GCD group dispatch and wait

implementation.m

  dispatch_queue_t queue = dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0);
  dispatch_group_t group = dispatch_group_create();
    
  dispatch_group_enter(group);     // pair 1 enter
  [self computeInBackground:1 completion:^{
    NSLog(@"1 done");
    dispatch_group_leave(group); // pair 1 leave
  }];

  dispatch_group_enter(group);     // pair 2 enter
  [self computeInBackground:2 completion:^{
    NSLog(@"2 done");
    dispatch_group_leave(group); // pair 2 leave
  }];

  dispatch_group_notify(group, queue, ^{
    NSLog(@"All jobs are done");
  });