Kotlin: How to create a Flow of nothing

user_813951 0
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I'm using a Flow just to communicate from the ViewModel with an activity, but I dont really need to pass any parameter to the activity. I just need the activity to react to the call.

Right now I have a flow of a nullable type:

val myFlow: Flow<Nothing?> = someChannel.receiveAsFlow()

And I need to emit a null, since Nothing cannot be instantiated:

someChannel.send(null)

What I'd like to do:

val myFlow: Flow<Nothing>

and then:

someChannel.send()

But of course that doesn't compile.

Is there a better way of doing this?

kotlin-nothing kotlin-flow kotlin android generics
2 Answers

Like mentioned in another answer, Nothing has a different purpose. You can also consider using Unit for that:

val myFlow: Flow<Unit> = someChannel.receiveAsFlow()
someChannel.send(Unit)

If you have a couple of places in the code where you call someChannel.send(Unit) you can create an extension function on SendChannel and call it instead of someChannel.send(Unit):

suspend fun SendChannel<Unit>.send() = send(Unit)

And then just call:

someChannel.send()
user_17316260

Nothing is intended for a different purpose. From the kotlin docs:

Nothing has no instances. You can use Nothing to represent "a value that never exists": for example, if a function has the return type of Nothing, it means that it never returns (always throws an exception).

But you try to emit a null value (it isn't Nothing). So, it looks like you are looking for a workaround where you really don't need it.

You can just create for example an EmptyState type and use it

val myFlow: Flow<EmptyState>
user_22716510
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