When using a `callbackFlow` is it necessary to call `close e

Question

When using a `callbackFlow` is it necessary to call `close exception ` if an operation inside it throws i e ```callbackFlow try val result = somethingThatThrows Try catch or just let it be offer result catch exception close exception awaitClose ```

Dominaezzz · Answer

It depends Your example doesn t look like it requires `callbackFlow` If the exception means that `offer` `send` will stop being called then yes you must call `close`

Tash · Answer

Gotcha That was a pseudo example The real use case would look something like ```fun processedEvents coordinator EventCoordinator Flow lt Event gt return callbackFlow val eventListener = object EventListener override fun onEvent event Event Can throw val processedEvent = coordinator processEvent event offer processedEvent coordinator addEventListener eventListener awaitClose coordinator removeEventListener eventListener ```

Tash · Answer

If `processEvent ` throws in that example the listener should be removed i e `awaitClose` block must run So sounds like needs to be surrounded with `try catch` + `close` thinking face

Dominaezzz · Answer

Ah in that case you re right it needs to be surrounded You might want to do this to make sure you re not catching exception thrown from `offer` ```val r = runCatching coordinator processEvent event r fold offer it close it ```

Dominaezzz · Answer

Or even better I think you should do this ```fun processedEvents coordinator EventCoordinator Flow lt Event gt return callbackFlow val eventListener = object EventListener override fun onEvent event Event offer event coordinator addEventListener eventListener awaitClose coordinator removeEventListener eventListener map coordinator processEvent it ```

Dominaezzz · Answer

better is debatable depending on how pure `coordinator processEvent` is

Tash · Answer

woah using `map` is kinda nice yeah `coordinator processEvent` internally calls a getter and maps the incoming argument to some new value it does not inflict any side effects

Tash · Answer

so it sounds like `awaitClose` is triggered only when `close` `cancel` have been explicitly called i e letting the `processEvent` exception propagate would not have caused `awaitClose` to be called

Dominaezzz · Answer

It would because the channel is closed when the downstream collector i e `map` cancels finishes collection i e throws exception

Tash · Answer

oh yes gotcha i meant without either the `map` or `try catch` `awaitClose` would not get called if `processEvent` threw

Tash · Answer

this is great thanks so much