reward.dart

class _GameRouteState extends State<GameRoute> implements QuizEventListener {

  ...

  // TODO: Add _isRewardedAdReady
  bool _isRewardedAdReady;

  ...

  // TODO: Implement _loadRewardedAd()
  void _loadRewardedAd() {
    RewardedVideoAd.instance.load(
      targetingInfo: MobileAdTargetingInfo(),
      adUnitId: AdManager.rewardedAdUnitId,
    );
  }

  // TODO: Implement _onRewardedAdEvent()
  void _onRewardedAdEvent(RewardedVideoAdEvent event,
      {String rewardType, int rewardAmount}) {
    switch (event) {
      case RewardedVideoAdEvent.loaded:
        setState(() {
          _isRewardedAdReady = true;
        });
        break;
      case RewardedVideoAdEvent.closed:
        setState(() {
          _isRewardedAdReady = false;
        });
        _loadRewardedAd();
        break;
      case RewardedVideoAdEvent.failedToLoad:
        setState(() {
          _isRewardedAdReady = false;
        });
        print('Failed to load a rewarded ad');
        break;
      case RewardedVideoAdEvent.rewarded:
        QuizManager.instance.useHint();
        break;
      default:
      // do nothing
    }
  }

  ...
}