97 Things Every Java Programmer Should Know

Chapter 81. Thinking in Coroutines

Dawn Griffiths and David Griffiths

Coroutines are functions or methods that can be suspended and resumed. In Kotlin, they can be used in place of threads for asynchronous work because many coroutines can run efficiently on a single thread.

To see how coroutines work, we’re going to create an example program that plays these drum sequences in parallel:

Instrument	Sequence
Toms	x-x-x-x-x-x-x-x-
High hat	x-x-x---x-x-x---
Crash cymbal	----------------x----

We could use threads to do this, but in most systems, the sound is played by the sound subsystem, while the code pauses until it can play the next sound. It’s wasteful to block a valuable resource like a thread in this way.

Instead, we’re going to create a set of coroutines: one for each of the instruments. We’ll have a method called playBeats, which takes a drum sequence and the name of a sound file. The full code is at https://oreil.ly/6x0GK; a simplified version looks like this:

suspend fun playBeats(beats: String, file: String) {
  for (...) { // for each beat
    ...
    playSound(file)
    ...
    delay(<time in milliseconds>)
    ...
  }
}

Call this with playBeats("x-x-x---x-x-x---", "high_hat.aiff"), and it will play the sequence using the high_hat.aiff sound file. There are two things in this code ...