Text affinity

main.dart

import 'package:flutter/material.dart';

void main() => runApp(MyApp());

class MyApp extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      home: Scaffold(
        body: HomeWidget(),
        backgroundColor: Colors.white,
      ),
    );
  }
}

class HomeWidget extends StatefulWidget {
  @override
  _HomeWidgetState createState() => _HomeWidgetState();
}

class _HomeWidgetState extends State<HomeWidget> {
  @override
  Widget build(BuildContext context) {
    return Padding(
      padding: const EdgeInsets.all(100.0),
      child: CustomPaint(
        size: Size(300, 300),
        painter: LineWrappingAffinity(),
        // painter: BidirectionalAffinity(), //   <-- try this too
      ),
    );
  }
}

class LineWrappingAffinity extends CustomPainter {
  @override
  void paint(Canvas canvas, Size size) {
    // text
    const text = '0123456789';
    final textSpan = const TextSpan(
      text: text,
      style: TextStyle(fontSize: 50, color: Colors.black),
    );

    // draw text
    final TextPainter textPainter = TextPainter()
      ..textDirection = TextDirection.ltr
      ..text = textSpan
      ..layout(maxWidth: 280);
    textPainter.paint(canvas, Offset(0, 0));

    // draw cursor
    final cursorPaint = Paint()
      ..color = Colors.red
      ..style = PaintingStyle.fill
      ..strokeWidth = 1;
    var caretOffset = textPainter.getOffsetForCaret(
      TextPosition(offset: 9, affinity: TextAffinity.upstream),
      Rect.zero,
    );
    final dx = caretOffset.dx;
    final dy = caretOffset.dy;
    final rect = Rect.fromLTRB(dx, dy, dx + 3, dy + textPainter.height / 2);
    canvas.drawRect(rect, cursorPaint);
  }

  @override
  bool shouldRepaint(CustomPainter old) {
    return false;
  }
}

class BidirectionalAffinity extends CustomPainter {
  @override
  void paint(Canvas canvas, Size size) {
    // text
    const text = 'Helloשלום';
    final textSpan = const TextSpan(
      text: text,
      style: TextStyle(fontSize: 50, color: Colors.black),
    );

    // painters
    final TextPainter textPainter = TextPainter()
      ..textDirection = TextDirection.ltr
      ..text = textSpan
      ..layout();
    final cursorPaint = Paint()
      ..color = Colors.red
      ..style = PaintingStyle.fill
      ..strokeWidth = 1;

    // loop through each caret offset
    var dy = 0.0;
    for (var offset = 0; offset <= text.length; offset++) {
      // draw text
      textPainter.paint(canvas, Offset(0, dy));

      // draw cursor
      var caretOffset = textPainter.getOffsetForCaret(
        TextPosition(offset: offset, affinity: TextAffinity.upstream),
        Rect.zero,
      );

      final dx = caretOffset.dx;
      final rect = Rect.fromLTRB(dx, dy, dx + 3, dy + textPainter.height);
      canvas.drawRect(rect, cursorPaint);

      dy += textPainter.height;
    }
  }

  @override
  bool shouldRepaint(CustomPainter old) {
    return false;
  }
}