功能：CustomPaint

• 相当于在一个画布上面画画，可以自己绘制不同的颜色形状等
• 在各种widget 或者是插件不能满足到需求的时候，可以自己定义一些形状

---

使用实例和代码：

CustomPaint： 能使你绘制的东西显示在你的ui 上面，
painter=》child=》oregroundPainter，foregroundPainter最外面的一层会覆盖painter，child 层里面的widget.

 return Container(
   //painter 绘制完成之后需要再CustomPaint 里面构建称为widget
   child: Center(
     child: CustomPaint(
       child: Icon(
         Icons.abc,
         size: 40,
         color: Colors.red,
       ),
       painter: MybackGroudnPaiter(), //最内一层
       //  child: ,//子组件，在中间
       foregroundPainter: ForegroundPainter(), //最外面一层，也是  CustomPainter
     ),
   ),
 );

CustomPainter 绘制

class MybackGroudnPaiter extends CustomPainter {
  @override
  void paint(Canvas canvas, Size size) {
    //canvas 画布，size 画布的尺寸
    //在这边绘制
    // canvas.drawColor(Colors.red, BlendMode.color);
    var centerp = size / 2;
    //定义画笔是什么颜色,样式，画笔的宽度
    var paint = Paint()
      ..color = Colors.teal
      ..style = PaintingStyle.fill
      ..strokeWidth = 2.0;
    canvas.drawRect(
        Rect.fromLTWH(centerp.width - 100, centerp.height - 100, 200, 200),
        paint);
  }

  @override
  bool shouldRepaint(covariant CustomPainter oldDelegate) {
    return true;
  }
}

class ForegroundPainter extends CustomPainter {
  @override
  void paint(Canvas canvas, Size size) {
    var centerp = size / 2;
    //定义画笔是什么颜色,样式，画笔的宽度
    var paint = Paint()
      ..color = Colors.pink
      ..style = PaintingStyle.fill
      ..strokeWidth = 2.0;
    canvas.drawRect(
        Rect.fromLTWH(centerp.width - 130, centerp.height - 130, 200, 200),
        paint);
  }

  @override
  bool shouldRepaint(covariant CustomPainter oldDelegate) {
    // TODO: implement shouldRepaint
    return true;
  }
}

---

绘画的能力

我们绘制1000个彩色点点，从上往下掉落，看看性能，没有任何的卡顿，真给力

import 'dart:math';
import 'package:flutter/material.dart';

void main() => runApp(const MyApp());

class MyApp extends StatelessWidget {
  const MyApp({Key? key}) : super(key: key);

  @override
  Widget build(BuildContext context) {
    return const MaterialApp(
      home: Scaffold(
        backgroundColor: Colors.white,
        body: MyHomePage(
          title: 'jack ma',
        ),
      ),
    );
  }
}

class MyHomePage extends StatefulWidget {
  const MyHomePage({Key? key, required this.title}) : super(key: key);

  final String title;

  @override
  State<MyHomePage> createState() => _MyHomePageState();
}

class _MyHomePageState extends State<MyHomePage>
    with SingleTickerProviderStateMixin {
  late AnimationController _controller;
  final List<Snowflake> _snowflakes =
      List.generate(1000, (index) => Snowflake());

  @override
  void initState() {
    ;
    _controller =
        AnimationController(vsync: this, duration: const Duration(seconds: 5))
          ..repeat();
    super.initState();
  }

  @override
  Widget build(BuildContext context) {
    return Scaffold(
        body: Center(
      child: SizedBox(
        width: double.infinity,
        height: double.infinity,
        child: AnimatedBuilder(
          animation: _controller,
          builder: (BuildContext context, Widget? child) {
            //一直在这边build，5s 动画运行时间
            for (var snow in _snowflakes) {
              snow.fall();
            }
            return CustomPaint(
              painter: MyPainter(_snowflakes),
            );
          },
        ),
      ),
    ));
  }
}

class MyPainter extends CustomPainter {
  final List<Snowflake> _snowflakes;
  MyPainter(this._snowflakes);

  @override
  void paint(Canvas canvas, Size size) {
    final whitePaint = Paint()..color = Colors.grey;

    for (int i = 0; i < 1000; i++) {
      whitePaint.color = Colors.primaries[i % Colors.primaries.length];
      canvas.drawCircle(Offset(_snowflakes[i].x, _snowflakes[i].y),
          _snowflakes[i].radius, whitePaint);
    }
  }

  @override
  bool shouldRepaint(covariant CustomPainter oldDelegate) => true;
}

class Snowflake {
  double x = Random().nextDouble() * 400;
  double y = Random().nextDouble() * 800;
  double radius = Random().nextDouble() * 2 + 2;
  double velocity = Random().nextDouble() * 4 + 2;
  void fall() {
    y += velocity;
    if (y > 800) {
      y = 0;
      x = Random().nextDouble() * 400;
      radius = Random().nextDouble() * 2 + 2;
      velocity = Random().nextDouble() * 4 + 2;
    }
  }
}