A dart package that helps implement the BLoC pattern.

Learn more at bloclibrary.dev!

This package is built to work with:

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 writing the business logic.

A Cubit is the base for Bloc (in other words Bloc extends Cubit ). Cubit is a special type of Stream which can be extended to manage any type of state. Cubit requires an initial state which will be the state before emit has been called. The current state of a cubit can be accessed via the state getter and the state of the cubit can be updated by calling emit with a new state .

State changes in cubit begin with predefined function calls which can use the emit method to output new states. onChange is called on each state change and contains the current and next state.

Creating a Cubit

/// A `CounterCubit` which manages an `int` as its state. class CounterCubit extends Cubit<int> { /// The initial state of the `CounterCubit` is 0. CounterCubit() : super(0); /// When increment is called, the current state /// of the cubit is accessed via `state` and /// a new `state` is emitted via `emit`. void increment() => emit(state + 1); }

Using a Cubit

void main() { /// Create a `CounterCubit` instance. final cubit = CounterCubit(); /// Access the state of the `cubit` via `state`. print(cubit.state); // 0 /// Interact with the `cubit` to trigger `state` changes. cubit.increment(); /// Access the new `state`. print(cubit.state); // 1 /// Close the `cubit` when it is no longer needed. cubit.close(); }

Observing a Cubit

onChange can be overridden to observe state changes for a single cubit .

onError can be overridden to observe errors for a single cubit .

class CounterCubit extends Cubit<int> { CounterCubit() : super(0); void increment() => emit(state + 1); @override void onChange(Change<int> change) { print(change); super.onChange(change); } @override void onError(Object error, StackTrace stackTrace) { print('$error, $stackTrace'); super.onError(error, stackTrace); } }

BlocObserver can be used to observe all cubits .

class MyBlocObserver extends BlocObserver { @override void onChange(Cubit cubit, Change change) { print(change); super.onChange(cubit, change); } @override void onError(Cubit cubit, Object error, StackTrace stackTrace) { print('$error, $stackTrace'); super.onError(cubit, error, stackTrace); } }

void main() { Bloc.observer = MyBlocObserver(); // Use cubits... }

A Bloc is a more advanced type of Cubit which relies on events to trigger state changes rather than functions. Bloc extends Cubit which means it has the same public API as Cubit . However, rather than calling a function on a Bloc and directly emitting a new state , Blocs receive events and convert the incoming events into outgoing states .

State changes in bloc begin when events are added which triggers onEvent . The events are then funnelled through transformEvents . By default, transformEvents uses asyncExpand to ensure each event is processed in the order it was added but it can be overridden to manipulate the incoming event stream. mapEventToState is then invoked with the transformed events and is responsible for yielding states in response to the incoming events. transitions are then funnelled through transformTransitions which can be overridden to manipulation the outgoing state changes. Lastly, onTransition is called just before the state is updated and contains the current state, event, and next state.

Creating a Bloc

/// The events which `CounterBloc` will react to. enum CounterEvent { increment } /// A `CounterBloc` which handles converting `CounterEvent`s into `int`s. class CounterBloc extends Bloc<CounterEvent, int> { /// The initial state of the `CounterBloc` is 0. CounterBloc() : super(0); @override Stream<int> mapEventToState(CounterEvent event) async* { switch (event) { /// When a `CounterEvent.increment` event is added, /// the current `state` of the bloc is accessed via the `state` property /// and a new state is emitted via `yield`. case CounterEvent.increment: yield state + 1; break; } } }

Using a Bloc

void main() async { /// Create a `CounterBloc` instance. final bloc = CounterBloc(); /// Access the state of the `bloc` via `state`. print(bloc.state); // 0 /// Interact with the `bloc` to trigger `state` changes. bloc.add(CounterEvent.increment); /// Wait for next iteration of the event-loop /// to ensure event has been processed. await Future.delayed(Duration.zero); /// Access the new `state`. print(bloc.state); // 1 /// Close the `bloc` when it is no longer needed. bloc.close(); }

Observing a Bloc

Since all Blocs are Cubits , onChange and onError can be overridden in a Bloc as well.

In addition, Blocs can also override onEvent and onTransition .

onEvent is called any time a new event is added to the Bloc .

onTransition is similar to onChange , however, it contains the event which triggered the state change in addition to the currentState and nextState .

enum CounterEvent { increment } class CounterBloc extends Bloc<CounterEvent, int> { CounterBloc() : super(0); @override Stream<int> mapEventToState(CounterEvent event) async* { switch (event) { case CounterEvent.increment: yield state + 1; break; } } @override void onEvent(CounterEvent event) { print(event); super.onEvent(event); } @override void onChange(Change<int> change) { print(change); super.onChange(change); } @override void onTransition(Transition<CounterEvent, int> transition) { print(transition); super.onTransition(transition); } @override void onError(Object error, StackTrace stackTrace) { print('$error, $stackTrace'); super.onError(error, stackTrace); } }

BlocObserver can be used to observe all blocs as well.

class MyBlocObserver extends BlocObserver { @override void onEvent(Bloc bloc, Object event) { print(event); super.onEvent(bloc, event); } @override void onChange(Cubit cubit, Change change) { print(change); super.onChange(cubit, change); } @override void onTransition(Bloc bloc, Transition transition) { print(transition); super.onTransition(bloc, transition); } @override void onError(Cubit cubit, Object error, StackTrace stackTrace) { print('$error, $stackTrace'); super.onError(cubit, error, stackTrace); } }

void main() { Bloc.observer = MyBlocObserver(); // Use blocs... }

Dart Versions #

Dart 2: >= 2.6.0