Flutter Bloc Feature Pattern

Scaffolding a Bloc Feature

Defines the canonical shape of a Bloc-driven feature: three files (<feature>_event.dart, <feature>_state.dart, <feature>_bloc.dart), sealed states with freezed, and an explicit EventTransformer per handler. Every other skill in the family (flutter-bloc-async-api, flutter-bloc-stream-tracking, flutter-bloc-testing, flutter-bloc-forms) builds on this shape. Prerequisite: flutter-bloc-setup.

Contents

• The Event/State/Bloc trio
• Sealed states with freezed
• Event handlers and concurrency
• Wiring the View
• Persistence (optional)
• Workflow: Scaffold a new feature
• Applied to Talabat-clone
• Examples

The Event/State/Bloc trio

Every feature has exactly three files under lib/ui/features/<feature>/bloc/.

Events and states are never mutable. The Bloc constructor registers handlers via on<Event>(handler, transformer: ...) and emits new state instances. Views observe state and dispatch events; they never read fields off the Bloc directly.

Sealed states with freezed

Use @freezed with a sealed class so a switch over state variants is exhaustive at compile time. Forgetting to handle a variant becomes a compile error rather than a runtime UI bug.

// lib/ui/features/cart/bloc/cart_state.dart
import 'package:freezed_annotation/freezed_annotation.dart';
import 'package:my_app/domain/models/cart_item.dart';
import 'package:my_app/domain/models/money.dart';

part 'cart_state.freezed.dart';
part 'cart_state.g.dart'; // required when the state is also `HydratedBloc`-persisted

@Freezed(unionKey: 'type') // stable discriminator key across class renames
sealed class CartState with _$CartState {
  const factory CartState.initial() = CartInitial;
  const factory CartState.loading() = CartLoading;
  const factory CartState.loaded({
    required List<CartItem> items,
    required Money subtotal,
    required Money total,
  }) = CartLoaded;
  const factory CartState.failure({required String message}) = CartFailure;

  // Required for `HydratedBloc` round-trip — `fromJson` is **not** generated
  // automatically; you must declare this factory yourself. `CartItem` and
  // `Money` must each be JSON-serializable (typically also `@freezed` classes).
  factory CartState.fromJson(Map<String, dynamic> json) =>
      _$CartStateFromJson(json);
}

The View consumes the sealed state with a Dart 3 pattern switch:

final widget = switch (state) {
  CartInitial()    => const _EmptyCart(),
  CartLoading()    => const Center(child: CircularProgressIndicator()),
  CartLoaded(:final items, :final total) => _CartList(items: items, total: total),
  CartFailure(:final message)            => _ErrorBanner(message: message),
};

If a future PR adds a CartLoaded.suspended variant, every switch in the codebase fails to compile until handled. That is the point.

Event handlers and concurrency

bloc_concurrency provides four EventTransformers. Pick the one that matches the semantic of the event, not the one that "feels safe".

class CartBloc extends HydratedBloc<CartEvent, CartState> {
  CartBloc({required CartRepository repo})
      : _repo = repo,
        super(const CartState.initial()) {
    on<CartItemAdded>(_onItemAdded, transformer: sequential());
    on<CartItemQtyChanged>(_onQtyChanged, transformer: sequential());
    on<CartCleared>(_onCleared, transformer: droppable());
  }

  final CartRepository _repo;
  // ...handlers
}

Sequential is not the default. The default concurrent() will reorder cart mutations — adding then removing the same item could leave the item in the cart if the remove handler finishes first. Always state the transformer explicitly for any handler that mutates shared state.

Wiring the View

Four widgets bridge a Bloc to the widget tree. Pick the narrowest one that does the job.

Common mistake: wrapping the entire screen in a BlocBuilder that rebuilds 30 widgets when only the cart total changed. Use BlocSelector for the total and let the rest of the tree stay stable.

Persistence (optional)

For state that must survive a cold start (cart, auth tokens, recently-viewed list), extend HydratedBloc<E, S> instead of Bloc<E, S> — or add HydratedMixin to an existing Cubit/Bloc and call hydrate() in its constructor when changing the parent class isn't an option. Override fromJson and toJson on the Bloc. With freezed, the serialization helpers are generated — but only when the state declares an explicit factory State.fromJson(...) factory and the file declares part 'state.g.dart';. See the CartState snippet above; missing either of these makes _$CartStateFromJson undefined and the file won't compile.

Always override storagePrefix to a stable string literal:

@override
String get storagePrefix => 'CartBloc';

The default storage key is runtimeType.toString(), which gets mangled by Dart's minifier in flutter build apk --release / --obfuscate. A user upgrading the app would find their persisted cart unreadable because the storage key changed names between builds. Pin the prefix in source so the key survives minification.

import 'package:bloc_concurrency/bloc_concurrency.dart';
import 'package:hydrated_bloc/hydrated_bloc.dart';

class CartBloc extends HydratedBloc<CartEvent, CartState> {
  CartBloc({required CartRepository repo})
      : _repo = repo,
        super(const CartState.initial()) {
    // Persistence and transformers coexist — the example is deliberately
    // explicit about both so readers see them together.
    on<CartHydrated>(_onHydrated, transformer: droppable());
    on<CartItemAdded>(_onItemAdded, transformer: sequential());
    on<CartItemQtyChanged>(_onQtyChanged, transformer: sequential());
    on<CartCleared>(_onCleared, transformer: droppable());
  }

  final CartRepository _repo;

  // ...handlers (omitted; each emits state.copyWith(...) per business rule)

  // Pin the storage key so release builds (which may minify class names)
  // continue reading the same persisted state across upgrades.
  @override
  String get storagePrefix => 'CartBloc';

  @override
  CartState? fromJson(Map<String, dynamic> json) => CartState.fromJson(json);

  @override
  Map<String, dynamic>? toJson(CartState state) =>
      (state as dynamic).toJson() as Map<String, dynamic>?;
}

toJson is cast through dynamic because freezed's generated toJson lives on each variant (e.g. CartLoaded.toJson()), not on the sealed parent's static analysis surface. The runtime resolution picks the right variant; the cast just gets past static type checking.

Limitations

Persisting a Bloc whose handler uses restartable() or concurrent() has two distinct ordering subtleties — don't conflate them:

1. restartable() and in-flight emits. When event B arrives during event A's handler, restartable() marks A's emitter as done. Any emit(...) A tries after that is silently dropped (good). The persisted state reflects whatever was emitted before B took over — so persistence sees the last successful emit, not necessarily the last dispatched event.
2. Process death, separately. If the app is killed while A is mid-await, A never gets the chance to emit at all — there's no "cancellation" because there's no process. The persisted state is whatever was emitted before A started.

Either way: treat persisted state as a starting point. When the screen mounts after a cold start, re-fetch from the API rather than assuming the persisted state reflects an in-flight request's eventual result.

Workflow: Scaffold a new feature

Task Progress

• Step 1 — Pick a feature name. Lowercase, singular noun (e.g., cart, auth, restaurant_list).
• Step 2 — Create the directory. mkdir -p lib/ui/features/<feature>/{bloc,view}.
• Step 3 — Write <feature>_event.dart. Sealed <Feature>Event class with one factory per user intent. No logic.
• Step 4 — Write <feature>_state.dart. Sealed <Feature>State with Initial, Loading, Loaded(...), Failure(message) at minimum. Add domain-specific variants as needed.
• Step 5 — Write <feature>_bloc.dart. Extend Bloc<E, S> (or HydratedBloc<E, S> if persisting). Inject the repository via constructor.
• Step 6 — Register handlers with explicit transformers. Use sequential() for mutations, restartable() for queries, droppable() for submits, concurrent() only for independent fetches.
• Step 7 — If persisting. Override fromJson / toJson on the Bloc using freezed-generated helpers.
• Step 8 — Generate code. dart run build_runner build --delete-conflicting-outputs (or rely on the watch -d side terminal from flutter-bloc-setup).
• Step 9 — Wire the View. Wrap the route in BlocProvider(create: (_) => <Feature>Bloc(repo: context.read<<Feature>Repository>())). Use BlocBuilder for rendering, BlocListener for side effects, BlocSelector for fine-grained rebuilds.
• Step 10 — Feedback loop. Run flutter analyze → review compile errors → if a switch over state is non-exhaustive, handle the missing variant → re-run.

Applied to Talabat-clone

The cart in this app has the most rules-heavy state in the system: one-cart / one-store / one-city, persistence across cold starts, and a "switch store" guard (PRD_catalog.md §15.5, PRD_states.md §7).

// pseudocode sketch — see the Counter example below for runnable form
// CartEvent
sealed class CartEvent
  CartHydrated()                                              // app startup, deserialize from disk
  CartItemAdded(Product product, int qty)                     // user taps "+" on a product card
  CartItemQtyChanged(String itemId, int qty)                  // bottom-sheet qty stepper
  CartItemRemoved(String itemId)                              // swipe-to-delete
  CartCleared()                                               // checkout success or explicit clear
  CartStoreSwitchRequested(Product fromOtherStore)            // user added from a different store
  CartStoreSwitchConfirmed()                                  // user confirmed "discard current cart"
  CartStoreSwitchCancelled()                                  // user backed out

// CartState
sealed class CartState
  CartInitial()
  CartLoaded(items, store, subtotal, discount, total)
  CartStoreSwitchPending(currentLoaded, pendingProduct)        // PRD_catalog.md §15.5
  CartFailure(message)

CartBloc extends HydratedBloc because the cart must survive cold start. CartItemAdded uses sequential() so two rapid taps on "+" never interleave with CartItemQtyChanged. CartStoreSwitchRequested is a pure state transition — no API call — that puts the Bloc in CartStoreSwitchPending until the user confirms or cancels. CartCleared happens at checkout success when the API confirms Cart: active → converted (PRD_states.md §7).

StoreListBloc does not extend HydratedBloc — store availability depends on operational gating (PRD_states.md §4) and must be re-fetched on launch.

Examples

A complete Counter feature illustrates the trio with no async dependencies. It uses sequential() because increment-then-decrement order matters.

lib/ui/features/counter/bloc/counter_event.dart

import 'package:freezed_annotation/freezed_annotation.dart';

part 'counter_event.freezed.dart';

@freezed
sealed class CounterEvent with _$CounterEvent {
  const factory CounterEvent.incremented() = CounterIncremented;
  const factory CounterEvent.decremented() = CounterDecremented;
  const factory CounterEvent.reset() = CounterReset;
}

lib/ui/features/counter/bloc/counter_state.dart

import 'package:freezed_annotation/freezed_annotation.dart';

part 'counter_state.freezed.dart';

// We use `sealed` uniformly across the codebase, even for one-variant states.
// `abstract` would also compile here but the family standardizes on `sealed`
// so future variants don't trigger a class-shape change.
@freezed
sealed class CounterState with _$CounterState {
  const factory CounterState({required int value}) = _CounterState;
}

lib/ui/features/counter/bloc/counter_bloc.dart

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

import 'counter_event.dart';
import 'counter_state.dart';

class CounterBloc extends Bloc<CounterEvent, CounterState> {
  CounterBloc() : super(const CounterState(value: 0)) {
    on<CounterIncremented>(
      (e, emit) => emit(CounterState(value: state.value + 1)),
      transformer: sequential(),
    );
    on<CounterDecremented>(
      (e, emit) => emit(CounterState(value: state.value - 1)),
      transformer: sequential(),
    );
    on<CounterReset>(
      (e, emit) => emit(const CounterState(value: 0)),
      transformer: droppable(),
    );
  }
}

lib/ui/features/counter/view/counter_view.dart

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

import '../bloc/counter_bloc.dart';
import '../bloc/counter_event.dart';
import '../bloc/counter_state.dart';

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

  @override
  Widget build(BuildContext context) {
    return BlocProvider(
      create: (_) => CounterBloc(),
      child: const _CounterScaffold(),
    );
  }
}

class _CounterScaffold extends StatelessWidget {
  const _CounterScaffold();

  @override
  Widget build(BuildContext context) {
    return Scaffold(
      appBar: AppBar(title: const Text('Counter')),
      body: Center(
        child: BlocSelector<CounterBloc, CounterState, int>(
          selector: (state) => state.value,
          builder: (context, value) => Text(
            '$value',
            style: Theme.of(context).textTheme.displayLarge,
          ),
        ),
      ),
      floatingActionButton: Column(
        mainAxisAlignment: MainAxisAlignment.end,
        children: [
          FloatingActionButton(
            heroTag: 'inc',
            onPressed: () =>
                context.read<CounterBloc>().add(const CounterEvent.incremented()),
            child: const Icon(Icons.add),
          ),
          const SizedBox(height: 8),
          FloatingActionButton(
            heroTag: 'dec',
            onPressed: () =>
                context.read<CounterBloc>().add(const CounterEvent.decremented()),
            child: const Icon(Icons.remove),
          ),
        ],
      ),
    );
  }
}

BlocSelector<CounterBloc, CounterState, int> rebuilds only when state.value changes, not when the Bloc emits a new CounterState instance with the same value. For a single-field state this is overkill, but the pattern matters for screens with many fields where one cell shouldn't trigger a full rebuild.

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