Using CheckedContinuation to communicate between concurrent tasks.

TaskSignaling.swift

// This is the code for https://forums.swift.org/t/communicating-between-two-concurrent-tasks/54240

import _Concurrency

actor Buffer {
  var elements: [Int] = []
  private var isNotEmpty: CheckedContinuation<Void, Never>? = nil

  deinit {
    // TODO: If the continuation is not nil,
    // resume it by throwing cancellation error?
  }

  func enqueue(_ value: Int) {
    elements.append(value)
    // Signal dequeue if needed.
    isNotEmpty?.resume()
    isNotEmpty = nil
  }

  func dequeue() async -> Int {
    while true {
      if !elements.isEmpty {
        return elements.removeFirst()
      } else {
        // Suspend until another element is enqueued.
        print("dequeue waiting")
        await withCheckedContinuation { cont in
          self.isNotEmpty = cont
        }
      }
    }
  }
}

Task {
  let buffer = Buffer()

  // Producer task
  Task {
    await buffer.enqueue(1)
    try! await Task.sleep(nanoseconds: 400_000_000)
    await buffer.enqueue(2)
    await buffer.enqueue(3)
  }

  // Consumer task
  Task {
    while true {
      let next = await buffer.dequeue()
      print(next)
      try! await Task.sleep(nanoseconds: 100_000_000)
    }
  }
}

import PlaygroundSupport
PlaygroundPage.current.needsIndefiniteExecution = true