bloc-pattern

name	bloc-pattern
description	Implements BLoC state management using AppBlocBase, an abstract State hierarchy, and a freezed _StateData
license	MIT
compatibility	all
metadata	{"audience":"flutter-developers","framework":"flutter","pattern":"bloc"}

BLoC Pattern Skill

When to use

• Adding state management to a new module.
• Modifying events/states for an existing bloc.

Conventions enforced

This template uses a specific bloc shape that is not the typical "freezed sealed union" pattern. The state hierarchy is hand-written abstract class siblings sharing a freezed _StateData; events are hand-written subclasses of an abstract event. Follow this shape exactly — state.copyWith<T>() and the _factories map depend on it.

1. Bloc extends AppBlocBase<E, S> (defined in apps/main/lib/presentation/base/bloc_base.dart, ultimately CoreBlocBase from core/).
2. Bloc is annotated @Injectable(). Use @factoryParam for runtime args.
3. Event classes inherit from a single abstract <X>Event — no freezed union.
4. State classes inherit from a single abstract <X>State and share a freezed _StateData. The base provides copyWith<T extends <X>State>({_StateData? data}) backed by a _factories map.
5. _StateData is @freezed sealed class _StateData with _$StateData — generator declares it that way; do not change.
6. Imports: package:core/core.dart (re-exports flutter_bloc, AppBlocBase, helpers), package:freezed_annotation/freezed_annotation.dart, package:injectable/injectable.dart.
7. Run make gen_all after editing — <feature>_bloc.freezed.dart is generated.

Reference: bloc file (<feature>_bloc.dart)

import 'dart:async';

import 'package:core/core.dart';
import 'package:freezed_annotation/freezed_annotation.dart';
import 'package:injectable/injectable.dart';

import '../../../../domain/usecases/<feature>/<feature>_usecase.dart';
import '../../../base/base.dart';

part '<feature>_bloc.freezed.dart';
part '<feature>_event.dart';
part '<feature>_state.dart';

@Injectable()
class FeatureBloc extends AppBlocBase<FeatureEvent, FeatureState> {
  final FeatureUsecase _usecase;

  FeatureBloc(
    @factoryParam Item? initial,
    this._usecase,
  ) : super(FeatureInitial(data: _StateData(initial: initial))) {
    on<GetFeatureEvent>(_onGet);
  }

  Future<void> _onGet(GetFeatureEvent event, Emitter<FeatureState> emit) async {
    final detail = await _usecase.getById(event.id);
    emit(state.copyWith<FeatureInitial>(
      data: state.data.copyWith(detail: detail),
    ));
  }
}

Reference: state file (<feature>_state.dart)

// ignore_for_file: unused_element
part of '<feature>_bloc.dart';

@freezed
sealed class _StateData with _$StateData {
  const factory _StateData({
    Item? detail,
    @Default([]) List<Item> items,
    @Default(false) bool canLoadMore,
  }) = __StateData;
}

abstract class FeatureState {
  final _StateData data;

  FeatureState(this.data);

  T copyWith<T extends FeatureState>({_StateData? data}) {
    return _factories[T == FeatureState ? runtimeType : T]!(
      data ?? this.data,
    );
  }

  // Forward common reads off _StateData.
  Item? get detail => data.detail;
  List<Item> get items => data.items;
  bool get canLoadMore => data.canLoadMore;
}

class FeatureInitial extends FeatureState {
  FeatureInitial({required _StateData data}) : super(data);
}

class FeatureLoaded extends FeatureState {
  FeatureLoaded({required _StateData data}) : super(data);
}

final _factories = <Type, Function(_StateData data)>{
  FeatureInitial: (data) => FeatureInitial(data: data),
  FeatureLoaded: (data) => FeatureLoaded(data: data),
};

Add a new state class? Add it to _factories in the same edit — copyWith<T>() will throw at runtime otherwise.

Reference: event file (<feature>_event.dart)

part of '<feature>_bloc.dart';

abstract class FeatureEvent {}

class GetFeatureEvent extends FeatureEvent {
  final String id;
  GetFeatureEvent(this.id);
}

class LoadMoreEvent extends FeatureEvent {}

For events that need to surface a result back to the caller (e.g. login flows), pass a Completer<T> on the event and complete it inside the handler. Choose T to carry the useful result object when the caller needs refreshed state; avoid returning only bool and forcing an immediate duplicate query.

Loading + error handling

StateBase<T> (the screen base) wires delegate?.addLoadingHandler and addErrorHandler against the bloc's CoreDelegate mixin during initState. So a bloc generally does not call showLoading/hideLoading itself — the screen does, and CoreBlocBase.onError already routes thrown errors through the screen's error UI.

When you do need explicit control inside a handler:

Future<void> _onSubmit(SubmitEvent event, Emitter<FeatureState> emit) async {
  showLoading();   // CoreDelegate fan-out — wakes the screen's EasyLoading
  try {
    await _usecase.submit(event.payload);
    emit(state.copyWith<FeatureSuccess>());
  } finally {
    hideLoading();
  }
}

showLoading() / hideLoading() here are the CoreDelegate versions inherited via CoreBlocBase. They notify every registered screen (usually one) which then calls its own showLoading() / hideLoading() on EasyLoading. There's a deliberate name collision with the screen's methods — see error-handling for the full picture.

Throw or rethrow on errors — CoreBlocBase.onError converts them to ErrorData and forwards to the screen.

Using state in the UI

BlocBuilder works against the runtime type:

BlocBuilder<FeatureBloc, FeatureState>(
  builder: (context, state) {
    if (state is FeatureLoaded) return _buildList(state.items);
    return _buildEmpty();
  },
)

Action files (see extension-action) typically use _blocListener(BuildContext context, FeatureState state) for side effects and dispatch events via bloc.add(...).

Checklist

• Bloc extends AppBlocBase<E, S> and is @Injectable().
• <feature>_bloc.dart declares the three part directives (freezed, event, state).
• _StateData is @freezed sealed class and uses @Default(...) for non-nullable defaults.
• Every concrete state subclass has a matching entry in _factories.
• Events extend the abstract <X>Event; no freezed unions for events or states.
• Long-running handlers call showLoading() and pair it with hideLoading() in a finally block (CoreDelegate fan-out).
• make gen_all run after changes.

Common mistakes

• Treating FeatureState as a freezed union (FeatureState.loading()/.loaded()) — this template does not.
• Forgetting to register a new concrete state in _factories (causes Null check operator used on a null value from copyWith<T>).
• Importing package:flutter_bloc/flutter_bloc.dart directly — go through package:core/core.dart.
• Doing both emit(...error) and rethrow — CoreBlocBase.onError already handles thrown errors; emit OR throw, don't double-fire.

Related

• module-scaffold
• extension-action
• error-handling
• data-layer