swift - lock and unlock dispatch_queue_t in async task -

i want lock lockqueue2 complete process , when unlock lockqueue2 show "number :" number in dispatch_after.

i need output :

number set value 1
number 1
number set value 2
number 2
number set value 3
number 3

let lockqueue = dispatch_queue_create("lockqueue", nil)  func test(num :int){   var number = 0   let lockqueue2 = dispatch_queue_create("lockqueue2", nil)    dispatch_async(lockqueue){     lockqueue2.lock()     // process or waiting     sleep(2)     dispatch_sync(lockqueue2) {              number = num              print("number set value ", number)            lockqueue2.unlock()     }   }    dispatch_async(dispatch_get_global_queue(qos_class_background, 0)){      // process or waiting      sleep(3)      dispatch_after(lockqueue2) {          print("number :", number)//*** show number set in last dispatch ***                  }    }            }  test(1) test(2) test(3) 

if you're dealing simple synchronous tasks, are, can use serial queue:

let serialqueue = dispatch_queue_create("com.domain.app.serialqueue", nil)  func simpleserialqueue(num: int) {     var number = 0      dispatch_async(serialqueue){         sleep(2)         number = num         print("number set value: ", number)     }      dispatch_async(serialqueue) {         print("number is:", number)//*** show number set in last dispatch ***     }           }  simpleserialqueue(1) simpleserialqueue(2) simpleserialqueue(3) 

or if had asynchronous task wanted wait finish, use semaphore:

let serialqueue = dispatch_queue_create("com.domain.app.serialqueue", nil)  func semaphoreexample(num: int) {     var number = 0      let semaphore = dispatch_semaphore_create(0)      // fire in 2 seconds      dispatch_after(dispatch_time(dispatch_time_now, int64(nsec_per_sec * 2)), serialqueue) {         number = num         print("number set value: ", number)         dispatch_semaphore_signal(semaphore);     }      // start immediately, wait until above semaphore signaled      dispatch_async(dispatch_get_global_queue(dispatch_queue_priority_default, 0)) {         print("going wait semaphore")         dispatch_semaphore_wait(semaphore, dispatch_time_forever)         print("number is:", number)     } }  semaphoreexample(1) 

frankly, semaphores solution of last resort, generally, other approaches handle more elegantly. example, might use asynchronous nsoperation subclass when dealing asynchronous processes: requires little more set up-front, handles asynchronous tasks gracefully. see wwdc 2015 advanced nsoperations robust example.