How can I cancel flow collection from within the `collect{}` operator? The docs say that it should b...

# coroutines

[Lukas Lechner]

How can I cancel flow collection from within the collect{} operator? The docs say that it should be possible to call cancel() , but that doesn’t seem to work….

[simon.vergauwen]

Where is the cancel() coming from in that case? 🤔

throw CancellationException is the first thing that comes to mind, which then probably ends up in a launch or launchIn and then the behavior depend on the surrounding CoroutineScope impl.

What is your use-case for this?

[Lukas Lechner]

Ahh, okay, the docs use runBlocking{} , and there we have coroutineScope as receiver, on which cancel can be called.

[Lukas Lechner]

https://kotlinlang.org/docs/flow.html#flow-cancellation-checks

[Lukas Lechner]

However I wonder why there is a crash in this case when a CancellationException is thrown 🤔 Aren’t CancellationExceptions special kind of exceptions that don’t lead to crashes?

[Robert Williams]

runBlocking needs to throw because it can return a result which won't be available if the coroutine is cancelled

[Jhonatan Sabadi]

It's important to cancel and use "ensureactive" on the emitter, if you don't do that, you can cancel and flow still send

[simon.vergauwen]

You can also compose this after the fact by calling cancellable it will inject ensureActive() in between calls to emit.
https://kotlinlang.org/api/kotlinx.coroutines/kotlinx-coroutines-core/kotlinx.coroutines.flow/cancellable.html