Flutter Forms: Get the list of fields in error

I don't think it is possible to get this kind of information from a Form object or a FormState.

But here is a way around to obtain the result you want (focus on the field in error) :

class _MyWidgetState extends State<MyWidget> {
  FocusNode _fieldToFocus;
  List<FocusNode> _focusNodes;

  final _formKey = GlobalKey<FormState>();
  final _numberOfFields = 3;

  String _emptyFieldValidator(String val, FocusNode focusNode) {
    if (val.isEmpty) {
      _fieldToFocus ??= focusNode;
      return 'This field cannot be empty';
    }
    return null;
  }

  @override
  void initState() {
    super.initState();
    _focusNodes =
        List<FocusNode>.generate(_numberOfFields, (index) => FocusNode());
  }

  @override
  Widget build(BuildContext context) {
    return Scaffold(
      appBar: AppBar(actions: [
        IconButton(
          icon: Icon(Icons.check),
          onPressed: () {
            if (_formKey.currentState.validate()) {
              print('Valid form');
            } else {
              _fieldToFocus?.requestFocus();
              _fieldToFocus = null;
            }
          },
        ),
      ]),
      body: Form(
        key: _formKey,
        child: Column(children: [
          ...List<TextFormField>.generate(
            _numberOfFields,
            (index) => TextFormField(
              decoration: InputDecoration(hintText: "Field $index"),
              focusNode: _focusNodes[index],
              validator: (val) => _emptyFieldValidator(val, _focusNodes[index]),
            ),
          ),
        ]),
      ),
    );
  }
}

You simply need to create a FocusNode for each one of your fields, thanks to that you will be abla to call requestFocus on a precise field (in your case a field considered as invalid). Then in the validator property of your form field, as it is the method called by the FormState.validate(), you need to set a temporary variable which will contains the right FocusNode. In my example I only set the variable _fieldToFocus if it was not already assigned using the ??= operator. After requesting the focus on the node I set _fieldToFocus back to null so it will still works for another validation.

You can try the full test code I have used on DartPad.

Sorry if I have derived a bit from your question but I still hope this will help you.

Expanding on Guillaume's answer, I've wrapped the functionality into a reusable class.

You can view a working example on DartPad here: https://www.dartpad.dev/61c4ccddbf29a343c971ee75e60d1038

import 'package:flutter/material.dart';

class FormValidationManager {
  final _fieldStates = Map<String, FormFieldValidationState>();

  FocusNode getFocusNodeForField(key) {
      _ensureExists(key);

      return _fieldStates[key].focusNode;
  }

  FormFieldValidator<T> wrapValidator<T>(String key, FormFieldValidator<T> validator) {
      _ensureExists(key);

      return (input) {
          final result = validator(input);

          _fieldStates[key].hasError = (result?.isNotEmpty ?? false);

          return result;
      };
  }

  List<FormFieldValidationState> get erroredFields =>
      _fieldStates.entries.where((s) => s.value.hasError).map((s) => s.value).toList();

  void _ensureExists(String key) {
      _fieldStates[key] ??= FormFieldValidationState(key: key);
  }

  void dispose() {
    _fieldStates.entries.forEach((s) {
        s.value.focusNode.dispose();
    });
  }
}

class FormFieldValidationState {
  final String key;

  bool hasError;
  FocusNode focusNode;

  FormFieldValidationState({@required this.key})
      : hasError = false,
      focusNode = FocusNode();
}

To use it, create your forms as usual, but add a FormValidationManager to your state class, and then use that instance to wrap your validation methods.

Usage:

class _MyWidgetState extends State<MyWidget> {
  final _formKey = GlobalKey<FormState>();
  final _formValidationManager = FormValidationManager();

  @override
  Widget build(BuildContext context) {
    return Form(
      key: _formKey,
      child: Column(
        children: [
          TextFormField(
              focusNode: _formValidationManager.getFocusNodeForField('field1'),
              validator: _formValidationManager.wrapValidator('field1', (value) {
                if (value.isEmpty) {
                  return 'Please enter a value';
                }

                return null;
              })),
          TextFormField(
              focusNode: _formValidationManager.getFocusNodeForField('field2'),
              validator: _formValidationManager.wrapValidator('field2', (value) {
                if (value.isEmpty) {
                  return 'Please enter a value';
                }

                return null;
              })),
          ElevatedButton(
              onPressed: () {
                if (!_formKey.currentState.validate()) {
                  _formValidationManager.erroredFields.first.focusNode.requestFocus();
                }
              },
              child: Text('SUBMIT'))
        ],
      ),
    );
  }

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