Call two or more async services using Dispatch Group

// https://developer.apple.com/library/ios/documentation/General/Conceptual/ConcurrencyProgrammingGuide/OperationQueues/OperationQueues.html

// for details see chapter of Apple's iOS Developer Library's Concurrency Programming Guide

//create dispatch group

dispatch_group_t  group = dispatch_group_create();

dispatch_group_async(group,dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_HIGH, 0), ^ {

    // block1

    NSLog(@"Block1");

    [NSThread sleepForTimeInterval:5.0];

    NSLog(@"Block1 End");

});

dispatch_group_async(group,dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_HIGH, 0), ^ {

    // block2

    NSLog(@"Block2");

    [NSThread sleepForTimeInterval:8.0];

    NSLog(@"Block2 End");

});

dispatch_group_notify(group,dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_HIGH, 0), ^ {

    // block3

    NSLog(@"Block3");

});

dispatch_release(group);

and could produce output like this:

2012-08-11 16:10:18.049 Dispatch[11858:1e03] Block1

2012-08-11 16:10:18.052 Dispatch[11858:1d03] Block2

2012-08-11 16:10:23.051 Dispatch[11858:1e03] Block1 End

2012-08-11 16:10:26.053 Dispatch[11858:1d03] Block2 End

2012-08-11 16:10:26.054 Dispatch[11858:1d03] Block3

If you do not have control over the dispatch_async calls for your blocks, as might be the case for async completion blocks, you can use the GCD groups using dispatch_group_enter and dispatch_group_leave directly.

// create a group

dispatch_group_t group = dispatch_group_create();

// pair a dispatch_group_enter for each dispatch_group_leave

dispatch_group_enter(group);       // pair 1 enter

[self computeInBackground:1 completion:^{

    NSLog(@"1 done");

    dispatch_group_leave(group);      // pair 1 leave

}];

// again... (and again...)

dispatch_group_enter(group);          // pair 2 enter

[self computeInBackground:2 completion:^{

    NSLog(@"2 done");

    dispatch_group_leave(group);         // pair 2 leave

}];

// dispatch your final block after all the dispatch_group_enters

dispatch_async(dispatch_get_main_queue(), ^{

    dispatch_group_wait(group, DISPATCH_TIME_FOREVER);

    NSLog(@"finally!");

});

In this example, computeInBackground:completion: is implemented as:

- (void)computeInBackground:(int)no completion:(void (^)(void))block {

    dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_HIGH, 0), ^{

        NSLog(@"%d starting", no);

        sleep(no*2);

        block();

    });

}

Output (with timestamps from a run):

12:57:02.574  2 running

12:57:02.574  1 running

12:57:04.590  1 done

12:57:06.590  2 done

12:57:06.591  finally!