如何触摸画布？

这里是颤振新手。我目前正在尝试使用 Flutter 构建一个简单的触摸绘图应用程序，但无法弄清楚如何触发画布重新绘制。

我所拥有的是这样的： 我有一个 CustomPaint 小部件，其中包含一个 GestureDetector 子部件。每当发生触摸事件时，CustomPaint 的绘制器都会收到一条消息，并存储触摸坐标以在重新绘制时绘制路径。问题是，paint 方法从未被调用。

这是我到目前为止的代码：

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,
    );
  }
}

根据自定义画家文档每当需要重新绘制时，您必须通知绘制小部件

触发重绘的最有效方法是扩展此类并向 CustomPainter 的构造函数提供重绘参数，其中该对象在需要重绘时通知其侦听器，或者扩展 Listenable（例如通过 ChangeNotifier）并实现CustomPainter，以便对象本身直接提供通知。无论哪种情况，CustomPaint 小部件或 RenderCustomPaint 渲染对象都将侦听 Listenable 并在动画发生时重新绘制，从而避免管道的构建和布局阶段。

E.g. KanjiPainter应该延长ChangeNotifier并实施CustomPainter。当你改变笔画时，调用notifyListeners

并且build函数总是创造新的KanjiPainter，这将删除所有旧数据。您可以在中初始化画家initState once.

工作示例：

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

class KanjiPainter extends ChangeNotifier implements CustomPainter {
  Color strokeColor;
  var strokes = <List<Offset>>[];

  KanjiPainter(this.strokeColor);

  bool hitTest(Offset position) => true;

  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 = 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;
  }

  @override
  // TODO: implement semanticsBuilder
  SemanticsBuilderCallback? get semanticsBuilder => null;

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

class _WriteScreenState extends State<WriteScreen> {
  late GestureDetector touch;
  late CustomPaint canvas;
  late 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,
    );
  }
}