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) {

  _time += time;

  if (_started == false) {
    if (_time > _delay) {
      _started = true;
      if (_onStart != null) _onStart();
    } else {
      return true;
    }
  }

  for (int i = 0; i < _animatables.length;) {
    if (_animatables[i].advanceTime(time) == false) {
      _animatables.removeAt(i);
    } else {
      i++;
    }
  }

  if (_animatables.length == 0) {
    _completed = true;
    if (_onComplete != null) _onComplete();
    return false;
  } else {
    return true;
  }
}