cancel method
      
void
cancel()
      
     
    
    
Implementation
void cancel() {
  if (!_inEffect) return;
  _inEffect = false;
  _isIdle = true;
  _isPaused = false;
  _finishedFlag = true;
  _currentTime = 0;
  _startTime = null;
  _effect!._calculateTiming(null);
  timeline?._removeAnimation(this);
  if (oncancel != null) {
    var event = AnimationPlaybackEvent(EVENT_CANCEL);
    event.timelineTime = _timeline!.currentTime;
    oncancel!(event);
  }
}