main.dart

import 'dart:ui';

import 'package:flutter/material.dart';

void main() {
  runApp(MyApp());
}

class MyApp extends StatelessWidget {
  // This widget is the root of your application.
  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      title: 'Flutter Demo',
      theme: ThemeData(
        // This is the theme of your application.
        //
        // Try running your application with "flutter run". You'll see the
        // application has a blue toolbar. Then, without quitting the app, try
        // changing the primarySwatch below to Colors.green and then invoke
        // "hot reload" (press "r" in the console where you ran "flutter run",
        // or simply save your changes to "hot reload" in a Flutter IDE).
        // Notice that the counter didn't reset back to zero; the application
        // is not restarted.
        primarySwatch: Colors.blue,
      ),
      home: MyHomePage(title: 'Flutter Demo Home Page'),
    );
  }
}

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

  // This widget is the home page of your application. It is stateful, meaning
  // that it has a State object (defined below) that contains fields that affect
  // how it looks.

  // This class is the configuration for the state. It holds the values (in this
  // case the title) provided by the parent (in this case the App widget) and
  // used by the build method of the State. Fields in a Widget subclass are
  // always marked "final".

  final String title;

  @override
  _MyHomePageState createState() => _MyHomePageState();
}

class _MyHomePageState extends State<MyHomePage> {
  List<Offset> _offsets = <Offset>[];
  @override
  Widget build(BuildContext context) {
    final mediaSize = MediaQuery.of(context).size;
    return Scaffold(
      appBar: AppBar(
        title: Text(widget.title),
      ),
      body: Center(
        child: GestureDetector(
          onPanUpdate: (details) {
            setState(() {
              _offsets.add(details.localPosition);
            });
          },
          onPanStart: (details) {
            setState(() {
              _offsets.add(details.localPosition);
            });
          },
          onPanEnd: (details) {
            setState(() {
              _offsets.add(null);
            });
          },
          child: CustomPaint(
            painter: FlipbookPainter(_offsets),
            child: Container(
              height: mediaSize.height,
              width: mediaSize.width,
              color: Color(0x88ffccdd),
            ),
          ),
        ),
      ),
    );
  }
}

enum PathState { start, middle, end }

class FlipbookPainter extends CustomPainter {
  final List<Offset> offsets;
  var offsetsChanged = true;
  final _paint = Paint()
    ..style = PaintingStyle.stroke
    ..color = Colors.black
    ..strokeWidth = 6
    ..strokeCap = StrokeCap.round
    ..strokeJoin = StrokeJoin.round
    ..strokeMiterLimit = 4;
  FlipbookPainter(this.offsets) : super();
  @override
  void paint(Canvas canvas, Size size) {
    var path = Path();
    var state = PathState.start;
    for (var i = 0; i < offsets.length - 1; i++) {
      final oi = offsets[i];
      print(oi.toString());
      switch (state) {
        case PathState.start:
          path.moveTo(oi.dx, oi.dy);
          state = PathState.middle;
          break;
        case PathState.middle:
          if (oi != null) {
            path.lineTo(oi.dx, oi.dy);
          } else {
            state = PathState.end;
          }
          break;
        case PathState.end:
          canvas.drawPath(path, _paint);
          path = Path();
          state = PathState.start;
          break;
      }

    }
  }


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