Flutterで雪を降らせる「雪の降るアニメーションの実装」

class Snow extends StatelessWidget {
  final double size;
  static const rate = 0.7;

  double x;
  double y;

  Snow({
    required this.size,
    required this.x,
    required this.y,
    Key? key,
  }) : super(key: key);

  @override
  Widget build(BuildContext context) {
    return Positioned(
      top: y,
      left: x,
      child: Stack(
        children: [
          Padding(
            padding: EdgeInsets.all(size * (1 - rate) / 2),
            child: Container(
              width: size * rate,
              height: size * rate,
              decoration: BoxDecoration(
                color: Colors.white.withOpacity(0.7),
                shape: BoxShape.circle,
              ),
            ),
          ),
          Container(
            width: size,
            height: size,
            decoration: BoxDecoration(
              color: Colors.white.withOpacity(0.4),
              shape: BoxShape.circle,
            ),
          ),
        ],
      ),
    );
  }
}

@freezed
class SnowData with _$SnowData {
  const factory SnowData({
    required Key key,
    required Key snowKey,
    required double x,
    required double y,
    required double size,
  }) = _SnowData;
}

class SnowLogic {
  Random _random = Random();

  final Size displaySize;
  SnowLogic(this.displaySize);

  SnowData generate(
    int index,
    double minSize,
    double maxSize,
  ) {
    return SnowData(
      key: ObjectKey('snow_$index'),
      snowKey: ObjectKey('snowKey_$index'),
      x: _random.nextDouble() * displaySize.width,
      y: _random.nextDouble() * displaySize.height,
      size: (maxSize - minSize) * _random.nextDouble() + minSize,
    );
  }

  SnowData fall(SnowData snow, double speed) {
    double x = snow.x + 0.3 * speed * (_random.nextDouble() - 0.5);
    double y = snow.y + speed / 3 + 2 * speed * _random.nextDouble() / 3;
    y = displaySize.height < y ? -5 : y;

    var newSnow = snow.copyWith(x: x, y: y);
    return newSnow;
  }
}

class _MyHomePageState extends State<MyHomePage> {
  List<Snow> snows = [];
  List<SnowData> snowData = [];
  Timer? _timer;

  late SnowLogic logic;

  @override
  void initState() {
    super.initState();
    WidgetsBinding.instance?.addPostFrameCallback((callback) {
      RenderBox scaffold =
          _keyScaffold.currentContext?.findRenderObject() as RenderBox;

      logic = SnowLogic(scaffold.size);

      for (int i = 0; i < NUMBER_OF_SNOW; i++) {
        snowData.add(logic.generate(i, MIN_SIZE_OF_SNOW, MAX_SIZE_OF_SNOW));
      }

      _timer =
          Timer.periodic(Duration(milliseconds: FRAME_RATE.ceil()), (timer) {
        List<Snow> newSnows = [];

        for (int i = 0; i < snowData.length; i++) {
          snowData[i] = logic.fall(snowData[i], 10);
        }

        for (var snow in snowData) {
          Snow newSnow = Snow(
            key: snow.snowKey,
            x: snow.x,
            y: snow.y,
            size: snow.size,
          );
          newSnows.add(newSnow);
          // print('${snow.x} ${snow.y}');
        }

        setState(() {
          snows = newSnows;
        });
      });
    });
  }

 @override
  Widget build(BuildContext context) {
    //print('build');
    return Scaffold(
      key: _keyScaffold,
      body: Stack(
        children: [
          Container(
            decoration: BoxDecoration(
              color: Colors.black,
              image: DecorationImage(
                image: AssetImage(Assets.imgp4604.assetName),
                fit: BoxFit.cover,
              ),
            ),
          ),
          ...snows
        ],
      ),
    );
  }
}