Producer-consumer written in Swift

ProducerConsumer.swift

// Note you can copy this code andn paste it in a Xcode Playground

// Simulates working and introduces some randomness
func randomSleep() {
    sleep(arc4random_uniform(2))
}

// Item to be produced/consumed
struct Item {
    let id: Int
}

// Waits some time and produce an item
class Producer {
    private var id = 0
    
    func produce() -> Item {
        randomSleep()
        id += 1
        let item = Item(id: id)
        print("PRODUCED: \(item.id)")
        return item
    }
}

// Waits some time and consume an item
class Consumer {
    func consume(_ item: Item) {
        randomSleep()
        print("consumed: \(item.id)")
    }
}

class Program {
    private let producer: Producer
    private let consumer: Consumer
    private let size: UInt
    
    // Serial queue that makes sure the items are consumed in order
    // and provides a system bound in case the size is too big
    private let queue = DispatchQueue(label: "serial-queue")
    private var buffer: UInt = 0
    
    init(producer: Producer, consumer: Consumer, size: UInt) {
        self.producer = producer
        self.consumer = consumer
        self.size = size
    }
    
    func start() {
        while true {
            // The producer doesn't produce if the buffer is full
            if buffer < size {
                buffer += 1
                let item = producer.produce()
                // When we have a produced item, we try to consume it in the
                // serial queue that will handle it asynchronously when possible,
                // and always in order. The producer can continue producing.
                queue.async { [unowned self] in
                    self.consumer.consume(item)
                    self.buffer -= 1
                }
            }
        }
    }
}

let producer = Producer()
let consumer = Consumer()
let program = Program(producer: producer, consumer: consumer, size: 5)
program.start()

You're accessing Program.buffer on both the main queue and your consumer queue.

Yeah, I don't remember exactly, but I think that was the point.