1. override
bool advanceTime(num time)

This method is called by the Juggler with the time past since the last call.

Returns true as long as this Animatable is not completed; false if it is completed.

Source 

@override
bool advanceTime(num time) {

  if (_currentTime < _totalTime || _started == false) {

    _currentTime = _currentTime + time;

    if (_currentTime > _totalTime) {
      _currentTime = _totalTime;
    }

    if (_currentTime >= 0.0) {

      // set startValues if this is the first start

      if (_started == false) {

        _started = true;

        for (int i = 0; i < _tweenPropertyList.length; i++) {
          _tweenPropertyList[i]._init();
        }
        if (_onStart != null) {
          _onStart();
        }
      }

      // calculate transition ratio and value

      num ratio = _currentTime / _totalTime;
      num transition = _transition(ratio).toDouble();

      for (int i = 0; i < _tweenPropertyList.length; i++) {
        _tweenPropertyList[i]._update(transition, _roundToInt);
      }
      if (_onUpdate != null) {
        _onUpdate();
      }
      if (_onComplete != null && _currentTime == _totalTime) {
        _onComplete();
      }
    }
  }

  return _currentTime < _totalTime;
}