Coupon code field

import 'package:flutter/material.dart';

class Coupon {
  const Coupon(this.code, this.percentOff);
  final String code;
  final int percentOff;
}

typedef CouponValidator = Future<Coupon> Function(String code); // throws String on rejection

enum _Phase { idle, checking, applied }

class CouponField extends StatefulWidget {
  const CouponField({super.key, required this.validate, required this.onChanged});
  final CouponValidator validate;
  final ValueChanged<Coupon?> onChanged; // null when removed

  @override
  State<CouponField> createState() => _CouponFieldState();
}

class _CouponFieldState extends State<CouponField> {
  final _text = TextEditingController();
  _Phase _phase = _Phase.idle;
  Coupon? _coupon;
  String? _error;

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

  Future<void> _apply() async {
    final code = _text.text.trim().toUpperCase();
    if (code.isEmpty) return;
    setState(() { _phase = _Phase.checking; _error = null; });
    try {
      final c = await widget.validate(code);
      if (!mounted) return;
      setState(() { _coupon = c; _phase = _Phase.applied; });
      widget.onChanged(c);
    } on String catch (reason) {
      if (mounted) setState(() { _phase = _Phase.idle; _error = reason; });
    } catch (_) {
      if (mounted) setState(() { _phase = _Phase.idle; _error = 'Could not check the code. Try again.'; });
    }
  }

  void _remove() {
    _text.clear();
    setState(() { _coupon = null; _phase = _Phase.idle; });
    widget.onChanged(null);
  }

  @override
  Widget build(BuildContext context) {
    if (_phase == _Phase.applied) {
      return ListTile(
        contentPadding: EdgeInsets.zero,
        leading: const Icon(Icons.sell_outlined),
        title: Text('${_coupon!.code} applied'),
        subtitle: Text('${_coupon!.percentOff}% off your order'),
        trailing: TextButton(onPressed: _remove, child: const Text('Remove')),
      );
    }
    return TextField(
      controller: _text,
      enabled: _phase != _Phase.checking,
      textCapitalization: TextCapitalization.characters,
      textInputAction: TextInputAction.done,
      onSubmitted: (_) => _apply(),
      decoration: InputDecoration(
        labelText: 'Promo code',
        errorText: _error,
        suffix: _phase == _Phase.checking
            ? const SizedBox(width: 16, height: 16, child: CircularProgressIndicator(strokeWidth: 2))
            : TextButton(onPressed: _apply, child: const Text('Apply')),
      ),
    );
  }
}