flatMap-maxPublishers.swift

import PlaygroundSupport
PlaygroundPage.current.needsIndefiniteExecution = true

import Combine
import Dispatch
import Foundation

(1...10)
  .publisher()
  .handleEvents(receiveOutput: { print("publisher sent \($0)") })
  .collect(2)
  .handleEvents(receiveOutput: { print("downstream collected \($0)") })
  .flatMap(maxPublishers: .max(1)) { values in
    return Publishers.Just(values)
      .delay(for: .seconds(1), scheduler: DispatchQueue.main)
      .handleEvents(receiveOutput: { print("downstream delayed \($0)") })
  }
  .sink(receiveCompletion: { _ in PlaygroundPage.current.finishExecution() }) {
    print("sink received \($0)")
  }

Output:

publisher sent 1
publisher sent 2
downstream collected [1, 2]
downstream delayed [1, 2]
sink received [1, 2]
publisher sent 3
publisher sent 4
downstream collected [3, 4]
downstream delayed [3, 4]
sink received [3, 4]
publisher sent 5
publisher sent 6
downstream collected [5, 6]
downstream delayed [5, 6]
sink received [5, 6]
publisher sent 7
publisher sent 8
downstream collected [7, 8]
downstream delayed [7, 8]
sink received [7, 8]
publisher sent 9
publisher sent 10
downstream collected [9, 10]
downstream delayed [9, 10]
sink received [9, 10]