main.dart

import 'package:flutter/material.dart';
import 'package:flutter/services.dart';

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

class MyApp extends StatefulWidget {
  const MyApp({super.key});

  @override
  State<MyApp> createState() => _MyAppState();
}

class _MyAppState extends State<MyApp> {
  late final TextControllerCustomView _controller;

  @override
  void initState() {
    _controller = TextControllerCustomView();
    super.initState();
  }

  @override
  void dispose() {
    _controller.dispose();
    super.dispose();
  }

  @override
  Widget build(BuildContext context) {
    return MaterialApp(
        title: 'Flutter Demo',
        theme: ThemeData(
          primarySwatch: Colors.blue,
        ),
        home: Scaffold(
          body: Center(
            child: TextFieldExample(
              controller: _controller,
            ),
          ),
        ));
  }
}

class TextFieldExample extends StatelessWidget {
  final TextControllerCustomView controller;
  const TextFieldExample({super.key, required this.controller});

  @override
  Widget build(BuildContext context) {
    return TextField(
      inputFormatters: [
        FilteringTextInputFormatter.allow(RegExp(r"(^\d*\.?\d*)")),
        TextInputFormatter.withFunction((oldValue, newValue) {
          var miv = MoneyInputValue(newValue.text);
          return miv.man().length <= 2 ? newValue : oldValue;
        })
      ],
      keyboardType: TextInputType.numberWithOptions(decimal: true),
      controller: controller,
      style: TextStyle(fontSize: 12),
    );
  }
}

class TextControllerCustomView extends TextEditingController {
  @override
  TextSpan buildTextSpan(
      {required BuildContext context,
      TextStyle? style,
      required bool withComposing}) {
    var miv = MoneyInputValue(super.text);
    return TextSpan(children: [
      TextSpan(
          text: '${miv.exp(fractionDigits: 2)}.',
          style: style?.copyWith(fontSize: (style.fontSize ?? 0) + 2)),
      TextSpan(text: miv.man(fractionDigits: 2), style: style)
    ]);
  }
}

class MoneyInputValue {
  final String text;

  MoneyInputValue(this.text);
  double parseValue() {
    double? d = double.tryParse(text);
    return d ?? 0;
  }

  List<String> getNumberParts({int? fractionDigits}) {
    String td = fractionDigits == null
        ? parseValue().toString()
        : parseValue().toStringAsFixed(fractionDigits);
    return td.split('.');
  }

  String exp({int? fractionDigits}) =>
      getNumberParts(fractionDigits: fractionDigits).first;
  String man({int? fractionDigits}) =>
      getNumberParts(fractionDigits: fractionDigits).last;
}