Swift Combine: How to collect while preserving order of publisher's results?

Question

I have an array of publishers that I want to wait on until all of the publishers have emitted a value, and then work with the results.

I tried using collect() with Publishers.MergeMany (edit: using MergeMany because I'm looking for a solution that could scale up to 40 publishers or more) , but collect just gathers the results in the order they are received but without respecting the order of the values as to correctly correlate the publisher and value.

To simplify the code and illustrate the issue, here's the Playground code:

import Combine

var cancellables = [AnyCancellable]()

let stringPublishers = [
    Just("1").delay(for: RunLoop.SchedulerTimeType.Stride(Double.random(in: 0.1...1.0)), scheduler: RunLoop.main),
    Just("2").delay(for: RunLoop.SchedulerTimeType.Stride(Double.random(in: 0.1...1.0)), scheduler: RunLoop.main),
    Just("3").delay(for: RunLoop.SchedulerTimeType.Stride(Double.random(in: 0.1...1.0)), scheduler: RunLoop.main),
    Just("4").delay(for: RunLoop.SchedulerTimeType.Stride(Double.random(in: 0.1...1.0)), scheduler: RunLoop.main),
    Just("5").delay(for: RunLoop.SchedulerTimeType.Stride(Double.random(in: 0.1...1.0)), scheduler: RunLoop.main)
]

Publishers.MergeMany(stringPublishers).collect().sink { value in
    print(value)
}.store(in: &cancellables)

Ran the code and the results are usually unordered like ["2", "4", "5", "3", "1"], ["1", "4", "5", "3", "2"], and so. How can I always get ["1", "2", "3", "4", "5"] ?

I tried adapting the solution here but I'm having trouble since my inputArray is already an array of Publishers, not plain values of integers or strings.

You may try a [Publishers.Zip](https://developer.apple.com/documentation/combine/publishers/zip) variant. But there are only variants with at max 4 publishers. To overcome this limitation, you may then look further here: https://stackoverflow.com/q/60345806/465677 — CouchDeveloper, Jan 12 '23 at 18:17
Oh yeah, forgot to specify that I need a flexible solution that could scale up to 40 publishers or more. I'll check that solution there to see if I can come up with something. Thanks @CouchDeveloper ! — Rodolfo Romero, Jan 12 '23 at 20:05

Daniel T. · Accepted Answer · 2023-01-13T12:53:24.877

Here is a function that will take any number of Publishers and combine them into a single publisher that contains an array of elements that respects the order:

func combineLatest<Pub>(_ pubs: [Pub]) -> AnyPublisher<[Pub.Output], Pub.Failure> where Pub: Publisher {
    guard !pubs.isEmpty else { return Empty().eraseToAnyPublisher() }
    return pubs.dropFirst().reduce(pubs[0].map { [$0] }.eraseToAnyPublisher()) { partial, next in
        partial.combineLatest(next)
            .map { $0 + [$1] }
            .eraseToAnyPublisher()
    }
}

It can be used like this:

combineLatest(stringPublishers).sink(receiveValue: {
    print($0)
})
.store(in: &cancellables)

If you don't like free functions, you can put it in an extension:

extension Collection where Element: Publisher {
    func combineLatest() -> AnyPublisher<[Element.Output], Element.Failure> {
        guard !isEmpty else { return Empty().eraseToAnyPublisher() }
        return dropFirst().reduce(first!.map { [$0] }.eraseToAnyPublisher()) { partial, next in
            partial.combineLatest(next)
                .map { $0 + [$1] }
                .eraseToAnyPublisher()
        }
    }
}

Swift Combine: How to collect while preserving order of publisher's results?

1 Answers1