⏸️ suspend Functions

📖 What is suspend?

suspend functions are special functions that can suspend a coroutine. They can wait without blocking threads, making them efficient!

💡 Basic Concepts

suspend Keyword

// suspend functions can call other suspend functions
suspend fun doSomething() {
    delay(1000)  // suspend function
    println("완료!")
}

fun main() = runBlocking {
    doSomething()  // Call within a coroutine
}

Regular Functions vs suspend Functions

// ❌ Cannot call suspend functions from regular functions
fun normalFunction() {
    // delay(1000)  // Compile error!
}

// ✅ Possible in suspend functions
suspend fun suspendFunction() {
    delay(1000)  // OK!
}

🎯 Practical Examples

Network Request

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

suspend fun fetchUser(userId: String): User {
    delay(1000)  // Simulate network delay
    return User(userId, "홍길동")
}

fun main() = runBlocking {
    println("요청 시작")
    val user = fetchUser("user123")
    println("받은 유저: ${user.name}")
}

Sequential Execution

suspend fun prepareFood(): String {
    delay(1000)
    return "음식"
}

suspend fun setTable(): String {
    delay(500)
    return "테이블"
}

suspend fun prepareMeal() {
    println("준비 시작")

    val food = prepareFood()
    println("$food 준비 완료")

    val table = setTable()
    println("$table 준비 완료")

    println("모든 준비 끝!")
}

fun main() = runBlocking {
    val time = measureTimeMillis {
        prepareMeal()
    }
    println("소요 시간: ${time}ms")  // ~1500ms
}

Parallel Execution

suspend fun prepareMealParallel() = coroutineScope {
    println("준비 시작")

    val foodDeferred = async { prepareFood() }
    val tableDeferred = async { setTable() }

    val food = foodDeferred.await()
    val table = tableDeferred.await()

    println("$food, $table 준비 완료")
}

fun main() = runBlocking {
    val time = measureTimeMillis {
        prepareMealParallel()
    }
    println("소요 시간: ${time}ms")  // ~1000ms (병렬!)
}

🔧 withContext

Dispatcher Switching

suspend fun loadData(): String {
    return withContext(Dispatchers.IO) {
        // Run on IO thread
        delay(1000)
        "데이터"
    }
}

fun main() = runBlocking {
    println("메인 쓰레드: ${Thread.currentThread().name}")

    val data = loadData()

    println("다시 메인: ${Thread.currentThread().name}")
    println("데이터: $data")
}

Multiple Contexts

suspend fun processData() {
    // IO operation
    val data = withContext(Dispatchers.IO) {
        readFromFile()
    }

    // CPU-intensive operation
    val processed = withContext(Dispatchers.Default) {
        heavyComputation(data)
    }

    // Main thread (UI update, etc.)
    withContext(Dispatchers.Main) {
        updateUI(processed)
    }
}

suspend fun readFromFile(): String {
    delay(500)
    return "파일 내용"
}

suspend fun heavyComputation(data: String): String {
    delay(1000)
    return data.uppercase()
}

suspend fun updateUI(data: String) {
    println("UI 업데이트: $data")
}

🎨 Advanced Patterns

Conditional Waiting

suspend fun fetchDataIfNeeded(cache: String?): String {
    return cache ?: fetchFromNetwork()
}

suspend fun fetchFromNetwork(): String {
    delay(1000)
    return "새 데이터"
}

fun main() = runBlocking {
    // With cache - immediate return
    val cached = fetchDataIfNeeded("캐시 데이터")
    println(cached)

    // No cache - network request
    val fresh = fetchDataIfNeeded(null)
    println(fresh)
}

Chaining

suspend fun step1(): Int {
    delay(500)
    return 10
}

suspend fun step2(input: Int): Int {
    delay(500)
    return input * 2
}

suspend fun step3(input: Int): Int {
    delay(500)
    return input + 5
}

suspend fun processChain(): Int {
    val a = step1()
    val b = step2(a)
    val c = step3(b)
    return c
}

fun main() = runBlocking {
    val result = processChain()
    println("최종 결과: $result")  // 25 (10 * 2 + 5)
}

Error Handling

suspend fun riskyOperation(): String {
    delay(1000)
    throw Exception("네트워크 오류!")
}

suspend fun safeOperation(): String? {
    return try {
        riskyOperation()
    } catch (e: Exception) {
        println("에러 발생: ${e.message}")
        null
    }
}

fun main() = runBlocking {
    val result = safeOperation()
    println("결과: ${result ?: "실패"}")
}

🔥 Practical Functions

Timeout Handling

suspend fun fetchWithTimeout(): String? {
    return try {
        withTimeout(2000) {
            delay(3000)  // Takes too long
            "데이터"
        }
    } catch (e: TimeoutCancellationException) {
        println("시간 초과!")
        null
    }
}

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

Retry Logic

suspend fun <T> retry(
    times: Int = 3,
    initialDelay: Long = 100,
    factor: Double = 2.0,
    block: suspend () -> T
): T {
    var currentDelay = initialDelay
    repeat(times - 1) { attempt ->
        try {
            return block()
        } catch (e: Exception) {
            println("시도 ${attempt + 1} 실패: ${e.message}")
        }
        delay(currentDelay)
        currentDelay = (currentDelay * factor).toLong()
    }
    return block()  // Last attempt
}

var attemptCount = 0

suspend fun unstableAPI(): String {
    attemptCount++
    if (attemptCount < 3) {
        throw Exception("일시적 오류")
    }
    return "성공!"
}

fun main() = runBlocking {
    val result = retry(times = 5) {
        unstableAPI()
    }
    println(result)
}

Cache Pattern

class DataRepository {
    private var cache: String? = null

    suspend fun getData(forceRefresh: Boolean = false): String {
        if (!forceRefresh && cache != null) {
            println("캐시에서 반환")
            return cache!!
        }

        println("네트워크에서 가져오는 중...")
        delay(1000)
        val data = "새 데이터"
        cache = data
        return data
    }
}

fun main() = runBlocking {
    val repo = DataRepository()

    // First call - network
    println(repo.getData())

    // Second call - cache
    println(repo.getData())

    // Force refresh
    println(repo.getData(forceRefresh = true))
}

🛡️ Cancellable Functions

Cancellation Check

suspend fun longRunningTask() {
    repeat(10) { i ->
        if (!isActive) {
            println("취소됨!")
            return
        }

        println("작업 $i")
        delay(500)
    }
}

fun main() = runBlocking {
    val job = launch {
        longRunningTask()
    }

    delay(2000)
    println("작업 취소!")
    job.cancelAndJoin()
}

ensureActive

suspend fun heavyWork() {
    repeat(10) { i ->
        ensureActive()  // Throws exception if cancelled

        println("무거운 작업 $i")
        Thread.sleep(500)  // Simulate CPU work
    }
}

fun main() = runBlocking {
    val job = launch {
        try {
            heavyWork()
        } catch (e: CancellationException) {
            println("작업이 취소되었습니다")
        }
    }

    delay(2000)
    job.cancel()
}

🤔 Frequently Asked Questions

Q1. Do suspend functions create threads?

A: No! suspend only suspends without blocking threads.

suspend fun example() {
    println("시작: ${Thread.currentThread().name}")
    delay(1000)
    println("끝: ${Thread.currentThread().name}")
}
// Can run on the same thread!

Q2. What if I make all functions suspend?

A: Unnecessary! Use it only when there are asynchronous operations.

// ❌ Unnecessary
suspend fun add(a: Int, b: Int) = a + b

// ✅ Necessary
suspend fun fetchData() = withContext(Dispatchers.IO) {
    // Network request
}

Q3. Can suspend functions call regular functions?

A: Of course!

fun normalFunction() = println("일반 함수")

suspend fun suspendFunction() {
    normalFunction()  // OK!
    delay(1000)
}

🎬 Conclusion

Write efficient asynchronous code with suspend functions!

Key Points:
✅ Suspend with suspend keyword
✅ Wait without blocking threads
✅ Switch threads with withContext
✅ Control sequential/parallel execution
✅ Cancellable operations

Next Step: Learn about asynchronous data streams in Flow!