Forms and validation

class SignupForm extends StatefulWidget {
  const SignupForm({super.key});
  @override
  State<SignupForm> createState() => _SignupFormState();
}

class _SignupFormState extends State<SignupForm> {
  final _formKey = GlobalKey<FormState>();
  final _name = TextEditingController();
  final _mobile = TextEditingController();

  @override
  void dispose() {
    _name.dispose();
    _mobile.dispose();
    super.dispose();
  }

  void _submit() {
    if (_formKey.currentState!.validate()) {
      ScaffoldMessenger.of(context).showSnackBar(
        SnackBar(content: Text('Welcome, ${_name.text.trim()}!')),
      );
    }
  }

  @override
  Widget build(BuildContext context) {
    return Form(
      key: _formKey,
      child: Column(
        children: [
          TextFormField(
            controller: _name,
            decoration: const InputDecoration(labelText: 'Full name'),
            validator: (v) => (v == null || v.trim().length < 2) ? 'Enter your name' : null,
          ),
          TextFormField(
            controller: _mobile,
            decoration: const InputDecoration(labelText: 'Mobile number', prefixText: '+91 '),
            keyboardType: TextInputType.phone,
            inputFormatters: [FilteringTextInputFormatter.digitsOnly, LengthLimitingTextInputFormatter(10)],
            validator: validateMobile,
          ),
          const SizedBox(height: 16),
          FilledButton(onPressed: _submit, child: const Text('Create account')),
        ],
      ),
    );
  }
}

String? validateMobile(String? value) {
  final v = value?.trim() ?? '';
  if (v.isEmpty) return 'Mobile number is required';
  if (v.length != 10) return 'Enter 10 digits';
  if (!RegExp(r'^[6-9]').hasMatch(v)) return 'Indian mobile numbers start with 6, 7, 8 or 9';
  return null;
}

String? validatePin(String? value) =>
    RegExp(r'^[1-9][0-9]{5}$').hasMatch(value ?? '') ? null : 'Enter a valid 6-digit PIN code';