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πŸŽ›οΈ Context & Dispatcher

πŸ“– What is CoroutineContext?​

CoroutineContext is a set of configurations that determines how a coroutine will execute. It specifies which thread to run on, what name to use, and more!

πŸ’‘ Dispatcher​

Basic Dispatchers​

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

    // IO - Network/file operations
    launch(Dispatchers.IO) {
        println("IO: ${Thread.currentThread().name}")
    }

    // Default - CPU-intensive tasks
    launch(Dispatchers.Default) {
        println("Default: ${Thread.currentThread().name}")
    }

    // Unconfined - Special cases
    launch(Dispatchers.Unconfined) {
        println("Unconfined: ${Thread.currentThread().name}")
    }

    delay(100)
}

Dispatchers.IO​

suspend fun readFile(): String = withContext(Dispatchers.IO) {
    // File reading, network requests, etc.
    delay(1000)
    "File contents"
}

suspend fun writeFile(content: String) = withContext(Dispatchers.IO) {
    // File writing
    delay(500)
    println("File saved: $content")
}

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

Dispatchers.Default​

suspend fun heavyComputation(): Int = withContext(Dispatchers.Default) {
    // CPU-intensive computation
    var result = 0
    repeat(1_000_000) {
        result += it
    }
    result
}

fun main() = runBlocking {
    val result = heavyComputation()
    println("Computation result: $result")
}

🎯 Practical Examples​

Dispatcher by Layer​

// Repository - IO
class UserRepository {
    suspend fun fetchUser(id: String): User = withContext(Dispatchers.IO) {
        delay(1000)  // Network request
        User(id, "John Doe")
    }
}

// UseCase - Default
class ProcessUserUseCase {
    suspend fun process(user: User): ProcessedUser = withContext(Dispatchers.Default) {
        // Data processing
        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("Result: ${processed.displayName}")
}

Parallel IO Operations​

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

    val posts = async(Dispatchers.IO) {
        delay(1500)
        "Posts data"
    }

    val comments = async(Dispatchers.IO) {
        delay(800)
        "Comments data"
    }

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

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

πŸ”§ Context Composition​

Naming​

fun main() = runBlocking {
    launch(CoroutineName("Task1")) {
        println("Name: ${coroutineContext[CoroutineName]}")
    }

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

    delay(100)
}

Adding Job​

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

    launch(job + Dispatchers.Default) {
        println("Task running")
        delay(1000)
        println("Task completed")
    }

    delay(500)
    println("Canceling task")
    job.cancel()
}

🎨 withContext​

Thread Switching​

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

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

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

    println("End: ${Thread.currentThread().name}")
    println("Result: $processed")
}

fun main() = runBlocking {
    complexTask()
}

Optimization Pattern​

// ❌ Unnecessary switching
suspend fun bad() {
    withContext(Dispatchers.IO) {
        val data1 = loadData1()
        withContext(Dispatchers.Default) {  // Unnecessary!
            process(data1)
        }
    }
}

// βœ… Efficient
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)

πŸ”₯ Practical Patterns​

Cache + Network​

class DataSource {
    private var cache: String? = null

    suspend fun getData(): String {
        // Check cache (fast)
        cache?.let { return it }

        // Network request (slow)
        return withContext(Dispatchers.IO) {
            delay(1000)
            "New data"
        }.also { cache = it }
    }
}

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

    // First call - Network
    val time1 = measureTimeMillis {
        println(source.getData())
    }
    println("First call: ${time1}ms")

    // Second call - Cache
    val time2 = measureTimeMillis {
        println(source.getData())
    }
    println("Second call: ${time2}ms")
}

Batch Processing​

suspend fun processBatch(items: List<Int>): List<Int> {
    return withContext(Dispatchers.Default) {
        items.map { item ->
            // Process each item
            item * 2
        }
    }
}

fun main() = runBlocking {
    val items = List(100) { it }
    val results = processBatch(items)
    println("Processing completed: ${results.size} items")
}

With Timeout​

suspend fun fetchWithTimeout(): String? {
    return try {
        withTimeout(2000) {
            withContext(Dispatchers.IO) {
                delay(3000)  // Takes too long
                "Data"
            }
        }
    } catch (e: TimeoutCancellationException) {
        null
    }
}

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

πŸ›‘οΈ Exception Handling​

CoroutineExceptionHandler​

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

    val job = launch(handler) {
        throw Exception("Problem occurred!")
    }

    job.join()
    println("Continues execution")
}

SupervisorJob​

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

    with(CoroutineScope(coroutineContext + supervisor)) {
        val job1 = launch {
            delay(500)
            throw Exception("Task1 failed")
        }

        val job2 = launch {
            delay(1000)
            println("Task2 succeeded!")
        }

        try {
            job1.join()
        } catch (e: Exception) {
            println("Task1 exception: ${e.message}")
        }

        job2.join()
    }
}

🎯 Custom Dispatcher​

Specifying Thread Pool Size​

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

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

    delay(3000)
}

Single Thread​

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

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

    delay(2000)
}

πŸ€” Frequently Asked Questions​

Q1. Which Dispatcher should I use?​

A: Choose based on the type of task!

// IO - Network, files, databases
suspend fun fetchData() = withContext(Dispatchers.IO) { }

// Default - CPU-intensive computations
suspend fun compute() = withContext(Dispatchers.Default) { }

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

Q2. Can I use withContext multiple times?​

A: Yes! Switch whenever needed.

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

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

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

Q3. What if I don't specify a Dispatcher?​

A: It inherits the parent coroutine's Context!

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

🎬 Conclusion​

Control your coroutines with Context and Dispatcher!

Key Takeaways:
βœ… Dispatchers.IO - Network/files
βœ… Dispatchers.Default - CPU tasks
βœ… Dispatchers.Main - UI updates
βœ… Switch threads with withContext
βœ… Fine-grained control with Context composition

Congratulations! You've completed the Coroutines series! πŸŽ‰

Next Step: Learn how to write tests in Unit Testing!