I have this minimal example:

import UIKit
import Combine

var values = [1,2,3,4,5]

var cancel = values.publisher
    .delay(for: 0.1, scheduler: DispatchQueue.global())
    .flatMap() { i in
    .sink { completion in
        print("Received Completion: (completion)")
    } receiveValue: { v in
        print("Received Value: (v)")

My expectation is that the source publisher emits the values from 1 to 5 into the stream. Each number gets transformed into (just for the sake of it) a new publisher that emits exactly the first value and then completes. Since this is done with each number, I would expect that all values reach the sink. This is not the case, however. Output looks like this:

request unlimited
receive value: (1)
Received Value: 1
receive value: (2)
Received Value: 2
receive value: (4)
Received Value: 4
receive finished
Received Completion: finished
receive value: (3)
receive value: (5)

In fact, only 3 values reach the sink before the completion event arrives. Why is this? The documentation states:

successful completion of the new Publisher does not complete the overall stream.

Even more curious, when you replace

.flatMap(maxPublishers: .max(1))
and add a
to the original source publisher only the first value makes it to the sink.

Any pointers are much appreciated!

Anonymous Asked question May 14, 2021