TextController money format util.

main.dart

// roipeker©2020
// demo codepen : https://codepen.io/roipeker/pen/oNbXRQz
// demo web     : https://dev.roipeker.website/flutter_demos/textcontroller_currency/

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

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

class MyApp extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return GestureDetector(
      onTap: () => WidgetsBinding.instance.focusManager.primaryFocus?.unfocus(),
      child: MaterialApp(
        debugShowCheckedModeBanner: false,
        darkTheme: ThemeData.dark(),
        themeMode: ThemeMode.dark,
        home: Scaffold(
          appBar: AppBar(
            title: Text('TextController Currency Format'),
            centerTitle: true,
            elevation: 0,
          ),
          body: Center(
            child: SizedBox(
              width: 200,
              child: TextInputMoney(),
            ),
          ),
        ),
      ),
    );
  }
}

class TextInputMoney extends StatefulWidget {
  @override
  _TextInputMoneyState createState() => _TextInputMoneyState();
}

class _TextInputMoneyState extends State<TextInputMoney> {
  TextEditingController _textControl;
  TextControllerFormat _formatControl;

  @override
  void initState() {
    super.initState();
    _textControl = TextEditingController();
    _formatControl = TextControllerFormat(_textControl);
    _textControl.addListener(_formatControl.formatMoney);
    // call one time, to refresh the text with the default.
    _formatControl.formatMoney();
  }

  @override
  void dispose() {
    _textControl.removeListener(_formatControl.formatMoney);
    _textControl.dispose();
    super.dispose();
  }

  @override
  Widget build(BuildContext context) {
    const style = TextStyle(
      fontFamily: 'JetbrainMono',
      fontFeatures: [
        FontFeature.proportionalFigures(),
        FontFeature.slashedZero()
      ],
    );

    return TextField(
      controller: _formatControl.controller,
      style: style,
      keyboardType: TextInputType.numberWithOptions(),
      textDirection: TextDirection.ltr,
      textAlign: TextAlign.end,
      maxLength: 18,
      decoration: InputDecoration(
        border: InputBorder.none,
        counter: SizedBox.shrink(),
        fillColor: Theme.of(context).backgroundColor,
        filled: true,
        contentPadding: EdgeInsets.all(12),
      ),
    );
  }
}

text_controller_format.dart

// roipeker©2020
// demo codepen : https://codepen.io/roipeker/pen/oNbXRQz
// demo web     : https://dev.roipeker.website/flutter_demos/textcontroller_currency/

import 'package:flutter/widgets.dart';

class TextControllerFormat {
  final TextEditingController controller;
  int _lastSelPos = 0;
  String _lastText;

  TextControllerFormat(this.controller);

  void formatMoney() {
    String str = controller.text;
    if (controller.selection.baseOffset > -1) {
      _lastSelPos = str.length - controller.selection.baseOffset;
    }
    if (_lastText == str) return;
    _lastText = str;
    str = str.replaceAll(RegExp(r'[^0-9]'), '').trim();
    var value = double.tryParse(str) ?? 0.0;
    if (value != 0) value /= 100;
    var numberParts = '$value'.split('.');
    if (numberParts.length == 1) {
      numberParts.add('0');
    }
    var intPart = numberParts[0];
    // make sure we have 2 digits in decimals when input 1 char.
    var decPart = numberParts[1].padLeft(2, '0');
    var output = '';
    var intList = intPart.split('').toList();
    final len = intList.length;
    intList.asMap().entries.forEach((e) {
      output += e.value;
      final j = len - e.key - 1;
      final isLast = e.key == len - 1;
      if (j % 3 == 0 && !isLast) {
        output += '.';
      }
    });
    output += ',' + decPart;

    controller.value = controller.value.copyWith(
      selection: TextSelection.collapsed(offset: output.length - _lastSelPos),
      text: output,
    );
  }
}