Flutter 画板实现

import 'package:flutter/material.dart';

class DrawingBoard extends StatefulWidget {
  @override
  _DrawingBoardState createState() => _DrawingBoardState();
}

class _DrawingBoardState extends State<DrawingBoard> {
  List<List<Offset>> _path = [];

  @override
  Widget build(BuildContext context) {
    return Listener(
      onPointerDown: (PointerDownEvent pointerDownEvent){
        setState(() { _path.add([pointerDownEvent.localPosition]); });
      },
      onPointerMove: (PointerMoveEvent pointerMoveEvent){
        setState(() { _path[_path.length-1].add(pointerMoveEvent.localPosition); });
      },
      onPointerUp: (PointerUpEvent pointerUpEvent){
        setState(() { _path[_path.length-1].add(pointerUpEvent.localPosition); });
      },
      onPointerCancel: (PointerCancelEvent pointerCancelEvent){
        setState(() { _path[_path.length-1].add(pointerCancelEvent.localPosition); });
      },
      child: Container( width: double.infinity, height: double.infinity,
        child: CustomPaint( painter: DrawingBoardPainter(_path), )),
    );
  }
}

class DrawingBoardPainter extends CustomPainter {
  final List<List<Offset>> path;
  DrawingBoardPainter(this.path);
  Paint _paint = Paint() ..color = Colors.red ..style = PaintingStyle.stroke ..strokeWidth = 3;

  @override
  void paint(Canvas canvas, Size size) {
    // TODO: implement paint
    path.forEach((list) { Path _path = Path();
    for (int i = 0; i < list.length; i++) {
      if (i == 0)
        { _path.moveTo(list[i].dx, list[i].dy); }
      else
        { _path.lineTo(list[i].dx, list[i].dy); } }
    canvas.drawPath( _path, _paint); });
  }

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