Bloc Package

Build Status codecov Pub Awesome Flutter License: MIT Gitter

A dart package that helps implement the BLoC pattern.

This package is built to work with flutter_bloc and angular_bloc.


Bloc Architecture

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.


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).

Transitions occur when an Event is dispatched after mapEventToState has been called but before the Bloc's state has been updated. A Transition consists of the currentState, the event which was dispatched, and the nextState.

BlocSupervisor oversees Blocs and delegates to BlocDelegate.

BlocDelegate handles events from all Blocs which are delegated by the BlocSupervisor. Can be used to observe all Bloc Transitions. It is a great way to handle logging/analytics universally.

Bloc Interface

initialState is the state before any events have been processed (before mapEventToState has ever been called). initialState must be implemented.

mapEventToState is a method that must be implemented when a class extends Bloc. The function takes two arguments: state and event. mapEventToState is called whenever an event is dispatched by the presentation layer. mapEventToState must convert that event, along with the current state, into a new state and return the new state in the form of a Stream which is consumed by the presentation layer.

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

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.

onTransition is a method that can be overridden to handle whenever a Transition occurs. A Transition occurs when a new Event is dispatched and mapEventToState is called. onTransition is called before a Bloc's state has been updated. It is a great place to add bloc-specific logging/analytics.

BlocDelegate Interface

onTransition is a method that can be implemented to handle whenever a Transition occurs from any Bloc. It is a great place to add universal logging/analytics.


For simplicity we can create a CounterBloc like:

class CounterBloc extends Bloc<CounterEvent, int> {
  int get initialState => 0;

  Stream<int> mapEventToState(int currentState, CounterEvent event) async* {
    switch (event) {
      case CounterEvent.decrement:
        yield currentState - 1;
      case CounterEvent.increment:
        yield currentState + 1;

Our CounterBloc converts CounterEvents to integers.

As a result, we need to define our CounterEvent like:

enum CounterEvent { increment, decrement }

Then we can dispatch events to our bloc like so:

void main() {
  final counterBloc = CounterBloc();



As our app grows and relies on multiple Blocs, it becomes useful to see the Transitions for all Blocs. This can easily be achieved by implementing a BlocDelegate.

class SimpleBlocDelegate implements BlocDelegate {
  void onTransition(Transition transition) {

Now that we have our SimpleBlocDelegate, we just need to tell the BlocSupervisor to use our delegate in our main.dart.

void main() {
  BlocSupervisor().delegate = SimpleBlocDelegate();

  final counterBloc = CounterBloc();

  counterBloc.dispatch(CounterEvent.increment); // { currentState: 0, event: CounterEvent.increment, nextState: 1 }
  counterBloc.dispatch(CounterEvent.increment); // { currentState: 1, event: CounterEvent.increment, nextState: 2 }
  counterBloc.dispatch(CounterEvent.increment); // { currentState: 2, event: CounterEvent.increment, nextState: 3 }

  counterBloc.dispatch(CounterEvent.decrement); // { currentState: 3, event: CounterEvent.decrement, nextState: 2 }
  counterBloc.dispatch(CounterEvent.decrement); // { currentState: 2, event: CounterEvent.decrement, nextState: 1 }
  counterBloc.dispatch(CounterEvent.decrement); // { currentState: 1, event: CounterEvent.decrement, nextState: 0 }

At this point, all Bloc Transitions will be reported to the SimpleBlocDelegate and we can see them in the console after running our app.

Dart Versions

  • Dart 2: >= 2.0.0