What’s the difference between `coroutineScope` and `withContext(Dispatchers.Default)`? They both lau...

# coroutines

[Viktor Qvarfordt]

What’s the difference between coroutineScope and withContext(Dispatchers.Default)? They both launch a suspend block with context Dispatchers.Default. Is withContext the same as coroutineScope { ... }.join()? Is there a version of withContext that defaults to use Dipsatchers.Default similar to how coroutineScope uses Dispatchers.Default?

[octylFractal]

withContext does not create a new Job -- so all started coroutines are children of the parent Scope, instead of a new scope with coroutineScope

[elizarov]

coroutineScope == withContext(EmptyCoroutineContext)

[elizarov]

coroutineScope != withContext(Dispatchers.Default) since the former inherits the dispatcher, while the latter replaces it with Dispatchers.Default.

[octylFractal]

isn't it more like withContext(Job())? I don't see withContext as specifying `Job`: https://kotlin.github.io/kotlinx.coroutines/kotlinx-coroutines-core/kotlinx.coroutines/with-context.html
but coroutineScope does: https://kotlin.github.io/kotlinx.coroutines/kotlinx-coroutines-core/kotlinx.coroutines/coroutine-scope.html

[elizarov]

No. withContext(Job()) is a weird thing that you should never do as it breaks structured concurrency (we should have forbidden it, but we cannot due to backwards compatibility)

[octylFractal]

hmm, okay

[elizarov]

Every function that accepts suspends CoroutineScope.() -> T as its parameter creates a Job, because that's the key part of the CoroutineScope intended usage. More details here: https://kotlin.github.io/kotlinx.coroutines/kotlinx-coroutines-core/kotlinx.coroutines/-coroutine-scope/index.html

[elizarov]

Quote from there:

Every coroutine builder (like launch, async, etc) and every scoping function (like coroutineScope, withContext, etc) provides its own scope with its own Job instance into the inner block of code it runs. By convention, they all wait for all the coroutines inside their block to complete before completing themselves, thus enforcing the discipline of structured concurrency.

[Viktor Qvarfordt]

Is it correct that creating a block with either withContext(Dispatchers.Default) or coroutineScope runs the suspend block on a thread pool of numCpus number of threads?

[louiscad]

@Viktor Qvarfordt No, coroutineScope { … } doesn't change the dispatcher. If the current coroutineContext is Dispatchers.Main for example, it will stay here, on that single thread backed dispatcher.

[Viktor Qvarfordt]

@louiscad Oh, I think I see now! I’m thinking of your example suspend fun main() = coroutineScope { … } which does use Dispatcher.Default. It uses Dispatchers.Default rather than Dispatchers.Main because it is the outer-most block?

[louiscad]

@Viktor Qvarfordt Yes, suspend fun main() runs on a dispatcher, and it's the default one, as one can expect.