I'm trying to execute a coroutine in a specific Context, but if I have understood it correctly, when using the

withContext

function, it waits for that coroutine to finish before continuing. But I want to launch multiple coroutines into another context and then later on, I will call join on that coroutine when I know I'm finished. How would I achieve something like this? I wrote a bare minimum example of what I'm trying to achieve here: https://pl.kotl.in/Ns6Hwa_8u

is

launch(coroutineContext)

not good enough?

withContext

is usually used to change the Dispatcher, so the coroutines execute on a different thread. The lambda of that function is packed and tossed in a queue on the specified dispatcher, and the parent coroutine will continue executing in its original context. Since you’re using

runBlocking

as the parent context, the

join()

will wait for the

doLaunch

calls to complete. But since

runBlocking

is single-threaded, it cannot exit the parent context and process the new jobs added to the queue from the

withContext

call. It’s deadlocked.

Alright that wasn't the best example. The problem I'm having is that I'm doing recursive coroutines and somehow some of them are not waited for. I will write a new example with bit more complicated code.

Like a fork-join approach? You’d need to launch the sub-tasks on a different, async dispatcher. It is based on both the parent and child contexts you’re launching in. If you launch child-jobs from

Dispatchers.Main

onto

<http://Dispatchers.IO|Dispatchers.IO>

, you’ll be just fine, and you can safely join the child jobs to the parent in the

Main

context. But launching from

Main

onto

Main

will cause problems, again, because it is single-threaded, and the queue cannot complete the parent task to start executing the child-tasks

kotlinx.serialization isn't supported on the playground, but this is exactly what I'm trying to achieve: https://pl.kotl.in/7p7j_A47S

why are you doing

withContext

before

async

? that doesn't really affect anything,

async

isn't a suspend function so

withContext

doesn't affect it

If I change it to only

async(this.coroutineContext)

instead of

withContext(this.coroutineContext){async(this.coroutineContext){block(..)}}

all data get's lost

what do you expect this playground code to do with

loadAllAtOnce(it).await()

? you never complete any of the deferreds, so of course that suspends forever?

The logic here is a bit off. The

DeferredJsonMap

is a map of objects, but you’re initializing it with deferred objects. That means at any given node in that tree, it’s going to synchronously wait for all objects in that node to be ready before continuing to populate the rest. But you’re awaiting a result in the middle that won’t be completed until later. Or, if you run the jobs async, the tree will be fully constructed before they get executed, and so cannot be added to the tree when they finish. What you need to do is first construct a tree of purely deferred objects. Once all the entire deferred tree has been constructed, then go through the entire thing and join to wait for the results of all of them to be finished. I’ve updated your example to populate the tree with deferreds first, and then complete. I think this is doing what you’re wanting. I’ve also added some logging traces which help illustrate how it’s executing https://pl.kotl.in/-BRc6p9wD

the core problem here is that

withContext

provides a

CoroutineScope

as

this

as well, so

withContext { async {} }

will suspend until the async block completes, rather than launching on the

DeferredJsonMap

scope

My previous link wasn’t entirely correct. I’ve added more logging and updated the example. It’s now clear exactly when things are set up and when they are executed, and the result should be as you expect https://pl.kotl.in/ap0xJOw0y

@Casey Brooks Thanks a lot! 🤩 I will try this out and report back 👍

It’s mostly the same, but create a

DeferredJsonArray

class which holds its data in lists instead of maps, and set it up similar to the Map one. And then add a

toDeferredArray

method to the map class which will create the builder for it. Here’s an updated example (it could probably be cleaned up/abstracted a bit, but it looks like it works just fine) https://pl.kotl.in/FUE07Z7Lu

Thanks for everything! We should make this code to a library 😉