tolerant-timer.cc

struct Timer {
  dispatch_source_t source;
  dispatch_time_t interval = DISPATCH_TIME_FOREVER;
  dispatch_time_t delay = DISPATCH_TIME_FOREVER; // == same as interval
  dispatch_time_t tolerance = DISPATCH_TIME_FOREVER; // == 10% of interval

  Timer(dispatch_queue_t queue, dispatch_time_t interval, dispatch_block_t block) : source{dispatch_source_create(DISPATCH_SOURCE_TYPE_TIMER, 0, 0, queue)} {
    dispatch_source_set_event_handler(source, block);
  }
  Timer(dispatch_time_t interval, dispatch_block_t block) : Timer{dispatch_get_main_queue(), interval, block} {}
  Timer(dispatch_block_t block) : Timer{dispatch_get_main_queue(), 0, block} {}

  void resume() {
    uint64_t leeway = (tolerance == DISPATCH_TIME_FOREVER) ? (interval / 10) : tolerance;
    if (delay == DISPATCH_TIME_FOREVER) {
      if (interval == DISPATCH_TIME_FOREVER) {
        // TODO: Error: Neither `delay` nor `interval` is defined
      } else {
        dispatch_source_set_timer(source, dispatch_time(DISPATCH_TIME_NOW, interval), interval, leeway);
        dispatch_resume(source);
      }
    } else if (interval == DISPATCH_TIME_FOREVER) {
      // TODO: One-shot timer based off of `delay`
    } else {
      dispatch_source_set_timer(source, dispatch_time(DISPATCH_TIME_NOW, delay), interval, leeway);
      dispatch_resume(source);
    }
  }

  void suspend() {
    dispatch_suspend(source);
  }
};


- (void)applicationDidFinishLaunching:(NSNotification*)notification {
  static auto timer = Timer{dispatch_get_main_queue(), ^{
    NSLog(@"timer fired");
  }};
  timer.delay = 1 * NSEC_PER_SEC;
  timer.interval = 5 * NSEC_PER_SEC;
  timer.tolerance = 1 * NSEC_PER_SEC;
  timer.resume();
}