bloc 0.2.1

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A dart package that helps implement the Bloc pattern.

This package is built to work with RxDart.dart 0.18.1+.

Bloc #

The goal of this package is to make it easy to implement the Bloc Design Pattern (Business Logic Component).

This design pattern helps to separate presentation from business logic. Following the Bloc pattern facilitates testability and reusability. This package abstracts reactive aspects of the pattern allowing developers to focus on converting events into states.

Glossary #

Events are the input to a Bloc. They are commonly UI events such as button presses. Events are dispatched and then converted to States.

States are the output of a Bloc. Presentation components can listen to the stream of states and redraw portions of themselves based on the given state (see BlocBuilder for more details).

Bloc Interface #

mapEventToState is a method that must be implemented when a class extends Bloc. The function takes a single argument, event. mapEventToState is called whenever an event is dispatched by the presentation layer. mapEventToState must convert that event into a state and return the state in the form of a Stream so that it can be consumed by the presentation layer.

dispatch is a method that takes an event and triggers mapEventToState. dispatch maybe be called from the presentation layer or from within the Bloc (see examples) and notifies the Bloc of a new event.

initialState is the state before any events have been processed (before mapEventToState has ever been called). initialState is an optional getter. If unimplemented, initialState will be null.

transform is a method that can be overridden to transform the Stream<Event> before mapEventToState is called. This allows for operations like distinct() and debounce() to be used.

Bloc Widgets #

BlocBuilder is a Flutter widget which requires a Bloc and a builder function. BlocBuilder handles building the widget in response to new states. BlocBuilder is very similar to StreamBuilder but has a more simple API to reduce the amount of boilerplate code needed.

Usage #

For simplicity we can create a Bloc that always returns a stream of static strings in response to any event. That would look something like:

class SimpleBloc extends Bloc<dynamic, String> {
  @override
  Stream<String> mapEventToState(event) async* {
    yield 'data';
  }
}

That isn't a very realistic use-case so let's take something more practical like a login flow.

We're going to need to define what our different LoginStates are going to be. For simplicity, let's say we only have 4 states:

• initial
• loading
• error
• success

class LoginState {
  final bool isLoading;
  final bool isLoginButtonEnabled;
  final String error;
  final String token;

  const LoginState({
    @required this.isLoading,
    @required this.isLoginButtonEnabled,
    @required this.error,
    @required this.token,
  });

  factory LoginState.initial() {
    return LoginState({
      isLoading: false,
      isLoginButtonEnabled: true,
      error: '',
      token: '',
    });
  }

  factory LoginState.loading() {
    return LoginState({
      isLoading: true,
      isLoginButtonEnabled: false,
      error: '',
      token: '',
    });
  }

  factory LoginState.error(String error) {
    return LoginState({
      isLoading: false,
      isLoginButtonEnabled: true,
      error: error,
      token: '',
    });
  }

  factory LoginState.success(String token) {
    return LoginState({
      isLoading: false,
      isLoginButtonEnabled: true,
      error: '',
      token: token,
    });
  }
}

Next we need to define the different events that our Bloc will respond to. Again, for simplicity, let's say there is just a single event we will handle: LoginButtonPressed.

abstract class LoginEvent {}

class LoginButtonPressed extends LoginEvent {
  final String username;
  final String password;

  const LoginButtonPressed({@required this.username, @required this.password});
}

Now that we've identified our states and events, our LoginBloc should look something like:

class LoginBloc extends Bloc<LoginEvent, LoginState> {
  LoginState get initialState => LoginState.initial();

  void onLoginButtonPressed({String username, String password}) {
    dispatch(
      LoginButtonPressed(
        username: username,
        password: password,
      ),
    );
  }

  @override
  Stream<LoginState> mapEventToState(LoginEvent event) async* {
    if (event is LoginButtonPressed) {
      yield LoginState.loading();

      try {
        final token = await _authenticate(event.username, event.password);
        yield LoginState.success(token);
      } catch (error) {
        yield LoginState.error(error.toString());
      }
    }
  }
}

Now that we have the LoginBloc lets take a look at how to use BlocBuilder to hook up our LoginForm widget to our LoginBloc.

  class LoginForm extends StatelessWidget {
    final LoginBloc loginBloc;
    final usernameController = TextEditingController();
    final passwordController = TextEditingController();

    const LoginForm({Key key, @required this.loginBloc}): super(key: key);

    @override
    Widget build(BuildContext context) {
      return BlocBuilder<LoginState>(
        bloc: loginBloc,
        builder: (
          BuildContext context,
          LoginState loginState,
        ) {
          if (loginState.token.isNotEmpty) {
            // user is authenticated do something...
          }

          return Form(
            child: Column(
              children: [
                Text(
                  loginState.error,
                ),
                TextFormField(
                  controller: usernameController,
                ),
                TextFormField(
                  controller: passwordController,
                  obscureText: true,
                ),
                RaisedButton(
                  onPressed: loginState.isLoginButtonEnabled
                      ? _onLoginButtonPressed
                      : null,
                  child: Text('Login'),
                ),
                Container(
                  child: loginState.isLoading
                      ? CircularProgressIndicator()
                      : null,
                ),
              ],
            ),
          );
        },
      );
    }

    _onLoginButtonPressed() {
      loginBloc.onLoginButtonPressed(
        username: usernameController.text,
        password: passwordController.text,
      );
    }
  }

At this point we have sucessfully separated our presentational layer from our business logic layer. Notice that the LoginForm widget knows nothing about what happens when a user taps the button. The form simply tells the LoginBloc that the user has pressed the button via dispatch. From that point, the LoginBloc tells the LoginForm to be in the loading state and proceeds to authenticate the user. If the user is successfully authenticated, the LoginBloc tells the LoginForm to be in the LoginSuccess state. If authentication failed, the LoginBloc tells the LoginForm to be in the LoginError state.

Dart Versions #

• Dart 2: >= 2.0.0

Examples #

• Simple Theme Example - an example of how to create a ThemeBloc to manage dynamically changing the theme of your flutter app.

Contributors #

• Felix Angelov