Combine Publishers.Merge but with Publishers.combineLatest behaviour

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.

Solution

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.