Simple form managment

main.dart

/// http://dartpad.dev/981a8d2145a9878dae2b52aebd9fae88

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

void main() => runApp(App());

class App extends StatelessWidget {
  @override
  Widget build(BuildContext context) => const MaterialApp(
        title: 'FORM DATA',
        home: Scaffold(
          body: SafeArea(
            child: Center(
              child: Card(
                margin: EdgeInsets.all(16),
                elevation: 4,
                clipBehavior: Clip.antiAlias,
                shape: RoundedRectangleBorder(
                  borderRadius: BorderRadius.all(
                    Radius.circular(
                      8,
                    ),
                  ),
                ),
                child: Padding(
                  padding: EdgeInsets.all(18),
                  child: FoxyForm(),
                ),
              ),
            ),
          ),
        ),
      );
}

/// Виджет формы
@immutable
class FoxyForm extends StatefulWidget {
  const FoxyForm({Key key}) : super(key: key);
  @override
  State<FoxyForm> createState() => _FoxyFormState();
}

class _FoxyFormState extends State<FoxyForm> {
  final ValueNotifier<bool> _allFilled = ValueNotifier(false);
  FormData _formData;
  FieldStatus<String, FormData> _nameField;
  FieldStatus<String, FormData> _ageField;
  Iterable<FieldStatus> _fields;
  final FocusNode _buttonFocusNode = FocusNode();

  @override
  void initState() {
    super.initState();
    _formData = FormData(
      name: 'Иван',
      age: '15',
    );
    _nameField = FieldStatus<String, FormData>(
      formData: _formData,
      value: (data) => data.name,
      onChanged: (data, name) => data.name = name,
      validator: (data, name) => name?.isEmpty ?? true ? 'Поле имени не должно быть пустым' : null,
    );
    _ageField = FieldStatus<String, FormData>(
      formData: _formData,
      value: (data) => data.age,
      onChanged: (data, age) => data.age = age,
      validator: (data, age) => age?.isEmpty ?? true ? 'Поле возраста не должно быть пустым' : null,
    );
    _fields = <FieldStatus>[_nameField, _ageField];
    _setValidationStatus();
  }

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

  @override
  Widget build(BuildContext context) => Form(
        autovalidateMode: AutovalidateMode.onUserInteraction,
        onChanged: _setValidationStatus,
        child: Builder(
          builder: (context) => SingleChildScrollView(
            child: Column(
              mainAxisSize: MainAxisSize.min,
              children: <Widget>[
                TextFormField(
                  autofocus: true,
                  focusNode: _nameField.focusNode,
                  onEditingComplete: _nextEmpty,
                  initialValue: _formData.name,
                  onSaved: _nameField.onChanged,
                  onChanged: _nameField.onChanged,
                  validator: _nameField.validator,
                  keyboardType: TextInputType.name,
                  inputFormatters: [
                    FilteringTextInputFormatter.allow(RegExp('[A-Za-zА-Яа-я]+')),
                  ],
                  decoration: const InputDecoration(
                    labelText: 'Name',
                  ),
                ),
                const SizedBox(height: 13),
                TextFormField(
                  focusNode: _ageField.focusNode,
                  onEditingComplete: _nextEmpty,
                  initialValue: _formData.age,
                  onSaved: _ageField.onChanged,
                  onChanged: _ageField.onChanged,
                  validator: _ageField.validator,
                  keyboardType: TextInputType.number,
                  inputFormatters: [
                    FilteringTextInputFormatter.digitsOnly,
                  ],
                  decoration: const InputDecoration(
                    labelText: 'Age',
                  ),
                ),
                const SizedBox(height: 13),
                ValueListenableBuilder<bool>(
                  valueListenable: _allFilled,
                  builder: (context, value, _) => RaisedButton(
                    focusNode: _buttonFocusNode,
                    onPressed: value ? () => _submit(context) : null,
                    child: const Text('SUBMIT FORM'),
                  ),
                ),
              ],
            ),
          ),
        ),
      );

  void _nextEmpty() {
    final fieldNode = _fields.firstWhere((field) => !field.filled, orElse: () => null)?.focusNode;
    if (fieldNode == null) {
      _unfocusAll();
      //_buttonFocusNode.requestFocus();
    } else {
      fieldNode.requestFocus();
    }
  }

  void _unfocusAll() =>
      _fields.map<FocusNode>((field) => field.focusNode).forEach((node) => node.unfocus());

  void _setValidationStatus() => _allFilled.value = _fields.every((field) => field.filled);

  void _submit(BuildContext context) {
    _unfocusAll();
    final form = Form.of(context)..save();
    if (!form.validate()) {
      _nextEmpty();
      return;
    }
    Scaffold.of(context).showSnackBar(
      SnackBar(
        content: Text('SUBMIT: $_formData'),
      ),
    );
  }
}

/// Данные формы
class FormData {
  String name;
  String age;
  FormData({this.name = '', this.age = ''});

  @override
  String toString() => '{"name": "$name", "age": "$age"}';
}

/// Класс в который инкапсулировано изменение и верификация поля формы
@immutable
class FieldStatus<FieldType, FormDataType> {
  /// Данные общие для формы
  final FormDataType _formData;

  /// Заполнено ли поле
  bool get filled => !hasValidator || validator(value(_formData)) == null;

  /// Получение текущего значения поля
  final FieldType Function(FormDataType data) value;

  /// Нода для перехода к полям (например незаполненным при [onEditingComplete]
  final FocusNode focusNode;

  /// При изменении поля
  ValueChanged<FieldType> get onChanged => (value) => _onChanged(_formData, value);
  final void Function(FormDataType data, FieldType value) _onChanged;

  /// Проверка поля
  /// validator может быть опциональный (для необязательных полей его не указывать)
  FormFieldValidator<FieldType> get validator => (value) => hasValidator ? _validator(_formData, value) : null;
  final String Function(FormDataType data, FieldType value) _validator;

  /// Обладает ли валидатором
  final bool hasValidator;

  FieldStatus({
    @required FormDataType formData,
    @required this.value,
    @required void Function(FormDataType data, FieldType value) onChanged,
    String Function(FormDataType data, FieldType value) validator,
    FocusNode focusNode,
  })  : _formData = formData,
        focusNode = focusNode ?? FocusNode(),
        _onChanged = onChanged,
        _validator = validator,
        hasValidator = validator != null;
}