🎯 Coroutine Basics

📖 Getting Started with Coroutines

To use coroutines, you first need to understand coroutine builders. Let's learn about the three most basic builders!

💡 runBlocking

Basic Usage

import kotlinx.coroutines.*

fun main() = runBlocking {  // Start coroutine!
    println("Hello")
    delay(1000)  // Wait 1 second
    println("World!")
}

Characteristics

fun main() {
    println("Before")

    runBlocking {
        delay(1000)
        println("Inside coroutine")
    }

    println("After")  // Wait until coroutine finishes
}
// Before
// (wait 1 second)
// Inside coroutine
// After

Note: runBlocking blocks the current thread! Use it mainly for testing or in the main function only.

🚀 launch

Starting Asynchronous Tasks

fun main() = runBlocking {
    launch {
        delay(1000)
        println("Task 1")
    }

    launch {
        delay(500)
        println("Task 2")
    }

    println("Main")
}
// Main
// (after 500ms) Task 2
// (after 1000ms) Task 1

Job Return

fun main() = runBlocking {
    val job = launch {
        repeat(5) { i ->
            println("Working... $i")
            delay(500)
        }
    }

    delay(1300)
    println("Canceling task!")
    job.cancel()  // Cancel task
    job.join()    // Wait for cancellation to complete
}

Managing Multiple Tasks

fun main() = runBlocking {
    val jobs = List(5) { i ->
        launch {
            delay(1000L * i)
            println("Task $i completed")
        }
    }

    jobs.forEach { it.join() }  // Wait for all tasks
    println("All tasks finished!")
}

⚡ async

Receiving Results

fun main() = runBlocking {
    val deferred = async {
        delay(1000)
        return@async "Result value"
    }

    println("Working...")
    val result = deferred.await()  // Wait for result
    println("Received value: $result")
}

launch vs async

fun main() = runBlocking {
    // launch - no result
    launch {
        delay(1000)
        println("launch completed")
    }

    // async - returns result
    val result = async {
        delay(1000)
        "async completed"
    }

    println(result.await())
}

🎯 Practical Examples

Parallel Processing

import kotlin.system.measureTimeMillis

fun main() = runBlocking {
    val time = measureTimeMillis {
        val one = async { fetchData1() }
        val two = async { fetchData2() }

        println("Results: ${one.await()}, ${two.await()}")
    }

    println("Time elapsed: ${time}ms")  // ~1000ms (parallel execution)
}

suspend fun fetchData1(): String {
    delay(1000)
    return "Data1"
}

suspend fun fetchData2(): String {
    delay(1000)
    return "Data2"
}

Sequential vs Parallel

fun main() = runBlocking {
    // ❌ Sequential execution (slow)
    val time1 = measureTimeMillis {
        val one = fetchData1()
        val two = fetchData2()
        println("$one, $two")
    }
    println("Sequential: ${time1}ms")  // ~2000ms

    // ✅ Parallel execution (fast)
    val time2 = measureTimeMillis {
        val one = async { fetchData1() }
        val two = async { fetchData2() }
        println("${one.await()}, ${two.await()}")
    }
    println("Parallel: ${time2}ms")  // ~1000ms
}

Exception Handling

fun main() = runBlocking {
    val deferred = async {
        delay(1000)
        throw Exception("Error occurred!")
    }

    try {
        deferred.await()
    } catch (e: Exception) {
        println("Caught error: ${e.message}")
    }
}

🔥 Practical Patterns

Timeout

fun main() = runBlocking {
    try {
        withTimeout(1300) {
            repeat(3) { i ->
                println("Task $i")
                delay(500)
            }
        }
    } catch (e: TimeoutCancellationException) {
        println("Timed out!")
    }
}

Multiple API Calls

data class UserData(
    val profile: String,
    val posts: List<String>,
    val friends: List<String>
)

suspend fun loadUserData(userId: String): UserData = coroutineScope {
    val profile = async { fetchProfile(userId) }
    val posts = async { fetchPosts(userId) }
    val friends = async { fetchFriends(userId) }

    UserData(
        profile.await(),
        posts.await(),
        friends.await()
    )
}

suspend fun fetchProfile(id: String): String {
    delay(500)
    return "Profile($id)"
}

suspend fun fetchPosts(id: String): List<String> {
    delay(800)
    return listOf("Post1", "Post2")
}

suspend fun fetchFriends(id: String): List<String> {
    delay(600)
    return listOf("Friend1", "Friend2")
}

Retry Logic

suspend fun <T> retryIO(
    times: Int = 3,
    delay: Long = 1000,
    block: suspend () -> T
): T {
    repeat(times - 1) { attempt ->
        try {
            return block()
        } catch (e: Exception) {
            println("Attempt ${attempt + 1} failed")
            delay(delay)
        }
    }
    return block()  // Last attempt
}

fun main() = runBlocking {
    val result = retryIO(times = 3) {
        fetchDataFromAPI()
    }
    println(result)
}

var attempt = 0
suspend fun fetchDataFromAPI(): String {
    attempt++
    if (attempt < 3) {
        throw Exception("Network error")
    }
    return "Success!"
}

🛠️ coroutineScope

Structured Concurrency

suspend fun doWork() = coroutineScope {
    launch {
        delay(1000)
        println("Task 1")
    }

    launch {
        delay(2000)
        println("Task 2")
    }

    println("All tasks started")
    // Wait until all children finish
}

fun main() = runBlocking {
    doWork()
    println("Done!")
}

Exception Propagation

suspend fun riskyWork() = coroutineScope {
    launch {
        delay(500)
        throw Exception("Error!")
    }

    launch {
        delay(1000)
        println("This code won't run")
    }
}

fun main() = runBlocking {
    try {
        riskyWork()
    } catch (e: Exception) {
        println("Error caught: ${e.message}")
    }
}

🤔 Frequently Asked Questions

Q1. What's the difference between launch and async?

A: Whether it returns a result!

fun main() = runBlocking {
    // launch - no result needed
    launch {
        println("Simple task")
    }

    // async - result needed
    val result = async {
        "Value to return"
    }
    println(result.await())
}

Q2. What's the difference between join() and await()?

A: join waits for completion, await waits for result!

fun main() = runBlocking {
    val job = launch {
        delay(1000)
    }
    job.join()  // Wait for completion only

    val deferred = async {
        delay(1000)
        "Result"
    }
    val result = deferred.await()  // Wait for result
}

Q3. Do coroutines cancel automatically?

A: When a parent is cancelled, children are cancelled too!

fun main() = runBlocking {
    val parent = launch {
        val child = launch {
            repeat(10) {
                println("Child task $it")
                delay(500)
            }
        }

        delay(1300)
        println("Parent completed")
    }

    delay(2000)
    parent.cancel()  // Children are cancelled together
}

🎬 Conclusion

You've mastered the basic coroutine builders!

Key Summary:
✅ runBlocking - for testing/main function
✅ launch - tasks that don't need results
✅ async - for returning results
✅ coroutineScope - structured concurrency
✅ Performance improvement with parallel processing

Next Step: Learn more about suspending functions in suspend functions!