bloc_component 0.1.8
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A library that helps structuring flutter apps using the BLoC library

bloc_component #

A library that helps structuring flutter apps using the BLoC library.
It makes it easier to:

  • structure your app into components and manage the state of each component separately.
  • Maintain your app
  • Add new pages and features

Getting Started #

This library is built upon the flutter bloc library. It allows developers to structure their app into components, where each component has its own bloc. The idea is to divide the app to components where each component has it's own BLoC and view.

The relation between the component and the bloc is described in the following diagram:

bloc component diagram

As we see in the diagram, the BLoC is used for managing the state of the component. Each component has its own BLoC and view.
The component view sends events to the bloc, and the bloc updates the view state.

The following diagram describes the relationship between the different components in the app

components relationship diagram

Because each component has one bloc only, sharing states between components should be done through services and listeners that are provided by the services.
For more info please check the examples

Library contents #

The library provides the following components in order to help structuring and manging the state of a flutter app:

  • BaseBloc: Used for managing the state of an app component
  • Component: Describes an app component. Can be a page or part of page. Each component has its own bloc. The Component describes how to create the bloc and the component's view
  • ComponentView: The component's view. Has direct access to the bloc created by the Component and uses StateBuilder in order to build widgets according to the bloc's states

BaseBloc #

The BaseBloc class is the class that each bloc should inherit from. It inherits from the Bloc class of the bloc library. The BaseBloc class adds additional functionality like initialization and error handling.

Example of MovieItemBloc that inherits from BaseBloc:
For the full example, please check the Movies App example in the library repository

// First we need to create the base bloc state which should extend from BlocState
class MovieItemState extends BlocState {}

// Then we create other states that are specific to this bloc
// The MovieItemStateFavoriteUpdated, is yielded when the movie is added or removed from the
// favorite list
class MovieItemStateFavoriteUpdated extends MovieItemState {
  final bool isFavorite;


  List<Object> get props => [isFavorite];

// we need to define a base bloc event that inherits from BlocEvent
class MovieItemEvent extends BlocEvent {}

// Then we need to create events that are specific to this bloc.
// MovieItemEventToggleFavorite is used for adding/removing movie from the favorite list
class MovieItemEventToggleFavorite extends MovieItemEvent {}

// Here we define the bloc. It extends BaseBloc which receives MovieItemEvent and yields
// MovieItemState.
// This bloc is used for managing the state of a single movie item in the movies list
class MovieItemBloc extends BaseBloc {
  final MoviesServices _moviesServices;
  final Movie movie;

  // This bloc depends on movies services and a specific movie

  // We need to override this eventToState() method in order to handle events and change the state
  // of the bloc
  Stream<MovieItemState> eventToState(MovieItemEvent event) async* {
    if (event is MovieItemEventToggleFavorite) {
      yield* _toggleFavorite();

  // We need to override the onInitialized() method in order to yield some state or add some
  // event upon bloc initialization
  Stream<MovieItemState> onInitialized() async* {
    yield* _notifyFavoriteUpdated();

  // We need to override the initialize() method if there is a need to initialize anything when
  // the bloc is created
  Stream<StateInitializing> initialize() async* {
    // here we wait until the movies service is initialized
    await _moviesServices.initialized;

  // This method handles the toggle favorite event. If the movie is in favorite list, then it is
  // removed, otherwise it's added to the favorite list
  Stream<MovieItemState> _toggleFavorite() async* {
    if (_moviesServices.isFavorite(movie)) {
      await _moviesServices.removeFavoriteMovie(movie);
    } else {
      await _moviesServices.addFavoriteMovie(movie);
    // in the end we notify listeners by yielding the new state
    yield* _notifyFavoriteUpdated();

  // a shortcut method for changing the state to MovieItemStateFavoriteUpdated in order to notify
  // listeners that the favorite state of the movie has changed
  Stream<MovieItemStateFavoriteUpdated> _notifyFavoriteUpdated() async* {
    yield MovieItemStateFavoriteUpdated(_moviesServices.isFavorite(movie));


Component #

The Component class is a StatelessWidget that is attached to one bloc. The component just describes how to create the bloc and how to create the view of this component.
Another utility that is provided by the Component class, is the stateListener which allows listening to state changes from the bloc that is attached to the component. The stateListener is simply a BlocListener from the BLoC library.

An Example of a Component that uses the MovieItemBloc from the example above:
For the full example, please check the Movies App example in the library repository

/// A MovieItem component that is used for showing a single component.
/// The bloc that is used by this component is the MovieItemBloc
class MovieItem extends Component<MovieItemBloc> {
  final Movie movie;


  MovieItemBloc createBloc(BuildContext context) {
    // in this method we create the MovieItemBloc
    return MovieItemBloc(MoviesServices.getInstance(), movie);

  ComponentView<MovieItemBloc> createView(MovieItemBloc bloc) {
    // in this method we create the component view and pass it the created bloc
    return MovieItemView(bloc);
  // override the stateListener method if you need to listen to states from the created bloc
  // (MovieItemBloc in our case).
  // Note that this is a listener and not a builder. It's useful for logging, navigation,
  // showing snack bars...
  void stateListener(BuildContext context, BlocState state) {
    if (state is StateError) {
      Scaffold.of(context).showSnackBar(SnackBar(content: Text(state.message)));


ComponentView #

The ComponentView is used for building the view of the component. It has direct access to the bloc created by the component, and it has some utilities for managing the state easily using the stateBuilder
You can also handle bloc initialization, and errors in the ComponentView. Check the Movies App example in the repository for more info

An Example of a ComponentView for the MovieItem component from the example above:
For the full example, please check the Movies App example in the library repository

/// The view of the MovieItem component
class MovieItemView extends ComponentView<MovieItemBloc> {
  MovieItemView(MovieItemBloc bloc) : super(bloc);

  // This method builds the content of the component's view
  Widget buildView(BuildContext context) {
    final Movie movie =;
    return ListTile(
      title: Text(movie.title),
      subtitle: Text(movie.year.toString()),
      trailing: _buildFavoriteIcon(),

  Widget _buildFavoriteIcon() {
    // The only state that is managed by the BlocItemBloc is the favorite state
    // Here we use a stateBuilder for rebuilding the favorite icon when the state is changed to
    // MovieIemStateFavoriteUpdated
    return stateBuilder<MovieItemStateFavoriteUpdated>(
        builder: (context, state) {
          return GestureDetector(
            child: Icon(state.isFavorite ? : Icons.star_border),
            // When tapping the favorite icon, a MovieItemEventToggleFavorite event is dispatched
            onTap: () => bloc.event(MovieItemEventToggleFavorite()),

The ComponentView class also allows to handle bloc initialization and errors.
Here is an example for handling initialization and error while initializing the MoviesView

/// Creates the view for the MoviesComponent
class MoviesView extends ComponentView<MoviesBloc> {

  MoviesView(MoviesBloc bloc) : super(bloc);

  // override onInitializing() method in order to show something while the component's bloc is
  // being initialized
  Widget onInitializing(BuildContext context, StateInitializing loadingData) {
    return Scaffold(
      body: Padding(
        padding: const EdgeInsets.all(15.0),
        child: Column(
          children: <Widget>[
            Text('Loading movies...'),
              value: loadingData.progress,

  Widget onInitializingError(BuildContext context, StateInitializationError error) {
    return Text("An error occurred while initializing movies: ${error.message}");

  Widget buildView(BuildContext context) {

More on stateBuilder #

stateBuilder is utility method provided by the ComponentView which helps in building and rebuilding widgets according to state changes. It wraps the StateBuilder class which wraps the BlocBuilder class of the bloc library

The ComponentView class provides a stateBuilder utility:


A state builder that calls the builder only on a specific state

  /// Used for building a widget on specific state S
  /// In most cases, the only parameter that you need to pass to the stateBuilder is the builder
  /// for building the widget when the bloc state is S
  Widget stateBuilder<S extends BlocState>({
    /// The builder to be called when the state is S
    @required BlocWidgetBuilder<S> builder,


StateBuilder config #

You can set default builders for onLoading, onError and onOther.
To do this, you need to extend StateBuilderConfig and pass an instance of it to StateBuilder.builderConfig.


class AppStateBuilderConfig extends StateBuilderConfig {
  Widget onError(BuildContext context, StateError error) {
    return Text(error.message);
  Widget onLoading(BuildContext context, StateLoading loading) {
    return Center(child: CircularProgressIndicator(),);
  Widget onOther(BuildContext context, BlocState state) {
    return Center(child: CircularProgressIndicator(),);

And in your main method simply override StateBuilder.builderConfig:

void main() {
  StateBuilder.builderConfig = AppStateBuilderConfig();

Examples #

Movies App demo

For more info please refer to the docs and API reference

pub points

A library that helps structuring flutter apps using the BLoC library

Repository (GitHub)
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API reference




bloc, equatable, flutter, flutter_bloc, provider


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