ThreadSafeOperation-1.swift

func threadSafeOperation() {
    let foo = Foo()
    let queue = DispatchQueue.global(qos: .default)
    let locker = NSLock()
    
    for _ in 0..<4 {
        queue.async {
            locker.lock()
            foo.doIt()
            locker.unlock()
        }
    }
}