Forms with Bloc and Formz

Pushes per-field validation into typed value objects (FormzInput<TValue, TError>) so the Bloc only orchestrates: hold the fields, derive overall validity, submit, and route server-side validation errors back to the right field. Form fields become reusable types (EmailInput, PhoneInput) shared across screens. Submission status (FormzSubmissionStatus.inProgress / success / failure) is decoupled from field validity so a valid form can still fail to submit, and vice versa. Builds on flutter-bloc-async-api for the underlying request/response flow.

Contents

• Why formz
• FormzInput per field
• Form Bloc state shape
• Mapping Laravel 422 errors to fields
• Workflow: Build a form with Bloc and formz
• Applied to Talabat-clone
• Examples

Why formz

A naive form Bloc holds raw String fields and runs validation in the submit handler. That has three failure modes: (1) validation rules duplicate across forms that share a field (email validator written three times); (2) the View has to know whether to display an error (don't show "required" before the user touches the field); (3) server-side and client-side errors are tracked separately and drift apart.

formz solves all three by making each field a value object:

• A FormzInput<TValue, TError> knows its current value, its validity, and whether it has been touched.
• displayError returns the error only if the field is dirty — the View can naively render it without an "if pure return null" check.
• Validation rules live on the input class, not the Bloc. Reuse EmailInput in login, sign-up, profile-edit, and forgot-password.

FormzInput per field

A field is a sealed enum of failure modes plus a FormzInput subclass.

import 'package:formz/formz.dart';

enum EmailValidationError { empty, malformed }

class EmailInput extends FormzInput<String, EmailValidationError> {
  const EmailInput.pure() : super.pure('');
  const EmailInput.dirty([super.value = '']) : super.dirty();

  static final _emailRegex = RegExp(r'^[^@\s]+@[^@\s]+\.[^@\s]+$');

  @override
  EmailValidationError? validator(String value) {
    if (value.isEmpty) return EmailValidationError.empty;
    if (!_emailRegex.hasMatch(value)) return EmailValidationError.malformed;
    return null;
  }
}

.pure(value) is for the initial state — field is untouched, no error displayed. .dirty(value) is for every subsequent update from user input — the validator runs and displayError will surface its result.

For one-off validators (a field used only in one form), inline the class in the same file as the Bloc. For reusable validators (EmailInput, PhoneInput, PasswordInput), put them in lib/core/forms/ so every form Bloc can import them.

Form Bloc state shape

The state holds all fields as inputs, an overall submission status, and a slot for server-level failures that aren't per-field.

@freezed
class LoginFormState with _$LoginFormState {
  const factory LoginFormState({
    @Default(EmailInput.pure()) EmailInput email,
    @Default(PasswordInput.pure()) PasswordInput password,
    @Default(FormzSubmissionStatus.initial) FormzSubmissionStatus status,
    String? serverError, // for 401, 500, network — never for 422
  }) = _LoginFormState;

  const LoginFormState._();

  bool get isValid => Formz.validate([email, password]);
}

A getter isValid derives from Formz.validate([...]) rather than being stored. The View consults state.isValid to decide whether the submit button is enabled — no duplicated truth to keep in sync.

The initial state with all-pure() inputs is not valid — pure inputs whose validator rejects empty strings are invalid-but-not-displayed. Submit is correctly disabled at start; displayError returns null until the field is dirty, so the user doesn't see "Email required" before typing.

Submission status is independent of validity. A form can be FormzSubmissionStatus.inProgress while isValid is true. After a 422, status flips to failure but isValid may still be true on the client (the server disagreed). Treat them as orthogonal axes.

formz exposes a FormzSubmissionStatusX extension with convenience getters: status.isInitial, status.isInProgress, status.isSuccess, status.isFailure, status.isCanceled, and the load-bearing status.isInProgressOrSuccess for "keep the submit button disabled while in flight and after success until navigation completes". Use them instead of raw status == FormzSubmissionStatus.inProgress comparisons.

Mapping Laravel 422 errors to fields

When flutter-bloc-async-api's repository returns Result.failure(Failure.validation(errors)) — the Failure taxonomy defined in flutter-bloc-async-api — the form Bloc splatters those errors back onto the corresponding inputs.

Laravel response:

{
  "message": "The given data was invalid.",
  "errors": {
    "email": ["The email has already been taken."],
    "phone": ["The phone must be at least 10 digits."]
  }
}

Each FormzInput subclass declares an extra final String? serverError field initialized by both .pure() and .dirty() constructors. The View prefers serverError over the client-side displayError. There is no extension, no withServerError(...) method, no freezed wrapper — just a constructor parameter.

The mapping in the Bloc's submission handler:

Future<void> _onSubmitted(
  LoginFormSubmitted event,
  Emitter<LoginFormState> emit,
) async {
  if (!state.isValid) return;
  emit(state.copyWith(status: FormzSubmissionStatus.inProgress));

  final result = await _repo.login(
    email: state.email.value,
    password: state.password.value,
  );

  emit(switch (result) {
    ResultSuccess() =>
        state.copyWith(status: FormzSubmissionStatus.success),
    ResultFailure(failure: ValidationFailure(:final errors)) =>
        state.copyWith(
          status: FormzSubmissionStatus.failure,
          email: errors.containsKey('email')
              ? EmailInput.dirty(state.email.value, errors['email']!.first)
              : state.email,
          password: errors.containsKey('password')
              ? PasswordInput.dirty(state.password.value, errors['password']!.first)
              : state.password,
        ),
    ResultFailure(:final failure) =>
        state.copyWith(
          status: FormzSubmissionStatus.failure,
          serverError: _messageFor(failure), // canonical mapper lives in flutter-bloc-async-api
        ),
  });
}

Stale-server-error invariant. The _onEmailChanged keystroke handler emits EmailInput.dirty(e.value) with no second argument, so serverError defaults to null and stale 422 messages clear automatically as the user types. The form re-renders without the obsolete error before the next submit even fires.

Workflow: Build a form with Bloc and formz

Task Progress

• Step 1 — Add the dep. flutter pub add formz.
• Step 2 — Decide which inputs are reusable. Inputs used in 2+ forms go in lib/core/forms/. One-offs stay in the feature folder.
• Step 3 — Write the FormzInput subclasses. Each has a pure() ctor, dirty() ctor, validation enum, and validator() override.
• Step 4 — Define the form state. @freezed class with one input per field, FormzSubmissionStatus, optional serverError, and a derived isValid getter using Formz.validate([...]).
• Step 5 — Define the events. One <Field>Changed(value) per field, plus Submitted and (optional) Reset.
• Step 6 — Implement field handlers. Each handler emits state.copyWith(<field>: <Input>.dirty(newValue)). Use transformer: sequential() for keystroke-driven fields (avoid concurrent() reordering).
• Step 7 — Implement the submit handler. Use transformer: droppable() so double-taps on submit are ignored. Guard with if (!state.isValid) return;. Emit inProgress, call the repository, pattern-match the Result.
• Step 8 — Map 422 to fields. In the failure branch, distribute ValidationFailure.errors map keys onto the matching inputs via <Input>.dirty(value) + a server-error attachment.
• Step 9 — Wire the View. TextField per input — decoration.errorText reads from state.<field>.displayError (mapped through a _message() helper). Submit button uses state.isValid && state.status != FormzSubmissionStatus.inProgress.
• Step 10 — Feedback loop. Run on device → submit with invalid data → assert per-field errors render → submit with data that triggers a server 422 → assert those errors render on the right fields. If a 422 error message shows in a snackbar instead of next to the field, the Bloc swallowed ValidationFailure into serverError instead of splatting it.

Applied to Talabat-clone

Two forms from the PRDs map directly.

AddressFormBloc (prd.md §2.1 selection, §27 fields)

Address selection (saved-addresses list, "use my current location", far-address warning) is prd.md §2.1. Address entry fields are prd.md §27: apartment number (رقم الشقة), floor (الطابق), landmark (علامة مميزة), plus special instructions (تعليمات خاصة بالعنوان). The form Bloc holds an ApartmentInput, FloorInput, LandmarkInput, SpecialInstructionsInput, and a CityInput populated automatically from a reverse-geocode lookup (the city field is shown read-only and is "dirty" on initial load — pure inputs whose validator rejects empty strings would block submit).

Submitting POSTs to /api/users/me/addresses. The Laravel side validates that apartment is non-empty and city_id exists in the cities table. A 422 with {"apartment": ["The apartment field is required."]} maps to state.copyWith(apartment: ApartmentInput.dirty(state.apartment.value, errors['apartment']!.first)) — the user sees the error inline.

OtpVerifyBloc (PRD_states.md §14)

The user lifecycle is pending_verification → active via phone/email OTP. The form has a single OtpCodeInput with a fixed-length validator (6 digits) and a submit. Server-side errors include 422 ({"code": ["Invalid code"]}); expired codes come back as Failure.unknown or a custom code your repository maps to Failure.unknown(...), surfaced as state.serverError rather than a per-field error. A Resend event triggers a separate repository call gated by a cooldown timer — Stream.periodic(Duration(seconds: 1)) consumed via a Ticked event handler is the lightest implementation; no real-time transport, so flutter-bloc-stream-tracking does not apply.

Examples

A LoginFormBloc complete with field inputs, state, events, handler, and view. The form has email and password; the API is POST /api/login returning either a 200 with a token or a 422 with field errors.

lib/core/forms/email_input.dart

import 'package:formz/formz.dart';

enum EmailValidationError { empty, malformed }

class EmailInput extends FormzInput<String, EmailValidationError> {
  // `pure` accepts an optional seed value so an edit-profile screen can
  // pre-populate the field without making it dirty. `serverError` is always
  // null for pure inputs (you only get a server error after submitting).
  // Dart requires every final field to be set by every constructor.
  const EmailInput.pure([String value = ''])
      : serverError = null,
        super.pure(value);
  const EmailInput.dirty([super.value = '', this.serverError]) : super.dirty();

  final String? serverError;

  static final _re = RegExp(r'^[^@\s]+@[^@\s]+\.[^@\s]+$');

  @override
  EmailValidationError? validator(String value) {
    if (value.isEmpty) return EmailValidationError.empty;
    if (!_re.hasMatch(value)) return EmailValidationError.malformed;
    return null;
  }

  // FormzInput's default `==` covers `isPure` and `value` only — we add
  // `serverError` so a `BlocSelector<EmailInput>` rebuilds when only the
  // server message changes (same value, same purity, new error string).
  @override
  bool operator ==(Object other) =>
      identical(this, other) ||
      (other is EmailInput &&
          other.value == value &&
          other.isPure == isPure &&
          other.serverError == serverError);

  @override
  int get hashCode => Object.hash(value, isPure, serverError);
}

lib/core/forms/password_input.dart

import 'package:formz/formz.dart';

enum PasswordValidationError { empty, tooShort }

class PasswordInput extends FormzInput<String, PasswordValidationError> {
  const PasswordInput.pure([String value = ''])
      : serverError = null,
        super.pure(value);
  const PasswordInput.dirty([super.value = '', this.serverError]) : super.dirty();

  final String? serverError;

  @override
  PasswordValidationError? validator(String value) {
    if (value.isEmpty) return PasswordValidationError.empty;
    if (value.length < 8) return PasswordValidationError.tooShort;
    return null;
  }

  @override
  bool operator ==(Object other) =>
      identical(this, other) ||
      (other is PasswordInput &&
          other.value == value &&
          other.isPure == isPure &&
          other.serverError == serverError);

  @override
  int get hashCode => Object.hash(value, isPure, serverError);
}

lib/ui/features/login/bloc/login_form_state.dart

import 'package:formz/formz.dart';
import 'package:freezed_annotation/freezed_annotation.dart';

import '../../../../core/forms/email_input.dart';
import '../../../../core/forms/password_input.dart';

part 'login_form_state.freezed.dart';

@freezed
class LoginFormState with _$LoginFormState {
  const factory LoginFormState({
    @Default(EmailInput.pure()) EmailInput email,
    @Default(PasswordInput.pure()) PasswordInput password,
    @Default(FormzSubmissionStatus.initial) FormzSubmissionStatus status,
    String? serverError,
  }) = _LoginFormState;

  const LoginFormState._();

  bool get isValid => Formz.validate([email, password]);
}

lib/ui/features/login/bloc/login_form_bloc.dart

import 'package:bloc_concurrency/bloc_concurrency.dart';
import 'package:flutter_bloc/flutter_bloc.dart';
import 'package:formz/formz.dart';

import '../../../../core/forms/email_input.dart';
import '../../../../core/forms/password_input.dart';
import '../../../../core/result/failure.dart';
import '../../../../core/result/result.dart';
import '../../../../data/repositories/auth_repository.dart';
import 'login_form_event.dart';
import 'login_form_state.dart';

class LoginFormBloc extends Bloc<LoginFormEvent, LoginFormState> {
  LoginFormBloc({required AuthRepository repo})
      : _repo = repo,
        super(const LoginFormState()) {
    on<LoginEmailChanged>(_onEmailChanged, transformer: sequential());
    on<LoginPasswordChanged>(_onPasswordChanged, transformer: sequential());
    on<LoginSubmitted>(_onSubmitted, transformer: droppable());
  }

  final AuthRepository _repo;

  void _onEmailChanged(LoginEmailChanged e, Emitter<LoginFormState> emit) {
    emit(state.copyWith(email: EmailInput.dirty(e.value)));
  }

  void _onPasswordChanged(LoginPasswordChanged e, Emitter<LoginFormState> emit) {
    emit(state.copyWith(password: PasswordInput.dirty(e.value)));
  }

  Future<void> _onSubmitted(
    LoginSubmitted event,
    Emitter<LoginFormState> emit,
  ) async {
    if (!state.isValid) return;
    emit(state.copyWith(status: FormzSubmissionStatus.inProgress));

    final result = await _repo.login(
      email: state.email.value,
      password: state.password.value,
    );

    emit(switch (result) {
      ResultSuccess() => state.copyWith(status: FormzSubmissionStatus.success),
      ResultFailure(failure: ValidationFailure(:final errors)) =>
          state.copyWith(
            status: FormzSubmissionStatus.failure,
            email: errors.containsKey('email')
                ? EmailInput.dirty(state.email.value, errors['email']!.first)
                : state.email,
            password: errors.containsKey('password')
                ? PasswordInput.dirty(state.password.value, errors['password']!.first)
                : state.password,
          ),
      ResultFailure(failure: UnauthorizedFailure()) =>
          state.copyWith(
            status: FormzSubmissionStatus.failure,
            serverError: 'Email or password is incorrect',
          ),
      ResultFailure(:final failure) =>
          state.copyWith(
            status: FormzSubmissionStatus.failure,
            serverError: _messageFor(failure),
          ),
    });
  }

  String _messageFor(Failure f) => switch (f) {
    NetworkFailure()      => 'No internet connection',
    TimeoutFailure()      => 'Server is taking too long. Try again.',
    ServerFailure(:final statusCode) => 'Server error ($statusCode)',
    _ => 'Something went wrong',
  };
}

lib/ui/features/login/view/login_view.dart

import 'package:flutter/material.dart';
import 'package:flutter_bloc/flutter_bloc.dart';
import 'package:formz/formz.dart';

import '../../../../core/forms/email_input.dart';
import '../../../../core/forms/password_input.dart';
import '../bloc/login_form_bloc.dart';
import '../bloc/login_form_event.dart';
import '../bloc/login_form_state.dart';

class LoginView extends StatelessWidget {
  const LoginView({super.key});

  @override
  Widget build(BuildContext context) {
    return BlocProvider(
      create: (ctx) => LoginFormBloc(repo: ctx.read()),
      child: const _LoginScaffold(),
    );
  }
}

class _LoginScaffold extends StatelessWidget {
  const _LoginScaffold();

  @override
  Widget build(BuildContext context) {
    return BlocListener<LoginFormBloc, LoginFormState>(
      listenWhen: (p, n) => p.status != n.status,
      listener: (ctx, state) {
        if (state.status == FormzSubmissionStatus.success) {
          Navigator.of(ctx).pushReplacementNamed('/home');
        } else if (state.serverError != null) {
          ScaffoldMessenger.of(ctx).showSnackBar(
            SnackBar(content: Text(state.serverError!)),
          );
        }
      },
      child: Scaffold(
        appBar: AppBar(title: const Text('Sign in')),
        body: Padding(
          padding: const EdgeInsets.all(16),
          child: Column(
            children: [
              BlocSelector<LoginFormBloc, LoginFormState, EmailInput>(
                selector: (s) => s.email,
                builder: (ctx, email) => TextField(
                  decoration: InputDecoration(
                    labelText: 'Email',
                    errorText: email.serverError ?? _emailMsg(email.displayError),
                  ),
                  onChanged: (v) => ctx.read<LoginFormBloc>().add(LoginEmailChanged(v)),
                ),
              ),
              const SizedBox(height: 12),
              BlocSelector<LoginFormBloc, LoginFormState, PasswordInput>(
                selector: (s) => s.password,
                builder: (ctx, password) => TextField(
                  obscureText: true,
                  decoration: InputDecoration(
                    labelText: 'Password',
                    errorText: password.serverError ?? _passwordMsg(password.displayError),
                  ),
                  onChanged: (v) => ctx.read<LoginFormBloc>().add(LoginPasswordChanged(v)),
                ),
              ),
              const SizedBox(height: 24),
              BlocBuilder<LoginFormBloc, LoginFormState>(
                builder: (ctx, state) => FilledButton(
                  onPressed: state.isValid && state.status != FormzSubmissionStatus.inProgress
                      ? () => ctx.read<LoginFormBloc>().add(const LoginSubmitted())
                      : null,
                  child: state.status == FormzSubmissionStatus.inProgress
                      ? const CircularProgressIndicator()
                      : const Text('Sign in'),
                ),
              ),
            ],
          ),
        ),
      ),
    );
  }

  String? _emailMsg(EmailValidationError? e) => switch (e) {
    EmailValidationError.empty     => 'Email is required',
    EmailValidationError.malformed => 'Enter a valid email',
    null                           => null,
  };

  String? _passwordMsg(PasswordValidationError? e) => switch (e) {
    PasswordValidationError.empty    => 'Password is required',
    PasswordValidationError.tooShort => 'At least 8 characters',
    null                             => null,
  };
}

errorText: email.serverError ?? _emailMsg(email.displayError) is the whole 422 round-trip in one line: server error wins when present, otherwise client-side validator output (or null when the field is still pure).

Source: abdallhMoukdad/flutter-bloc-skills — distributed by TomeVault.