Also, when exactly are coroutines checked for canc...

# coroutines

[Marc Knaup]

Also, when exactly are coroutines checked for cancelation?
Will a suspending function call check for cancelation before the function implementation is invoked?
Will withContext check for cancelation upon before and/or after invoking block?
Will yield check for cancelation immediately or on/after dispatch?

[bezrukov]

Will a suspending function call check for cancelation before the function implementation is invoked?

it's heavily depends on suspend fun implementation, by default suspend fun is not cancellable out of the box, so it needs to be designed to support cooperative cancellation (e.g. use built-in primitives or check for coroutine state).

Will withContext check for cancelation upon before and/or after invoking block?

Should be both in general

Will yield check for cancelation immediately or on/after dispatch?

yield will check it immediately, but will also resumed with exception if job was cancelled before re-dispatching. Also consider ensureActive for checking for cancellation in your non-cancellable code

[Marc Knaup]

Thank you for the clarification!
I didn’t know about ensureActive. That seems helpful given that suspend fun don’t check for cancelation by default (unless it calls others that do).

Do you happen to know a good open source example/library/app that properly manages various scopes & contexts and thus can serve as guidance?

[louiscad]

withContext since I don't recall which kotlinx.coroutines version does check for cancellation before and after invoking block using its given context (which, in case of NonCancellable, will be prevented).

[uli]

I'd expect all suspension points to check for cancelation, at least on resume. There are not so many ways to cause suspension: Suspend(Cancelable)Coroutine, yield, withContext, await, and funs calling one of the above. what did i forget?

[octylFractal]

all suspension points can't check cancelation because suspend (Continuations) is a language feature, but coroutines are a library implemented on top of that

e. g. sequences are suspend, but they do not cooperate with coroutines and do not dispatch, or handle cancelation. additionally, calling a suspend function will not result in a cancelation check

[octylFractal]

another example is (until recently) Flow basic mechanisms did not check for cancelation

[louiscad]

@octylFractal coroutines are not a library, kotlinx.coroutines is, and as of Kotlin 1.3, it brings cancellation support with Job.
FYI, the "x" in kotlinx stands for extension.

[octylFractal]

alright, I suppose that's true, but it doesn't change what I said -- cancellation is not a concept in the stdlib, so not all suspension points check for it

[octylFractal]

"kotlinx.coroutines" is what brings cancellation to the table

[louiscad]

That's true.

[Marc Knaup]

Interesting and makes sense. Thanks @octylFractal and @louiscad!