I am quite new to coroutines and I don't seem to u...

# coroutines

[tjohnn]

I am quite new to coroutines and I don't seem to understand some basic examples:

fun main() = runBlocking {
//sampleStart
    launch {
        repeat(1000) { i ->
            println("I'm sleeping $i ...")
            delay(500L)
        }
    }
    delay(1300L) // just quit after delay
//sampleEnd    
}

Prints 3 lines and exits because it uses a daemon thread

I'm sleeping 0 ...
I'm sleeping 1 ...
I'm sleeping 2 ...

But main waits for coroutineScope to finish execution

fun main() = runBlocking { // this: CoroutineScope
    launch { 
        delay(200L)
        println("Task from runBlocking")
    }

    coroutineScope { // Creates a new coroutine scope
        launch {
            delay(900L) 
            println("Task from nested launch")
        }

        delay(100L)
        println("Task from coroutine scope") // This line will be printed before nested launch
    }

    println("Coroutine scope is over") // This line is not printed until nested launch completes
}

And prints this

Task from coroutine scope
Task from runBlocking
Task from nested launch
Coroutine scope is over

Is there a magic done by coroutineScope that makes it block the main from finishing? Any help is appreciated.

[marstran]

The first example will print all numbers if you wait long enough. I got this when I reduced the repeat to 5 times:

I'm sleeping 0 ...
I'm sleeping 1 ...
I'm sleeping 2 ...
I'm sleeping 3 ...
I'm sleeping 4 ...

[marstran]

The coroutineScope will wait for all child-coroutines to complete before the scope completes. The runBlocking function also creates a coroutinescope that will behave that way.

[r4zzz4k]

coroutineScope { ... } is akin to runBlocking { ... }, with the difference that it suspends and not blocks. There are of course subtle differences under the hood as runBlocking sets up the bridge between blocking and non-blocking worlds in the absence of external event loop (and that's the only legitimate use case for this function), but you can skip that for the time being.

[tjohnn]

Marius and Andrew thanks for the quick response

[tjohnn]

So to my understanding now, it seems runBlocking waits for coroutineScope to finish executing its nested coroutines but doesn't wait for coroutines like launch to finish execution, is that right?

[marstran]

It waits for launch too.

[marstran]

Unless you call launch on another scope.

[tjohnn]

I edited the first example to this:

fun main() = runBlocking {
    //sampleStart
    coroutineScope{
        launch {
            repeat(20) { i ->
                println("I'm sleeping $i ...")
                delay(500L)
            }
        }
    }

    println("Finished with launch blocking")
    delay(1300L) // just quit after delay
    println("Exiting main")
//sampleEnd
}

It printed the whole 20 "I am sleeping" before exiting, But without wrapping the repeat in coroutineScope, it stoped at the third loop

[tjohnn]

Marius thanks I understand it now

[marstran]

That doesn't happen here if I remove the coroutineScope. I think you just need to wait longer.

[marstran]

See here (long link):
https://play.kotlinlang.org/#eyJ2ZXJzaW9uIjoiMS4zLjIxIiwicGxhdGZvcm0iOiJqYXZhIiwiYXJncyI6IiIsImpzQ29kZSI6IiIsIm5vbmVNYXJrZXJzIjp0cnVlLCJ0aGVtZSI6ImlkZWEiLCJmb2xkZWRCdXR0b24iOnRydWUsInJlYWRPbmx5IjpmYWxzZSwiY29kZSI6ImltcG9ydCBrb3RsaW54LmNvcm91dGluZXMuKlxuXG5cbmZ1biBtYWluKCkgPSBydW5CbG9ja2luZyB7XG4gICAgLy9zYW1wbGVTdGFydFxuICAgIGxhdW5jaCB7XG4gICAgICAgIHJlcGVhdCgyMCkgeyBpIC0+XG4gICAgICAgICAgICAgICAgcHJpbnRsbihcIkknbSBzbGVlcGluZyAkaSAuLi5cIilcbiAgICAgICAgICAgIGRlbGF5KDMwMEwpXG4gICAgICAgIH1cbiAgICB9XG5cbiAgICBwcmludGxuKFwiRmluaXNoZWQgd2l0aCBsYXVuY2ggYmxvY2tpbmdcIilcbiAgICBkZWxheSgxMzAwTCkgLy8ganVzdCBxdWl0IGFmdGVyIGRlbGF5XG4gICAgcHJpbnRsbihcIkV4aXRpbmcgbWFpblwiKVxuICAgIC8vc2FtcGxlRW5kXG59In0=

[tjohnn]

Yes that is true, but if you wrap the repeat in coroutineScope, "Exiting main" will be the last line to be printed. In your example without coroutineScope, it "Exiting main" is printed on the 6th line

[tjohnn]

https://pl.kotl.in/Skgwt9vu4

[tjohnn]

So can I conclude that coroutineScope doesn't allow runBlocking to execute next line until it finishes but launch does allow it ?

[marstran]

coroutineScope will only wait for child-coroutines when the scope exists (at the }). It does not have anything to do with launch running asynchronously.

[marstran]

So when you added the coroutineScope around launch, it was just waiting for launch to finish.

[tjohnn]

Okay I see 🙂. Thanks.