suspend and Builders

suspend Functions and the Coroutine Builders

A suspend fun can pause at a suspension point, a call to another suspending function, and resume later, possibly on another thread; only coroutines and other suspend functions can call it. Builders start coroutines: launch returns a Job to join() or cancel(); async returns a Deferred<T>, whose await() suspends for the result; runBlocking blocks its thread, so it belongs in main, never on Android's main thread.

builders.kt: one launch and two async calls on one threadKotlin

import kotlinx.coroutines.*
suspend fun fetchPrice(i: Int): Double { delay(300); return catalog[i].price }  // "network"
fun main() = runBlocking {
  log("start")
  val job = launch { log("launch got ${fetchPrice(5)}") }
  val a = async { fetchPrice(0) }; val b = async { fetchPrice(2) }
  log("async total ${a.await() + b.await()}; job completed: ${job.isCompleted}")
}

Output

   0 ms [main @coroutine#1] start
 431 ms [main @coroutine#2] launch got 21.3
 449 ms [main @coroutine#1] async total 38.99; job completed: true

The three 300 ms fetches overlapped on one thread, with no await keyword on fetchPrice. The compiler turns each suspend fun into a state machine resumed through a hidden Continuation parameter.

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