func add(_ a: Int, _ b: Int) -> Int {
sleep(3)
return a + b
}
let queue = DispatchQueue.global(
qos: DispatchQoS.QoSClass.background)
queue.async {
let result = add(3, 5)
print("result: \(result)")
}
print("main thread")
//printed:
//main thread
//result: 8