void ratio=(num value)

Source 

@override
set ratio(num value) {
  num oldRatio = ratio;
  super.ratio = value;
  if (_alarmsEnabled) {
    _alarms.forEach((alarmName, alarmRatio) {
      if (alarmRatio < oldRatio && alarmRatio >= ratio) {
        dispatchEvent(new Event(alarmName, true));
      }
    });
  }
}