Scheduler

This is another concept we discussed briefly in Chapter 8, Threads and Coroutines, in the Starting a coroutine section. 

Remember how our launch() or async() could receive CommonPool?

Here's an example to remind you that you could specify it explicitly:

// Same as launch {}
launch(CommonPool) {
...
}

// Same as async {}
val result = async(CommonPool) {
...
}

This CommonPool is a Scheduler design pattern in a bad disguise. Many async tasks may be mapped to the same Scheduler.

Run the following code:

val r1 = async(CommonPool) {
for (i in 1..1000) {
println(Thread.currentThread().name)
yield()
    }
}

r1.await()

What is interesting is the fact that the same coroutine is picked up by different threads:

ForkJoinPool.commonPool-worker-2
ForkJoinPool.commonPool-worker-3
...
ForkJoinPool.commonPool-worker-3
ForkJoinPool.commonPool-worker-1

You can also specify the context as Unconfined:

val r1 = async(Unconfined) {
    ...
}

This will run the coroutine on the main thread. It prints:

main
main
...