {
        delay(1500)
        "帖子数据"
    }

    val comments = async(Dispatchers.IO) {
        delay(800)
        "评论数据"
    }

    Triple(user.await(), posts.await(), comments.await())
}

fun main() = runBlocking {
    val time = measureTimeMillis {
        val (user, posts, comments) = loadAllData()
        println("$user, $posts, $comments")
    }
    println("耗时: ${time}ms")  // ~1500ms (并行)
}

🔧 Context 组合​

命名​

fun main() = runBlocking {
    launch(CoroutineName("任务1")) {
        println("名称: ${coroutineContext[CoroutineName]}")
    }

    launch(Dispatchers.IO + CoroutineName("IO任务")) {
        println("线程: ${Thread.currentThread().name}")
        println("名称: ${coroutineContext[CoroutineName]}")
    }

    delay(100)
}

添加 Job​

fun main() = runBlocking {
    val job = Job()

    launch(job + Dispatchers.Default) {
        println("任务执行")
        delay(1000)
        println("任务完成")
    }

    delay(500)
    println("任务取消")
    job.cancel()
}

🎨 withContext​

线程切换​

suspend fun complexTask() {
    println("开始: ${Thread.currentThread().name}")

    // IO 操作
    val data = withContext(Dispatchers.IO) {
        println("IO: ${Thread.currentThread().name}")
        "数据"
    }

    // CPU 操作
    val processed = withContext(Dispatchers.Default) {
        println("Default: ${Thread.currentThread().name}")
        data.uppercase()
    }

    println("结束: ${Thread.currentThread().name}")
    println("结果: $processed")
}

fun main() = runBlocking {
    complexTask()
}

优化模式​

// ❌ 不必要的切换
suspend fun bad() {
    withContext(Dispatchers.IO) {
        val data1 = loadData1()
        withContext(Dispatchers.Default) {  // 不必要！
            process(data1)
        }
    }
}

// ✅ 高效
suspend fun good() {
    val data1 = withContext(Dispatchers.IO) {
        loadData1()
    }

    withContext(Dispatchers.Default) {
        process(data1)
    }
}

suspend fun loadData1() = delay(100)
suspend fun process(data: Unit) = delay(100)

🔥 实用模式​

缓存 + 网络​

class DataSource {
    private var cache: String? = null

    suspend fun getData(): String {
        // 检查缓存 (快)
        cache?.let { return it }

        // 网络请求 (慢)
        return withContext(Dispatchers.IO) {
            delay(1000)
            "新数据"
        }.also { cache = it }
    }
}

fun main() = runBlocking {
    val source = DataSource()

    // 第一次调用 - 网络
    val time1 = measureTimeMillis {
        println(source.getData())
    }
    println("第一次调用: ${time1}ms")

    // 第二次 - 缓存
    val time2 = measureTimeMillis {
        println(source.getData())
    }
    println("第二次: ${time2}ms")
}

批处理​

suspend fun processBatch(items: List<Int>): List<Int> {
    return withContext(Dispatchers.Default) {
        items.map { item ->
            // 处理每个项目
            item * 2
        }
    }
}

fun main() = runBlocking {
    val items = List(100) { it }
    val results = processBatch(items)
    println("处理完成: ${results.size}个")
}

配合超时​

suspend fun fetchWithTimeout(): String? {
    return try {
        withTimeout(2000) {
            withContext(Dispatchers.IO) {
                delay(3000)  // 耗时太长
                "数据"
            }
        }
    } catch (e: TimeoutCancellationException) {
        null
    }
}

fun main() = runBlocking {
    val result = fetchWithTimeout()
    println("结果: ${result ?: "超时"}")
}

🛡️ 异常处理​

CoroutineExceptionHandler​

fun main() = runBlocking {
    val handler = CoroutineExceptionHandler { _, exception ->
        println("错误处理: ${exception.message}")
    }

    val job = launch(handler) {
        throw Exception("发生问题！")
    }

    job.join()
    println("继续执行")
}

SupervisorJob​

fun main() = runBlocking {
    val supervisor = SupervisorJob()

    with(CoroutineScope(coroutineContext + supervisor)) {
        val job1 = launch {
            delay(500)
            throw Exception("任务1失败")
        }

        val job2 = launch {
            delay(1000)
            println("任务2成功！")
        }

        try {
            job1.join()
        } catch (e: Exception) {
            println("任务1异常: ${e.message}")
        }

        job2.join()
    }
}

🎯 自定义 Dispatcher​

指定线程池大小​

fun main() = runBlocking {
    val customDispatcher = Dispatchers.IO.limitedParallelism(2)

    repeat(5) { i ->
        launch(customDispatcher) {
            println("任务 $i: ${Thread.currentThread().name}")
            delay(1000)
        }
    }

    delay(3000)
}

单线程​

fun main() = runBlocking {
    val singleThread = Dispatchers.Default.limitedParallelism(1)

    repeat(3) { i ->
        launch(singleThread) {
            println("任务 $i: ${Thread.currentThread().name}")
            delay(500)
        }
    }

    delay(2000)
}

🤔 常见问题​

Q1. 应该使用哪个 Dispatcher？​

A: 根据任务类型选择！

// IO - 网络、文件、数据库
suspend fun fetchData() = withContext(Dispatchers.IO) { }

// Default - CPU 密集型计算
suspend fun compute() = withContext(Dispatchers.Default) { }

// Main - UI 更新 (Android/Desktop)
suspend fun updateUI() = withContext(Dispatchers.Main) { }

Q2. 可以多次使用 withContext 吗？​

A: 可以！需要时随时切换。

suspend fun workflow() {
    val data = withContext(Dispatchers.IO) {
        loadFromNetwork()
    }

    val processed = withContext(Dispatchers.Default) {
        processData(data)
    }

    withContext(Dispatchers.Main) {
        updateUI(processed)
    }
}

Q3. 不指定 Dispatcher 会怎样？​

A: 继承父协程的 Context！

fun main() = runBlocking(Dispatchers.Default) {
    launch {  // 继承 Dispatchers.Default
        println(Thread.currentThread().name)
    }
}

🎬 总结​

使用 Context 和 Dispatcher 控制协程！

核心要点:
✅ Dispatchers.IO - 网络/文件
✅ Dispatchers.Default - CPU 任务
✅ Dispatchers.Main - UI 更新
✅ withContext 切换线程
✅ Context 组合实现精细控制

恭喜！您已完成 Coroutines 系列！🎉

下一步: 在单元测试中学习如何编写测试！