Combine Publishers.Merge but with Publishers.combineLatest behaviour

Question

I am currently trying to implement the merging of two publishers. But I can't find a solution for my use case.

I want to merge 2 publishers that both emit an array of structs of the same type. I want the combined publisher to emit values when either one of the merged publishers emit a new value.

Basically this would be a use case for Publishers.CombineLatest, but since my underlying publishers both emit values of the same type a merge would be more fitting here. But Publishers.Merge will not remember the last values of the merged publishers.

Therefore I would like to have a Publishers.CombineLatest behaviour with a Publishers.Merge operation. Is there something inside the Combine framework which can accomplish this kind of behaviour ?

Rough example what should happen:

Definitions:

PublisherA: emits -> [Value]
PublisherB emits -> [Value]

CombinedAB: -> [Value]


PublisherA changes: CombinedAB -> [NewA, OldB]
PublisherB changes: CombinedAB -> [OldA, NewB]

let a = CurrentValueSubject<[Int], Never>(["a", "b", "c"])
let b = CurrentValueSubject<[Int], Never>(["d", "e", "f"])

let combined = Publisher.AnyThing(a, b)

combined.sink {
   print($0)
}


b.send(["g", "h", "i"])


Outputs:
["a", "b", "c", "d", "e", "f"]
["a", "b", "c", "g", "h", "i"]

So it's basically a Publishers.CombineLatest but without emitting a tuple of (NewA,OldB) but instead already merged, because both values have the same type.

Any help is much appreciated.

Answer 1

Assuming that your combine operation is just concat of the subarrays you can do:

let a = CurrentValueSubject<[String], Never>(["a", "b", "c"])
let b = CurrentValueSubject<[String], Never>(["d", "e", "f"])

let combined = Publishers.CombineLatest(a, b).map(+)

combined.sink {
   print($0)     //["a", "b", "c", "d", "e", "f"] and ["a", "b", "c", "g", "h", "i"]
}


b.send(["g", "h", "i"])

I am not completely sure what you mean with "already merged". If you want to have the latest emitted array always at the end of the combined array then you might need a scan operator before the map(+) to be able to compare with previous emissions and swap them.