雪花动画：
    image.png

    1. import 'dart:math';
    3. import 'package:flutter/material.dart';
    5. class AnimatedSnowPage extends StatefulWidget {
    6.   const AnimatedSnowPage({Key? key}) : super(key: key);
    8.   @override
    9.   State<AnimatedSnowPage> createState() => _AnimatedSnowPageState();
    10. }
    12. class _AnimatedSnowPageState extends State<AnimatedSnowPage>
    13.     with SingleTickerProviderStateMixin {
    14.   AnimationController? _controller;
    15.   final List<Snowflake> _snowflakes =
    16.       List.generate(1000, (index) => Snowflake());
    18.   @override
    19.   void initState() {
    20.     _controller = AnimationController(
    21.       vsync: this,
    22.       duration: const Duration(seconds: 1),
    23.     )..repeat();
    24.     super.initState();
    25.   }
    27.   @override
    28.   void dispose() {
    29.     _controller!.dispose();
    30.     super.dispose();
    31.   }
    33.   @override
    34.   Widget build(BuildContext context) {
    35.     return Scaffold(
    36.       body: Center(
    37.         child: Container(
    38.           width: double.infinity,
    39.           height: double.infinity,
    40.           decoration: const BoxDecoration(
    41.             gradient: LinearGradient(
    42.               begin: Alignment.topCenter,
    43.               end: Alignment.bottomCenter,
    44.               colors: [Colors.blue, Colors.lightBlue, Colors.white],
    45.               stops: [0.0, 0.7, 0.95],
    46.             ),
    47.           ),
    48.           child: AnimatedBuilder(
    49.             animation: _controller!,
    50.             builder: (BuildContext context, Widget? child) {
    51.               for (var snow in _snowflakes) {
    52.                 snow.fall();
    53.               }
    54.               return CustomPaint(
    55.                 painter: MyPainter(_snowflakes),
    56.               );
    57.             },
    58.           ),
    59.         ),
    60.       ),
    61.     );
    62.   }
    63. }
    65. class MyPainter extends CustomPainter {
    66.   final whitePaint = Paint()..color = Colors.white;
    67.   final List<Snowflake> _snowflakes;
    69.   MyPainter(this._snowflakes);
    71.   @override
    72.   void paint(Canvas canvas, Size size) {
    73.     canvas.drawCircle(size.center(const Offset(0, 150)), 60, whitePaint);
    74.     canvas.drawOval(
    75.         Rect.fromCenter(
    76.           center: size.center(const Offset(0, 320)),
    77.           width: 200,
    78.           height: 250,
    79.         ),
    80.         whitePaint);
    82.     for (var snowflake in _snowflakes) {
    83.       canvas.drawCircle(
    84.           Offset(snowflake.x, snowflake.y), snowflake.radius, whitePaint);
    85.     }
    86.   }
    88.   @override
    89.   bool shouldRepaint(covariant CustomPainter oldDelegate) {
    90.     return true;
    91.   }
    92. }
    94. class Snowflake {
    95.   double x = Random().nextDouble() * 400;
    96.   double y = Random().nextDouble() * 800;
    97.   double radius = Random().nextDouble() * 2 + 2;
    98.   double velocity = Random().nextDouble() * 4 + 2;
    100.   fall() {
    101.     y += velocity;
    102.     if (y > 800) {
    103.       y = 0;
    104.       x = Random().nextDouble() * 400;
    105.       y = Random().nextDouble() * 800;
    106.       radius = Random().nextDouble() * 2 + 2;
    107.       velocity = Random().nextDouble() * 4 + 2;
    108.     }
    109.   }
    110. }