52. Coroutines Basics

A coroutine is a suspendable computation: it can pause without blocking a thread and resume later. runBlocking bridges normal code into the coroutine world, launch starts a new coroutine, and delay suspends it without blocking the underlying thread. Note: requires the kotlinx-coroutines-core dependency.

import kotlinx.coroutines.*

fun main() = runBlocking {
    // launch starts a coroutine that runs concurrently with the rest.
    launch {
        delay(100)              // suspends this coroutine, not the thread
        println("World!")
    }
    println("Hello,")           // runs while the launched coroutine is delayed
}

Because delay only suspends the launched coroutine, main continues and prints Hello, first; once the delay elapses, the coroutine resumes and prints World!. runBlocking waits for its children to finish before returning.

Coroutines are lightweight — far cheaper than threads — so you can launch thousands of them at once.

import kotlinx.coroutines.*

fun main() = runBlocking {
    // Launch 50_000 coroutines; each just prints a dot after a short delay.
    repeat(50_000) {
        launch {
            delay(10)
            print(".")
        }
    }
}

Running it:

$ kotlin run
Hello,
World!

← Prev

Index

Next →