绘制组件

CustomPaint类似一个画布。
CustomPainter类似一个画家，抽象类，需要继承并配置。
Paint类

class demo extends StatelessWidget{
  @override
  Widget build(BuildContext context) {
    return Container(
      color: Colors.white,
      width: 500,
      height: 500,
      child: CustomPaint(
        painter: MyPainter(),
      ),
    );
  }
}
class MyPainter extends CustomPainter{
  @override
  //size屏幕的宽高
  void paint(Canvas canvas, Size size) {
    //画笔
    var p = Paint();
    p.strokeWidth = 10.0;
    p.color = Colors.red;
    p.isAntiAlias = true;
    //油布              开始位置        结束位置          画笔
    canvas.drawLine(Offset(0, 0) ,Offset(100, 100), p);
  }
  @override
  bool shouldRepaint(covariant CustomPainter oldDelegate) {
    return true;
  }
}