Simple example of rich text in a TextField by overriding TextEditingController

main.dart

import 'package:flutter/material.dart';

// This example shows how to format input text in a TextField using TextSpans.

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

class MyApp extends StatelessWidget {
  const MyApp({Key? key}) : super(key: key);

  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      title: 'Rich TextEditingController Demo',
      theme: ThemeData(
        primarySwatch: Colors.blue,
      ),
      home: MyHomePage(title: 'Rich TextEditingController Demo'),
    );
  }
}

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

  final String title;

  final _BigBoldTextEditingController _bigBoldTextEditingController = _BigBoldTextEditingController();

  @override
  Widget build(BuildContext context) {
    return Scaffold(
      appBar: AppBar(
        title: Text(title),
      ),
      body: Center(
        child: Column(
          mainAxisAlignment: MainAxisAlignment.center,
          children: <Widget>[
            TextField(
              decoration: const InputDecoration(
                hintText: 'Type something with the word "big"',
              ),
              controller: _bigBoldTextEditingController,
            ),
          ],
        ),
      ),
    );
  }
}

/// A [TextEditingController] that makes the string "big" appear bold whenever
// it appears.
///
/// Doesn't bother with the default behavior of
/// [TextEditingController.buildTextSpan], which styles composing regions.
class _BigBoldTextEditingController extends TextEditingController {
  static const String _stringToEmbolden = 'big';

  static const TextStyle _emboldenedTextStyle = TextStyle(
    fontWeight: FontWeight.w900,
  );

  @override
  TextSpan buildTextSpan({required BuildContext context, TextStyle? style, required bool withComposing}) {
    return _embolden(text, style);
  }

  static TextSpan _embolden(String text, TextStyle? style) {
    final int index = text.indexOf(_stringToEmbolden);

    if (index < 0) {
      return TextSpan(
        text: text,
        style: style,
      );
    }

    final String before = text.substring(0, index);
    final String after = text.substring(index + _stringToEmbolden.length);
    final TextStyle emboldenedTextStyle = style == null
        ? _emboldenedTextStyle
        : style.copyWith(fontWeight: FontWeight.bold);

    return TextSpan(
      text: before,
      style: style,
      children: <TextSpan>[
        TextSpan(
          text: _stringToEmbolden,
          style: emboldenedTextStyle,
        ),
        _embolden(after, style),
      ],
    );
  }
}