iphone - What will happen if I have nested dispatch_async calls? -

it may dumb question need ask , clear myself.

to submit block onto queue execution, use functions dispatch_sync , dispatch_async. both take queue , block parameters. dispatch_async returns immediately, running block asynchronously, while dispatch_sync blocks execution until provided block returns. here situations:

situation 1

dispatch_queue_t queue = dispatch_get_global_queue(dispatch_queue_priority_default, 0ul);     dispatch_async(queue, ^{     [self godosomethinglongandinvolved];     dispatch_async(queue, ^{         nslog(@"this statement1");      }); }); 

situation 2

dispatch_queue_t queue = dispatch_get_global_queue(dispatch_queue_priority_default, 0ul);     dispatch_sync(queue, ^{     [self godosomethinglongandinvolved];     dispatch_sync(queue, ^{         nslog(@"this statement1");      }); }); 

situation 3

{     [super viewdidload];     dispatch_queue_t queue = dispatch_get_global_queue(dispatch_queue_priority_default, 0ul);         dispatch_async(queue, ^{         [self godosomethinglongandinvolved];         dispatch_sync(queue, ^{             nslog(@"this statement1");          });     }); 

situation 4

{     [super viewdidload];     dispatch_queue_t queue = dispatch_get_global_queue(dispatch_queue_priority_default, 0ul);         dispatch_sync(queue, ^{         [self godosomethinglongandinvolved];         dispatch_async(queue, ^{             nslog(@"this statement1");          });     }); 

}

and godosomethinglongandinvolved is

-(void)godosomethinglongandinvolved {     nslog(@"godosomethinglongandinvolved"); } 

i tried run them in xcode cant see difference @ all.

so questions are:

1. what's main difference between these situations?
2. what if replace queue dispatch_get_main_queue()?

the dispatch_sync statement waits until block covers executed completely. dispatch_async returns , proceeds next line of code, inside happening in parallel.

if queue serial queue created yourself, then:

situation 1 - root block returns immediately. inside waits [self go....], , goes dispatch_async, returns well.

situation 2 - if queue serial queue, there dead lock since wait finish executing. since dealing asynchronous one, block executed in parallel. (thanks, @ken thomases)

situation 3 - no need in dispatch_sync here. causes deadlock.

situation 4 - waits [self ...], returns immediately.

if replace queue main queue, remember not dispatch_sync on main queue, because cause deadlock (it not if dispatched not main thread, @ken thomases).

to understand better, replace function with:

-(void)godosomethinglongandinvolved:(nsstring *)message {     for(int = 0; < 50; ++i) {         nslog(@"%@ -> %d", message, i);      } } 

you see what's going on every time, whether waits or not. luck.