objective c - iOS: Don't return from function until multiple background threads complete -

this seems should simple i'm having lot of trouble. have function fires off bunch of other functions run in background , have completion blocks. want function wait until of completion blocks have been called before returns.

i not have control of function calling executes in background. otherwise modify use dispatch_async own queue , wait on queue finish.

example of situation looks like:

- (void)functionthatshouldbesynchronous {     (int = 0; < 10; i++) {         [self dosomethinginbackground:^{             nslog(@"completed!");         }];     }     // how wait until 10 threads have completed before returning? }  - (void)dosomethinginbackground:(void(^)())completion {     dispatch_async(dispatch_get_global_queue(dispatch_queue_priority_background, 0), ^{         [nsthread sleepfortimeinterval:1.0f]; // stuff         completion(); // execute completion block     }); } 

thanks in advance.

use dispatch group this:

- (void)functionthatshouldbesynchronous {     dispatch_group_t taskgroup = dispatch_group_create();      (int = 0; < 10; i++) {         dispatch_group_enter(taskgroup);         [self dosomethinginbackground:^{             nslog(@"completed!");             dispatch_group_leave(taskgroup);         }];     }      // waiting threads     dispatch_group_wait(taskgroup, dispatch_time_forever);     dispatch_release(taskgroup);      // background work complete } 

if want timeout waiting threads change dispatch_group_wait line this

// waiting 10 seconds before giving if (dispatch_group_wait(taskgroup, dispatch_time(dispatch_time_now, 10000000000)) != 0) {     // timeout } 

the parameter in nanoseconds.

as bbum saying, shouldn't block main thread. in case this:

typedef void(^mycompletionhandler)(); -(void)functiondoingbackgroundworkwithcompletionhandler:(mycompletionhandler)completionhandler {     dispatch_group_t taskgroup = dispatch_group_create();      (int = 0; < 10; i++) {         dispatch_group_enter(taskgroup);         [self dosomethinginbackground:^{             nslog(@"completed!");             dispatch_group_leave(taskgroup);         }];     }      dispatch_queue_t waitingqueue = dispatch_queue_create("com.mycompany.myapp.waitingqueue", dispatch_queue_concurrent);     dispatch_async(waitingqueue, ^{         // waiting threads         dispatch_group_wait(taskgroup, dispatch_time_forever);         dispatch_release(taskgroup);           // background work complete         dispatch_async(dispatch_get_main_queue(), ^{             // calling completion handler on main thread (if like)             completionhandler();         });         dispatch_release(waitingqueue);     }); }