line_number_controller.dart

import 'package:flutter/widgets.dart';

class LineNumberController extends TextEditingController {
  final TextSpan Function(int, TextStyle?)? lineNumberBuilder;

  LineNumberController(
    this.lineNumberBuilder,
  );

  @override
  TextSpan buildTextSpan({
    required BuildContext context,
    TextStyle? style,
    bool? withComposing,
  }) {
    final children = <InlineSpan>[];
    final list = text.split('\n');

    for (int k = 0; k < list.length; k++) {
      final el = list[k];
      // Blank lines are placeholders inserted to align with wrapped
      // visual lines. They should render as empty text spans to
      // preserve vertical spacing.
      if (el.trim().isEmpty) {
        children.add(TextSpan(text: '', style: style));
      } else {
        final number = int.tryParse(el) ?? 0;
        var textSpan = TextSpan(text: el, style: style);
        if (lineNumberBuilder != null && number != 0) {
          textSpan = lineNumberBuilder!(number, style);
        }
        children.add(textSpan);
      }
      if (k < list.length - 1) {
        children.add(const TextSpan(text: '\n'));
      }
    }

    return TextSpan(children: children, style: style);
  }
}