Using locks as signals and for mutual exclusion

Locks-room-counter.swift

import Foundation

/// A class to model a room.
final class Room {
  // Counter
  private(set) var count: Int = 0
  
  // Lock to protect mutation of our counter
  private var lock = os_unfair_lock_s()

  // Someone has entered the room; increment the counter  
  func enter() {
    os_unfair_lock_lock(&lock)
    count += 1
    os_unfair_lock_unlock(&lock)
  }
  
  // Someone has left the room; decrement the counter
  func leave() {
    os_unfair_lock_lock(&lock)
    count -= 1
    os_unfair_lock_unlock(&lock)
  }
}

let aspenBallroom = Room()

// Main door
let agnes = DispatchQueue(label: "Agnes")
let agnesLock = NSLock()
agnesLock.lock()
agnes.async {
  for _ in 1...6000 {
    aspenBallroom.enter()
  }
  print("Agnes is done")
  agnesLock.unlock()
}

// Side door
let tammy = DispatchQueue(label: "Tammy")
let tammyLock = NSLock()
tammyLock.lock()
tammy.async {
  for _ in 1...4000 {
    aspenBallroom.enter()
  }
  print("Tammy is done")
  tammyLock.unlock()
}

// Emergency exit
let sam = DispatchQueue(label: "sam")
let samLock = NSLock()
samLock.lock()
sam.async {
  for _ in 1...1000 {
    aspenBallroom.leave()
  }
  print("sam is done")
  samLock.unlock()
}

// Acquire all the locks, ensuring all our resources have completed their jobs
samLock.lock()
agnesLock.lock()
tammyLock.lock()

// Use the counter
print("Count: \(aspenBallroom.count)")

// Release the locks
samLock.unlock()
agnesLock.unlock()
tammyLock.unlock()