Combine: what are those multicast functions for?

Using Swift
12

Hi @DevAndArtist,

I've got that bug in my queue now and I think the behavior you're seeing is about the PassthroughSubject and not flatMap. PassthroughSubject will drop values if the downstream has not made any demand for them. Because flatMap with maxPublishers: .max(1) will only request one value at a time from upstream, and due to the delay, all values other than the first 1 are dropped.

This works (please forgive my slight reformatting as I adjusted this to fit into my test harness):

        let something = [1, 2, 3, 4, 5, 6, 7, 8, 9].publisher
            .flatMap(maxPublishers: .max(1)) { value in
                [Int].init(repeating: value, count: value).publisher
                    .flatMap { value in
                        Just(value)
                            .delay(for: .seconds(Double.random(in: 1 ... 4)), scheduler: RunLoop.main)
                    }
            }
            .sink { value in
                print(value)
            }
                
        DispatchQueue.main.asyncAfter(deadline: .now() + 300) {
            something.cancel()
        }
        RunLoop.main.run(until: Date() + 400)

or this:

        let subject = PassthroughSubject<Int, Never>()
        let something = subject
            .buffer(size: 9, prefetch: .byRequest, whenFull: .dropOldest)
            .flatMap(maxPublishers: .max(1)) { value in
                [Int].init(repeating: value, count: value).publisher
                    .flatMap { value in
                        Just(value)
                            .delay(for: .seconds(Double.random(in: 1 ... 4)), scheduler: RunLoop.main)
                    }
            }
            .sink { value in
                print(value)
            }
        
        let c = [1, 2, 3, 4, 5, 6, 7, 8, 9].publisher.subscribe(subject)
        
        DispatchQueue.main.asyncAfter(deadline: .now() + 300) {
            something.cancel()
        }
        RunLoop.main.run(until: Date() + 400)
        c.cancel()

I haven't yet checked the rest of the comparison to concatMap you put in the bug (thank you for all of that).

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