I want to make a generic class that combines and a...

# flow

[Mikael Alfredsson]

I want to make a generic class that combines and array of flows, runs a transformer and emits the result (the class should do more than this, so thats the reason to put it in a class rather than a function)
this small example explains the problem

class FlowGroup<T>(
    items:List<MutableSharedFlow<T>>,
    transform: suspend (Array<T>) -> T
) {
    val flow = MutableSharedFlow<T>()
    init {
        MainScope().launch {
            combine(items, transform).collect {
                flow.emit(it)
            }
        }
    }
}

combine wont compile in this code because it uses the reified qualifier, and I don’t know how to get that into this function. Any suggestions?

[Manuel Wrage]

You can do it likes this:

class FlowGroup<T> private constructor(combinedFlow: Flow<T>) {
    val flow = MutableSharedFlow<T>()
    init {
        MainScope().launch { 
            combinedFlow.collect {
                flow.emit(it)
            }
        }
    }
    companion object {
        inline operator fun <reified T> invoke(
            items:List<MutableSharedFlow<T>>,
            noinline transform: suspend (Array<T>) -> T
        ) = FlowGroup<T>(combine(items, transform))
    }
}

[Mikael Alfredsson]

Thanks.. It works, but if I want to make this an open class the code starts to get quite weird 🙂 Do you know why the combine function that takes a list of flow as input has to be reified (the ones with defined number of input flows are not)

[Manuel Wrage]

The combine function calls arrayOfNulls<T> and the T of it is also marked as reified .

[Mikael Alfredsson]

yes, I saw that, but I don’t know if there is any technical limitation that forces the function to set that

[Mikael Alfredsson]

or is this just a decision by the kotlin team