I have this function `suspend fun foo progressLambda Long gt

Question

I have this function `suspend fun foo progressLambda Long gt Unit ` and I want to convert this to a function returning a `StateFlow lt State gt ` where `State` is a sealed class providing `Progress` `Success` etc Initially I thought of using `callbackFlow` but that seems to be more for functions that run asynchronously Is there a more appropriate way

gildor · Answer

Looks that you need standard flow builder

Mark · Answer

But I cannot `emit` from within the lambda because it is not a suspend lambda

Mark · Answer

Another option if not using `callbackFlow` would be something like ```val progressFlow = MutableStateFlow State Progress 0L return flow emit null foo progressValue gt progressFlow value = State Progress progressValue emit State Success combine progressFlow invokeState progressState gt invokeState progressState ```

Mark · Answer

Alternatively without using `combine` ```flow val progressFlow = MutableStateFlow State Progress 0L coroutineScope val progressJob = launch progressFlow collect progress gt emit progress foo progressValue gt progressFlow value = State Progress progressValue progressJob cancel emit State Success ```

Mark · Answer

I think I prefer the `combine` approach because I am more confident there will never be a progress state emitted after the success state

gildor · Answer

if you wrap some callback indeed you need callbackFlow

Mark · Answer

Are you saying the other two solutions above don t work

gildor · Answer

no they will work

gildor · Answer

but looks overcomplicated for me

gildor · Answer

callbackFlow is probably way to go

Mark · Answer

```callbackFlow send State Progress 0L try foo progressValue gt offer State Progress progressValue send State Success close catch e Exception close e conflate we only care about the latest value```

Mark · Answer

Or if there is a Failure state ```callbackFlow send State Progress 0L try foo progressValue gt offer State Progress progressValue send State Success catch e Exception send State Failure e message we need to close or awaitClose but nothing to wait for close conflate we only care about the latest value```

Mark · Answer

But is there any guarantee that the progress state cannot be emitted after the Success Failure state

gildor · Answer

there is no such guarantee after all it purely related on foo implementation if it stop send progress before return then you are fine but if you not sure implementation is not under your control you always can just use something like AtomicBoolean as flag and prevent send progress before sending Success

gildor · Answer

I m a bit worry about this implementation though it looks that `foo` is blocking function so you probably want also add flowOn IO Computation

Mark · Answer

`foo` does not launch another coroutine so I think it s impossible for the lambda to be called completed after `foo` returns So `offer` can only be called before `foo` returns and so the question comes down to if `offer` is called before `send` potentially using a different dispatcher is the order of emitted values guaranteed to be the same

gildor · Answer

yes it guaranteed

gildor · Answer

also you always can use sendBlocking

gildor · Answer

this function anyway blocking

gildor · Answer

also allows you to avoid missing progress thought offer looks fine for progress

Mark · Answer

That s what I like about the `combine` solution You get a clear priority of the invoke flow over the progress flow `invokeState progressState` ```val progressFlow = MutableStateFlow State Progress 0L val invokeFlow = flow emit null try foo progressValue gt progressFlow value = State Progress progressValue emit State Success catch e Exception emit State Failure e message return invokeFlow combine progressFlow invokeState progressState gt invokeState progressState ```

gildor · Answer

I don t know callbackFlow looks fine too also has priority

Mark · Answer

Another benefit would be that `StateFlow` will swallow duplicates Also using separate progress flow allows us to do things like debounce the progress

gildor · Answer

also true