Hi all Need some help What is the best way to return result

Question

Hi all Need some help What is the best way to return result of Listener callback function as result of my function which create it Should I use channel to achive this or something like that ``` fun someFunction SomeResult val someObj = SomeObj SomeListenr result gt SomeResult result lt WANT THIS someObj start ```

gildor · Answer

You don t have return

gildor · Answer

what exactly do you want to achive Is it related to coroutines or more general question

iLobanov · Answer

I m think it more general but I try to solve in in courutine context Yes I don t have return because I don t shure hot to return SomeObject from someFunctuion

iLobanov · Answer

Actualy it look more like this ``` fun someFunction SomeResult launch UI val someObj = SomeObj SomeListenr result gt SomeResult result lt WANT THIS someObj start return SomeResult from listener ```

uli · Answer

iLobanov · Answer

< uliluckas> wow looking like what I want Thx Check it

uli · Answer

You still need to figure out how to bridge between the suspending and none suspending world

uli · Answer

I e do you want someFunction to wait for the callback

iLobanov · Answer

Yes and yes

uli · Answer

Then you could call the suspending wrapper from runBlocking But as it says it will block your thread waiting for the callback

iLobanov · Answer

SomeObj SomeListenr objects from library But operation working really fast so I should have no problems with bloking thread

gildor · Answer

There is also suspendCancellableCoroutine from Kotlinx Coroutines that allows you to support cancellation if your callback supports this

iLobanov · Answer

< gildor> Thanks for the advice I will know

iLobanov · Answer

Missed that need to collect the result of callback to list Here is my solution Partly compleated Maybe there is some better way to do that ``` private suspend fun baseFindCamera List lt SearchSDK SearchResult gt = suspendCoroutine cont gt val list ArrayList lt SearchSDK SearchResult gt = ArrayList lateinit var searchSDK SearchSDK var isResumed = false val timer = Timer var task = timerTask launch UI searchSDK = SearchSDK SearchSDK ISearchResult result gt Log d TAG searchSDK get result $ result uid if isResumed if result uid isEmpty list add result else cont resumeWithException NullPointerException task cancel task = timerTask cont resume list isResumed = true timer cancel timer purge searchSDK stop timer schedule task 2000 searchSDK search2 override fun findCameras Either lt Failure List lt MyCameraEntity gt gt val emptyCamera = MyCameraEntity val list = runBlocking CommonPool baseFindCamera list forEach r gt Log d TAG uid $ r uid | ip $ r ip | port $ r port | name $ r name | version $ r version return Either Right listOf emptyCamera ```

uli · Answer

This is hard to read inside the thread Could you provide a gist or slack snippet You could even post to <https try kotlinlang org >

uli · Answer

Without understanding what you are doing yet I would think that a `launch` inside `suspendCoroutine` is probably not what you want Do one thing at a time Wrap your callback into a coroutine Then take that coroutine and do other stuff schedule to UI thread stop the sdk

gildor · Answer

Agree with Uli in general your snippet doesn t look correct all thos runBlocking launch UI etc

uli · Answer

something like this should be your starting point Then you do your loop and list building using this suspending `search` function Your timer task probably reduces to a simple delay ``` private suspend fun search SearchSDK SearchResult = suspendCoroutine cont gt searchSDK = SearchSDK SearchSDK ISearchResult result gt if result uid isEmpty cont resumeWith result else cont resumeWithException NullPointerException searchSDK search2 ```

uli · Answer

Please ignore my last post Just looked at your code again on a big screen So you are getting continuous callbacks form your search which you want to collect until there is silence for 2 seconds right

uli · Answer

multiple values can be represented by channels in the coroutine world You could try something like this ``` private suspend fun findMyCameras Channel lt SearchSDK SearchResult gt val channel = Channel lt SearchSDK SearchResult gt val searchSDK = SearchSDK SearchSDK ISearchResult result gt Log d TAG searchSDK get result $ result uid if result uid isEmpty channel sendBlocking result else channel close NullPointerException launch UI searchSDK search2 channel invokeOnClose searchSDK stop return channel fun findCameras Either lt Authenticator Failure List lt MyCameraEntity gt gt val list ArrayList lt SearchSDK SearchResult gt = ArrayList val emptyCamera = MyCameraEntity runBlocking CommonPool findMyCameras use cameraChannel gt while true val nextCamera = withTimeoutOrNull 2 TimeUnit SECONDS cameraChannel receiveOrNull x if nextCamera == null break list add result list forEach r gt c Log d TAG uid $ r uid | ip $ r ip | port $ r port | name $ r name | version $ r version return Either Right listOf emptyCamera ```

uli · Answer

Not sure if you need to catch the potential exception from `sendBlocking`

iLobanov · Answer

< uli> Thanks for your help I ll use implementation you suggest

uli · Answer

be aware though it is not tested at all Especially installing `invokeOnClose` was a wild guess Maybe `searchSDK stop `needs to be called from a try catch around `channel sendBlocking result `

uli · Answer

if you get it to work please let us know slightly smiling face

iLobanov · Answer

Yes It works But I m replace findMyCameras use with consume because use marked as deprecated invokeOnClose seems also working but need more tests

iLobanov · Answer

< gildor> < uli> Working version for me looking like that Thx for help again First time using Gist

gildor · Answer

I think you don t need internal launch for send looks like unnecessary overhead I would just use channel with buffer and or use offer instead of send depends on required semantics check offer and send docs and channel buffering docs

iLobanov · Answer

< gildor> Ok offer looks like what i need I ll try it but for some reasons not always return the full data set Also working a little bit slowly then launch send

gildor · Answer

Sure it s not guaranteed because if channel is full and you don t suspend channel cannot receive this data but even send returns instantly in case of buffered channel

gildor · Answer

How do you measure speed Creating one more coroutine on each even and especially dispatching of this coroutine should be more expensive than just send to a channel directly

iLobanov · Answer

< gildor> Actually speed not actualy really problem in my case And I ll prefer to make more clean code Test it calculate system time ms diff before and after 5 times in row 2403 2581 2128 2178 2184 ms send wrapped with launch 2899 2863 2134 2127 2169 ms using offer But as I say offer also not return not full result And this is problem Have no experience with channels maybe with offer need to use other type of Channel instead of Channel lt T gt or change code which consume data

gildor · Answer

As s I mentioned in case of offer you need channel with buffer to cache events until delivery

gildor · Answer

5 measurements without any warming cannot be used for such benchmarking I would just ignore those results especially for this simple code for microbenchmarking you need proper test harness

iLobanov · Answer

I m dont try to prove something about this functions Just check it for interest Left to read about the channels slightly smiling face

uli · Answer

That s why I used sendBlocking in my example Your performance issues probably come from using a rendezvous channel If you pass a buffer size to the channel constructor sendBlocking will not block unless the buffer is full But opposed to offer it will also not loose data if the buffer is full This is a lot cleaner then using launch as with buffered channels you just queue the data not the whole coroutine to process it

iLobanov · Answer

used launch because was t know about buffer for channel And with default RendezvousChannel sendBlocking cause error Now when changed to buffered channel Both functions offer and sendBlocking working good

gildor · Answer

Why do you use sendBlocking in general

iLobanov · Answer

Currently I m using offer Send blocking was in the original exapmle by Uli and HiSearchSDK was out of launch UI corutine But HiSearchSDK must be initialized inside main thread only so after that I move it inide launch UI and it cause problem because sendBlocking should not call from the main tread So I change it to send wrapped with one more launch And finilly you tell me about offer and channel with buffer

gildor · Answer

I think it s pretty rare when you need sendBlocking only probably for tests or if you want to use coroutines from some existing blocking code

uli · Answer

I guess sending to a channel from a callback i e not suspending with backpressure is a usecase but only makes sense on a background thread

gildor · Answer

yes as an adapter it will work but usually writing coroutine wrapper for existing even blocking callback API would be more clean solution