Hi there :wave: :slightly_smiling_face: I’m playin...

# coroutines

[Dan Lukashevich]

Hi there 👋 🙂
I’m playing around with the coroutines basics doc and I noticed the useless of the delay in the very first example:

fun main() = runBlocking { // this: CoroutineScope
    launch { // launch a new coroutine and continue
        delay(1000L) // non-blocking delay for 1 second (default time unit is ms)
        println("World!") // print after delay
    }
    println("Hello") // main coroutine continues while a previous one is delayed
}

The code block above prints:

Hello
World!

However the code block below prints the same 🙃

fun main() = runBlocking {
    launch {
        println("World!")
    }
    println("Hello")
}

However the output changes if we’re using Dispatcher.Unconfined in the launch coroutine builder.
Do you have an idea how to update it without changing the default dispatcher and the same time using runBlocking so that delay is noticeable and the result could be different?

Hello
World!

If there is a delay

World!
Hello

If there is no delay

[Joffrey]

The point of this doc is to make people familiar with concurrency concepts: understanding that launch runs concurrently with the rest of the code. Conceptually, the delay is useful because you don't need the reader to understand the fact that runBlocking creates a single-threaded event loop that guarantees that the body of launch will run after the rest no matter whether it suspends or not.

[Joffrey]

You cannot use runBlocking with its default single-threaded event loop disptacher AND have non-deterministic / time-sensitive behaviour with delays. You would have to make it multi-threaded. For instance by passing Dispatchers.Default to runBlocking

[Joffrey]

One way to make it delay-sensitive could be to launch both print statements inside launch blocks, and add a delay in front of each print.

[Dan Lukashevich]

Come up with a single launch block just adding another delay to the runBlocking. Well I think it’s much more clear in terms of delays:

fun main() = runBlocking {
    launch {
        delay(2000L)
        println("World!")
    }
    delay(1000L)
    println("Hello")
}

It is also clear that runBlocking is a coroutine itself.