How do I attach a piece of code to run when a `Flo...

# coroutines

[efemoney]

How do I attach a piece of code to run when a Flow is cancelled/disposed. Something like ObservableEmitter.setCancellable { ... } in the rx world.

I want to use Flow to model an add/remove listener so that a listener is attached when the flow is “subscribed” to and detached when the subscription is “disposed”. Forgive the rx lingo, pretty new to Flow so I will also appreciate reading material that can tie or at least differenciate between my RxJava knowlege and Flow

[octylFractal]

perhaps onErrorCollect or onErrorReturn?

[octylFractal]

don't know if that's how cancelled exceptions work with Flow though

[octylFractal]

I'm not even sure what you mean by "cancelled/disposed" though, a Flow has no cancel operation

[efemoney]

Yeah I’m definitely missing a correct mental model.

[efemoney]

The second paragraph of my post is what I want to achieve. I’m wondering if that fits into the capabilities of Flow

[tseisel]

flowViaChannel is what you need : https://kotlin.github.io/kotlinx.coroutines/kotlinx-coroutines-core/kotlinx.coroutines.flow/flow-via-channel.html
You can't directly cancel a Flow, but you can cancel the coroutine it is running in.
This will cause the underlying channel to be closed, triggering its invokeOnClose block and thus removing your listener.

[efemoney]

Works like a charm. Thank you @tseisel 🙏

[elizarov]

If you don’t have callbacks from outside APIs, then you don’t need a channel and you can use a simple `try-finally`:

flow {
    try { 
        while (isActive) { 
             /* emit elements */ 
        }
    } finally { 
        /* called when it is cancelled */ 
    }
}