currency_text_input_formatter_example.dart

import 'dart:math';

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

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

class MyApp extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      title: 'Flutter Demo',
      theme: ThemeData(
        primarySwatch: Colors.blue,
      ),
      home: MyHomePage(),
    );
  }
}

class MyHomePage extends StatefulWidget {
  @override
  _MyHomePageState createState() => _MyHomePageState();
}

class _MyHomePageState extends State<MyHomePage> {
  // Customize number format pattern https://docs.oracle.com/javase/tutorial/i18n/format/decimalFormat.html
  final vnCurrencyFormat = NumberFormat.currency(
    locale: 'en-US',
    customPattern: '###,###',
    decimalDigits: 0,
  ); // VN currency format with ',' group separator. Ex: 9,999,999.

  final _controller = TextEditingController();
  int currentPrice = 9999999;

  @override
  void initState() {
    super.initState();

    _controller.text = vnCurrencyFormat.format(currentPrice);
    _controller.addListener(() {
      final price = _controller.text.isNotEmpty
          ? vnCurrencyFormat.parse(_controller.text).toInt()
          : 0;

      setState(() {
        currentPrice = price;
      });
    });
  }

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

    super.dispose();
  }

  @override
  Widget build(BuildContext context) {
    return Scaffold(
      body: Padding(
        padding: const EdgeInsets.all(16.0),
        child: Column(
          mainAxisAlignment: MainAxisAlignment.center,
          children: <Widget>[
            TextField(
              controller: _controller,
              keyboardType: TextInputType.number,
              inputFormatters: [
                CurrencyTextInputFormatter(currencyFormat: vnCurrencyFormat),
              ],
            ),
            SizedBox(height: 16.0),
            Text(
              'Price: $currentPrice',
            ),
          ],
        ),
      ),
    );
  }
}

// https://github.com/gtgalone/currency_text_input_formatter/blob/master/lib/currency_text_input_formatter.dart
class CurrencyTextInputFormatter extends TextInputFormatter {
  final int maxNumbers;
  NumberFormat currencyFormat;

  CurrencyTextInputFormatter({
    this.maxNumbers = 15,
    NumberFormat currencyFormat,
  }) {
    this.currencyFormat = currencyFormat ??
        NumberFormat.currency(
          locale: 'en-US',
          customPattern: '###,###',
          decimalDigits: 0,
        );
  }

  TextEditingValue formatEditUpdate(
    TextEditingValue oldValue,
    TextEditingValue newValue,
  ) {
    final newText = newValue.text.replaceAll(RegExp('[^0-9]'), '');

    if (newText.trim() == '' || newText.trim() == '0') {
      return newValue.copyWith(text: '');
    } else if (int.parse(newText) < 1) {
      return newValue.copyWith(text: '');
    } else if (newText.length > maxNumbers) {
      return TextEditingValue(
        text: oldValue.text,
        selection: TextSelection.collapsed(
          offset: oldValue.selection.end,
        ),
      );
    }

    dynamic newInt = int.parse(newText);
    var selectionIndexFromTheRight =
        newValue.text.length - newValue.selection.end;
    if (currencyFormat.decimalDigits > 0) {
      newInt /= pow(10, currencyFormat.decimalDigits);
    }
    final newString = currencyFormat.format(newInt);
    return TextEditingValue(
      text: newString,
      selection: TextSelection.collapsed(
        offset: newString.length - selectionIndexFromTheRight,
      ),
    );
  }
}