How to touch paint a canvas?

Flutter newbie here. I'm currently trying to build a simple touch-drawing app with Flutter, but cannot work out how to trigger a canvas re-draw.

What I have is this: I have a CustomPaint widget which contains a GestureDetector child. The CustomPaint's painter is getting a message whenever a touch event occurs, and stores the touch coordinates to draw a path on re-paint. Problem is, the paint method is never called.

This is the code I have so far:

import 'package:flutter/material.dart';

class WriteScreen extends StatefulWidget {
  @override
  _WriteScreenState createState() => new _WriteScreenState();
}


class KanjiPainter extends CustomPainter {
  Color strokeColor;
  var strokes = new List<List<Offset>>();

  KanjiPainter( this.strokeColor );

  void startStroke(Offset position) {
    print("startStroke");
    strokes.add([position]);
  }

  void appendStroke(Offset position) {
    print("appendStroke");
    var stroke = strokes.last;
    stroke.add(position);
  }

  void endStroke() {
  }

  @override
  void paint(Canvas canvas, Size size) {
    print("paint!");
    var rect = Offset.zero & size;
    Paint fillPaint = new Paint();
    fillPaint.color = Colors.yellow[100];
    fillPaint.style = PaintingStyle.fill;
    canvas.drawRect(
      rect, 
      fillPaint
    );

    Paint strokePaint = new Paint();
    strokePaint.color = Colors.black;
    strokePaint.style = PaintingStyle.stroke;

    for (var stroke in strokes) {
      Path strokePath = new Path();
      // Iterator strokeIt = stroke.iterator..moveNext();
      // Offset start = strokeIt.current;
      // strokePath.moveTo(start.dx, start.dy);
      // while (strokeIt.moveNext()) {
      //   Offset off = strokeIt.current;
      //   strokePath.addP
      // }
      strokePath.addPolygon(stroke, false);
      canvas.drawPath(strokePath, strokePaint);
    }
  }

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


class _WriteScreenState extends State<WriteScreen> {
  GestureDetector touch;
  CustomPaint canvas;
  KanjiPainter kanjiPainter;

  void panStart(DragStartDetails details) {
    print(details.globalPosition);
    kanjiPainter.startStroke(details.globalPosition);
  }

  void panUpdate(DragUpdateDetails details) {
    print(details.globalPosition);
    kanjiPainter.appendStroke(details.globalPosition);
  }

  void panEnd(DragEndDetails details) {
    kanjiPainter.endStroke();
  }

  @override
  Widget build(BuildContext context) {
    touch = new GestureDetector(
      onPanStart: panStart,
      onPanUpdate: panUpdate,
      onPanEnd: panEnd,
    );

    kanjiPainter = new KanjiPainter( const Color.fromRGBO(255, 255, 255, 1.0) );

    canvas = new CustomPaint(
      painter: kanjiPainter,
      child: touch,
      // child: new Text("Custom Painter"),
      // size: const Size.square(100.0),
    );

    Container container = new Container(
      padding: new EdgeInsets.all(20.0),
      child: new ConstrainedBox(
        constraints: const BoxConstraints.expand(),
        child: new Card(
          elevation: 10.0,
          child: canvas,
        )
      )
    );

    return new Scaffold(
      appBar: new AppBar(
        title: new Text("Draw!")
      ),
      backgroundColor: const Color.fromRGBO(200, 200, 200, 1.0),
      body: container,
    );
  }
}

According to CustomPainter docs you must notify paint widget whenever repainting is needed

The most efficient way to trigger a repaint is to either extend this class and supply a repaint argument to the constructor of the CustomPainter, where that object notifies its listeners when it is time to repaint, or to extend Listenable (e.g. via ChangeNotifier) and implement CustomPainter, so that the object itself provides the notifications directly. In either case, the CustomPaint widget or RenderCustomPaint render object will listen to the Listenable and repaint whenever the animation ticks, avoiding both the build and layout phases of the pipeline.

E.g. KanjiPainter should extend ChangeNotifier and implement CustomPainter. And when you change strokes, invoke notifyListeners

And also build function always creates new KanjiPainter, this will remove all old data. You can init painter in initState once.

Working example:

class WriteScreen extends StatefulWidget {
 @override
  _WriteScreenState createState() => new _WriteScreenState();
}

class KanjiPainter extends ChangeNotifier implements CustomPainter {
  Color strokeColor;
  var strokes = new List<List<Offset>>();

  KanjiPainter(this.strokeColor);

  bool hitTest(Offset position) => null;

  void startStroke(Offset position) {
    print("startStroke");
    strokes.add([position]);
    notifyListeners();
  }

  void appendStroke(Offset position) {
    print("appendStroke");
    var stroke = strokes.last;
    stroke.add(position);
    notifyListeners();
  }

  void endStroke() {
    notifyListeners();
  }

  @override
  void paint(Canvas canvas, Size size) {
    print("paint!");
    var rect = Offset.zero & size;
    Paint fillPaint = new Paint();
    fillPaint.color = Colors.yellow[100];
    fillPaint.style = PaintingStyle.fill;
    canvas.drawRect(rect, fillPaint);

    Paint strokePaint = new Paint();
    strokePaint.color = Colors.black;
    strokePaint.style = PaintingStyle.stroke;

    for (var stroke in strokes) {
      Path strokePath = new Path();
      // Iterator strokeIt = stroke.iterator..moveNext();
      // Offset start = strokeIt.current;
      // strokePath.moveTo(start.dx, start.dy);
      // while (strokeIt.moveNext()) {
      //   Offset off = strokeIt.current;
      //   strokePath.addP
      // }
      strokePath.addPolygon(stroke, false);
      canvas.drawPath(strokePath, strokePaint);
    }
  }

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

class _WriteScreenState extends State<WriteScreen> {
  GestureDetector touch;
  CustomPaint canvas;
  KanjiPainter kanjiPainter;

  void panStart(DragStartDetails details) {
    print(details.globalPosition);
    kanjiPainter.startStroke(details.globalPosition);
  }

  void panUpdate(DragUpdateDetails details) {
    print(details.globalPosition);
    kanjiPainter.appendStroke(details.globalPosition);
  }

  void panEnd(DragEndDetails details) {
    kanjiPainter.endStroke();
  }

  @override
  void initState() {
    super.initState();
    kanjiPainter = new KanjiPainter(const Color.fromRGBO(255, 255, 255, 1.0));
  }

  @override
  Widget build(BuildContext context) {
    touch = new GestureDetector(
      onPanStart: panStart,
      onPanUpdate: panUpdate,
      onPanEnd: panEnd,
    );

    canvas = new CustomPaint(
      painter: kanjiPainter,
      child: touch,
      // child: new Text("Custom Painter"),
      // size: const Size.square(100.0),
    );

    Container container = new Container(
        padding: new EdgeInsets.all(20.0),
        child: new ConstrainedBox(
            constraints: const BoxConstraints.expand(),
            child: new Card(
              elevation: 10.0,
              child: canvas,
            )));

    return new Scaffold(
      appBar: new AppBar(title: new Text("Draw!")),
      backgroundColor: const Color.fromRGBO(200, 200, 200, 1.0),
      body: container,
    );
  }
}