NSLock() example

let lock = NSLock()
var counter: Int = 0 {
    didSet{
        print("Counter value is \(counter)")
    }
}
let thrd1 = Thread {
    for _ in 0..<1000 {
        lock.lock()
        let temp = counter + 1
        counter = temp
        lock.unlock()
    }
}

let thrd2 = Thread {
    for _ in 0..<1000 {
        lock.lock()
        let temp = counter + 1
        counter = temp
        lock.unlock()
    }
}

thrd1.start()
thrd2.start()