A simple generic unbuffered channel using NSCondition

Channel.swift

import Foundation

class Channel<Message> {
  private enum State {
    case empty
    case readyToReceive
    case full(Message)
  }

  private let condition: NSCondition
  private var message: Message!
  private var state: State

  init() {
    self.condition = NSCondition()
    self.state = .empty
  }

  func send(_ message: Message) {
    condition.lock()
    defer { condition.unlock() }

    loop: do {
      switch state {
      case .empty, .full:
        condition.wait()
        continue loop

      case .readyToReceive:
        state = .full(message)
        condition.signal()
      }
    }
  }

  func receive() -> Message {
    condition.lock()
    defer { condition.unlock() }

    loop: do {
      switch state {
      case .empty:
        state = .readyToReceive
        condition.signal()
        condition.wait()
        continue loop

      case .readyToReceive:
        condition.wait()
        continue loop

      case let .full(message):
        state = .empty
        condition.signal()
        return message
      }
    }
  }
}

let channel = Channel<String>()
DispatchQueue.global().asyncAfter(deadline: .now() + 1) {
  channel.send("Hello, world!")
}
print(Date())
let message = channel.receive()
print(message)
print(Date())