Hi all I was trying to use `callbackFlow ` but the code insi

Question

Hi all I was trying to use `callbackFlow ` but the code inside the block never gets called am I doing something wrong

Joffrey · Answer

If you share how you declare and use your `callbackFlow` we should be able to answer this wink

ephemient · Answer

`flow ` and `channelFlow ` are cold flows they only run when collected

Sergio C. · Answer

already rewritten everything but I had a ViewModel with ```fun getSensors LiveData lt ReadResult gt return sensorsRepository getSensorsFLow asLiveData ``` in the repository ```private var data Flow lt ReadResult gt = MutableSharedFlow init CoroutineScope <http Dispatchers IO|Dispatchers IO> launch val dataStore DataStore lt AppPreferences gt = app appPreferencesStore dataStore data catch updateObservers AppPreferences getDefaultInstance collectLatest updateObservers it private fun updateObservers appPreferences AppPreferences stopSensors when appPreferences selectedDevice AppPreferences Device SUNI gt initSuniReader AppPreferences Device TZ702 gt initTZReader AppPreferences Device NEXPEK7 gt initNexpekReader else gt flowOf ReadResul private fun initNexpek Flow lt ReadResult gt return callbackFlow lt ReadResult gt init sensor and wait for callback trySend the result ```

Sergio C. · Answer

Sensor was not initiated

Sergio C. · Answer

Now I have ```private var data MutableSharedFlow lt ReadResult gt = MutableSharedFlow ``` and everyone sends data to it with tryEmit

Joffrey · Answer

You probably forgot the `awaitClose ` call at the end of the `callbackFlow` block

Joffrey · Answer

But note that `callbackFlow` is meant for multi shot callbacks If you want to encapsulate a single shot callback you should make a `suspend` function returning a single value instead of a flow and you can build that function using `suspendCancellableCoroutine` I m not sure which one is your case though

Sergio C. · Answer

just add `awaitClose ` inside the lock

Sergio C. · Answer

I can have multiple return events from the sensor

Sergio C. · Answer

I will try this later

Sergio C. · Answer

thanks for the tips

Joffrey · Answer

In general if you want to encapsulate a multi shot callback into a Flow you also want to unregister the callback stop the emitter when the flow is cancelled or completed This is what you would do inside the `awaitClose ` block Check the documentation of `callbackFlow`

Sergio C. · Answer

ok

Sergio C. · Answer

I need more practice with Flows

Sergio C. · Answer

also using tryEmit from different places to the same MutableSharedFlow seems like a valid approach what do you think

ephemient · Answer

updating a MutableStateFlow from different places would be the closest equivalent to LiveData without being tied to the UI thread

ephemient · Answer

having mutation from multiple places does not sound like a good design though

Sergio C. · Answer

ok any other approach I could take

gildor · Answer

Cold flow is usually better approach It really depends on your case