Examines an issue where the MergeMany operator in Combine may not perform background processing.

CombineMergeManyTest.swift

import Foundation
import Combine

// see: workaround https://gist.github.com/susieyy/95e8f55fd077162c7e02c13b541e8309

// Examines an issue where the MergeMany operator in Combine may not perform background processing.

// $ swift --version
// Apple Swift version 5.3.1 (swiftlang-1200.0.41 clang-1200.0.32.8)
// Target: x86_64-apple-darwin20.1.0

// $ xcodebuild -version
// Xcode 12.2
// Build version 12B5044c

let workers: [AnyPublisher<String, Never>] = (0..<20).map {
    Just<Int>($0)
        .receive(on: DispatchQueue.global())
        .map {
            print("Working: isMainThread [\(Thread.isMainThread)] [\($0)]")
            return String($0)
        }
        .eraseToAnyPublisher()
}

var cancellables: Set<AnyCancellable> = .init()

print("Workers initial count: \(workers.count)")

Publishers
    .MergeMany(workers)
    .collect()
    .receive(on: DispatchQueue.main)
    .sink(receiveValue: {
        print("Workers result count: \($0.count)")
        dump($0)
    })
    .store(in: &cancellables)

// - OUTPUT --------------------------------------------------------
//    Workers initial count: 20
//    Working: isMainThread [false] [0]
//    Working: isMainThread [false] [1]
//    Working: isMainThread [false] [2]
//    Working: isMainThread [false] [3]
//    Working: isMainThread [false] [5]
//    Working: isMainThread [false] [6]
//    Working: isMainThread [false] [7]
//    Working: isMainThread [false] [10]
//    Working: isMainThread [false] [9]
//    Working: isMainThread [false] [11]
//    Working: isMainThread [false] [12]
//    Working: isMainThread [false] [13]
//    Working: isMainThread [false] [14]
//    Working: isMainThread [false] [15]
//    Working: isMainThread [false] [17]
//    Working: isMainThread [false] [19]
//    Working: isMainThread [false] [18]
//    Workers result count: 17 ← Expect 20 counts
//    ▿ 17 elements
//      - "0"
//      - "3"
//      - "2"
//      - "1"
//      - "5"
//      - "10"
//      - "7"
//      - "6"
//      - "9"
//      - "11"
//      - "12"
//      - "14"
//      - "15"
//      - "13"
//      - "18"
//      - "17"
//      - "19"
// - OUTPUT --------------------------------------------------------