Suspend Functions

// Defining a suspend function
suspend fun functionName(param: Type): ReturnType {
    // Can only be called from within a coroutine
    delay(1000)  // Wait 1 second (does not block the thread)
    return result
}

// Calling a suspend function (from within a coroutine scope)
runBlocking {
    val result = functionName(param)
}

import kotlinx.coroutines.*

// Define suspend functions
suspend fun fetchData(id: Int): String {
    delay(500)  // Simulate a network request (wait 0.5 seconds)
    return "Data-$id"
}

suspend fun processData(data: String): String {
    delay(200)  // Simulate processing time
    return data.uppercase()
}

fun main() = runBlocking {
    println("Start")

    // Call suspend functions sequentially
    val data = fetchData(1)
    println("Fetched: $data")    // Fetched: Data-1

    val result = processData(data)
    println("Processed: $result")  // Processed: DATA-1

    // Group child coroutines with coroutineScope
    coroutineScope {
        val a = fetchData(2)
        val b = fetchData(3)
        println("$a, $b")     // Data-2, Data-3
    }

    println("Done")
}

A suspend function can only be called from within a coroutine or another suspend function. To call one from a regular function, you need to create a coroutine scope using runBlocking or launch.

Use delay() instead of Thread.sleep(), as it suspends the coroutine without blocking the thread. By launching multiple suspend functions concurrently inside a coroutine, you can achieve high throughput with fewer threads.

For launching coroutines concurrently, see launch / async. For retrieving asynchronous results, see await() / Deferred.