Is there a flow operator that provides previous em...

# flow

[Marek Kubiczek]

Is there a flow operator that provides previous emission? I would like to emit something based on current and previous emission. Tried runningReduce but it’s not 100% what I need as I always want two last values from upstream rather then the current Value and accumalated value.

[Marek Kubiczek]

Also scan is not an option as I do not want every historical emission but just last 2 ones.

[bezrukov]

you can adapt runningReduce for your case:

flow.runningReduce { (prev, _), next -> 
   // do what you want with prev,next
   val mappedResult = mapper(prev, next)
   next to mappedResult
}.map { it.second }

But if you use this pattern widely, you can write this function, it's pretty easy:

public fun <T> Flow<T>.zipWithNext(operation: suspend (value: T, next: T) -> R): Flow<R> = flow {
    val NULL = Any()
    var prev: Any? = NULL
    collect { value ->
        if (prev !== NULL) {
            emit(operation(prev as T, value))
        }
        prev = value
    }
}

[Marek Kubiczek]

Yes I already wrote something similar as this zipWithNext. Was just wondering if there is something provided by flow already.

[elizarov]

It would be great if you explain your zipWithNext use-case in more details in https://github.com/kotlin/kotlinx.coroutines/issues