dispatch_source_set_timer(timer,
dispatch_time(DISPATCH_TIME_NOW, 0), --------> Configure here the start time, put the interval with dispatch_time or dispatch_walltime
interval * NSEC_PER_SEC,
0);
....
dispatch_after(dispatch_time(DISPATCH_TIME_NOW, interval * NSEC_PER_SEC), queue, ^{
dispatch_resume(timer);
}); -----> just resume normally now, no more dispathc_after
