Is there an operator to combine the latest values ...

# coroutines

[Rob]

Is there an operator to combine the latest values from a List<Flow<T>>? Something that would have this signature (List<Flow<T>>) -> Flow<List<T>>? I’m updating a RecyclerView using AsyncListDiffer.submitIList().

[tateisu]

Flow is not a collection, so I think you'll have to implement a mechanism to remember recent values yourself.

[Rob]

There are Rx operators for this transformation so I was hoping Flow came with batteries 😉

[Rob]

My first attempt.

Untitled

[MiSikora]

You can use something like this.

internal fun <T> Iterable<Flow<T>>.combineLatest(): Flow<List<T>> {
  val emptyFlow = emptyFlow<List<T>>()
  return fold(emptyFlow) { xs, x ->
    if (xs === emptyFlow) {
      x.map(::listOf)
    } else {
      xs.combine(x) { a, b -> a + b }
    }
  }
}

[Rob]

Impressive but it doesn’t quite behave correctly for me. I’m not sure why. My implementation also doesn’t work 😕

[MiSikora]

What doesn't work? What is the incorrect behavior?

[Rob]

For some reason I only get the first value after one of the flows completes.

[Rob]

I’m dealing with other bugs so it might not be this code

[Dominaezzz]

I think you want combine(....) {...}.

[Rob]

It works as expected. It was just stupid Retrofit/OkHttp blocking preventing multiple concurrent requests.

[Rob]

This is another version that works. But I don’t think these are efficient. Is there an implementation is easy to reason about and reasonably efficient?

Untitled

[Dominaezzz]

fun <T> Iterable<Flow<T>>.combineLatest(): Flow<List<T>> = combine(this) { it.toList() }

[MiSikora]

Oh nice. I wasn’t aware of an overload that accepts an iterable.