If I have this: ```runBlocking { SMR() println("test") }``` it hangs when SMR is like: ```suspen...

# coroutines

[Exerosis]

If I have this:

runBlocking {
  SMR()
  println("test")
}

it hangs when SMR is like:

suspend fun SMR() = withContext(dispatcher) {
  launch { ... }
}

But not if it's like:

suspend fun CoroutineScope.SMR() {
  launch { ... }
}

From the look of things withContext should just grab the old context, merge with new context and dispatch if the dispatcher changed.

[bezrukov]

Is dispatcher here a single-threaded dispatcher? Is it the same dispatcher/thread where runBlocking was called?

[bezrukov]

e.g. Dispatchers.Main

[Trevor Stone]

This is expected behavior due to structured concurrency. The first example of withContext, the inner this is a new child coroutine scoped to the parent caller. The inner coroutine will not complete and return until all of its children are completed. Essentially if you stop the work inside of launch after some time, it will function closer to your expectation.

In the second example no new coroutineScope is created, thus the launch is bound to the caller's coroutine scope

[Exerosis]

@Trevor Stone what would the correct way to go about this be?

[Trevor Stone]

What are you trying to do?

[Joffrey]

@Exerosis withContext is not only switching the context here, it also creates a nested scope (like coroutineScope { ... } ) and waits for all children to finish.

If you just want to launch with added context, just pass the extra context directly to launch:

fun CoroutineScope.SMR() {
  launch(dispatcher) { ... }
}

Also, avoid making a suspend function with CoroutineScope receiver. It's confusing because suspend functions are usually expected to finish their work before returning, while CoroutineScope extensions are expected to return almost immediately and launch child coroutines

[Exerosis]

@Joffrey Ah I see, I feel like I keep making this same mistake. I don't really understand the advantages of it working in this way. However I think in my case I want a CoroutineScope extension and you are right I don't need to mark it as suspend.