How to use GCD group?

GCD_group.m

- (void)test_group {
  dispatch_queue_t queue = dispatch_queue_create("diamond-queue", DISPATCH_QUEUE_CONCURRENT);
  dispatch_group_t group = dispatch_group_create();
  
  for (int i = 0; i < 10; i++) {
    // dispatch_group_enter(group);
    dispatch_sync(queue, ^{
      dispatch_group_enter(group);
      NSLog(@"i: [%@]__LINE__: %@", @(i), @(__LINE__));
      [self do_async_operation:^{
        // Main Thread
        NSLog(@"In do_async_operation");
        dispatch_group_leave(group);
      }];
    });
  }
  
  dispatch_group_notify(group, queue, ^{
    NSLog(@"__LINE__: %@", @(__LINE__));
    dispatch_async(dispatch_get_main_queue(), ^{
      NSLog(@"__LINE__: %@", @(__LINE__));
    });
  });
  
  NSLog(@"__LINE__: %@", @(__LINE__));
}


- (void)do_async_operation:(void(^)(void))block {
  //
  dispatch_async(dispatch_get_global_queue(QOS_CLASS_BACKGROUND, 0), ^{
    sleep(2);
    dispatch_async(dispatch_get_main_queue(), ^{
      // Main Thread
      block();
    });
  });
}