Bloc

License: MIT Pub

Overview

The goal of this package is to make it easy to work with BLoC and Isolate.

The main difference from another BLoC pattern implementations is what blocs work in Isolate and don't slow down UI.

This package works on all flutter platforms.

You can read about BLoC pattern here.

Index

Attention

I recommend you to read about Isolates to get to know their weaknesses and strengths.

In brief, isolates share memory, so immutable objects are not copied when transferred to another isolate. You can now use them without being afraid but you shouldn't forget that there are still some limitations and overhead costs.

Bloc and Cubit

Data flow scheme

In Bloc, events are processed strictly in turn. It gets an event and responds to it with a stream of states in mapEventToState. Until the stream ends, the processing of a new event will not begin.

In Cubit, events are received in onEventReceived and processed asynchronously, and the state is returned by the emit function.

Creating

IsolateCubit

/// Cubit for counter with `CounterEvent` and `int` state.
class CounterCubit extends IsolateCubit<CountEvent, int> {
  /// The initial state of the `CounterCubit` is 0.
  CounterCubit() : super(0);

  /// When `CountEvent` is received, the current state
  /// of the bloc is accessed via `state` and
  /// a new `state` is emitted via `emit`.
  @override
  void onEventReceived(CountEvent event) {
    emit(event == CountEvent.increment ? state+1 : state-1);
  }
}

IsolateBloc

class CounterBloc extends IsolateBloc<CountEvent, int> {
  /// The initial state of the `CounterBloc` is 0.
  CounterBloc() : super(0);

  /// When `CountEvent` is received, the current state
  /// of the bloc is accessed via `state` and
  /// and a new state is emitted via `yield`.
  Stream<int> mapEventToState(CountEvent event) async* {
    yield event == CountEvent.increment ? state+1 : state-1;
  }
}

Registering a Bloc or Cubit

To be able to create Bloc you need to register it. You can do with the register function.

void main() async {
  await initialize(isolatedFunc);
  ...
}

/// Global function which is used to register blocs or cubits and called in Isolate
void isolatedFunc() {
  /// Register a bloc or cubit to be able to create it in main Isolate
  register<CounterBloc, int>(create: () => CounterBloc());
}

register function will create one instance of all registered blocs to get their initial states. To prevent this you may provide initial state to the register function.

register<CounterBloc, int>(
  create: () => CounterBloc(), 
  initialState: 0,
)

Using Bloc or Cubit in UI

YourWidget(
  /// Create CounterBloc and provide it down to the widget tree
  child: IsolateBlocProvider<CounterBloc, int>(
    child: CounterScreen(),
  ),
)
...
class CounterScreen extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return Scaffold(
      appBar: AppBar(
        title: Text('Counter'),
      ),
      body: Center(
        /// Listen for CounterBloc State
        child: IsolateBlocListener<CounterBloc, int>(
          listener: (context, state) => print("New bloc state: $state"),
          /// Build widget based on CounterBloc's State
          child: IsolateBlocBuilder<CounterBloc, int>(
            builder: (context, state) {
              return Text('You tapped $state times');
            },
          ),
        ),
      ),
      floatingActionButton: Column(
        mainAxisSize: MainAxisSize.min,
        children: [
          FloatingActionButton(
            heroTag: 'Increment',
            /// Get bloc using extension and add new event
            onPressed: () => context.isolateBloc<CounterBloc, int>().add(CountEvent.increment),
            child: Icon(Icons.add),
          ),
          SizedBox(height: 10),
          FloatingActionButton(
            heroTag: 'Decrement',
            /// Get bloc using provider class and add new event
            onPressed: () => IsolateBlocProvider.of<CounterBloc, int>(context).add(CountEvent.decrement),
            child: Icon(Icons.remove),
          ),
        ],
      ),
    );
  }
}

All Api

IsolateBlocWrapper

IsolateBlocWrapper works like a client for IsolateBloc. It receives IsolateBloc's states and sends events added by wrapperInstance.add(YourEvent()). So you can listen for origin bloc's state with wrapperInstance.listen((state) { }) and add events as shown above. createBloc<BlocA, BlocAState>() function creates IsolateBloc in Isolate and returns IsolateBlocWrapper.

// Create counter bloc and receive it's wrapper
IsolateBlocWrapper wrapper = createBloc<CounterBloc, int>();

 // Wrapper's initial state is the same as CounterBloc's initial state 
assert(wrapper.state == 0);

 // This event will be sent to the CounterBloc
wrapper.add(CounterEvent.increment);

wrapper.listen((state) => print('CounterBloc state: $state'));

Initialization

To create Isolate and register Blocs you need to call initialize and provide initialization (isolated) function. This function will be executed in Isolate and it MUST be a GLOBAL or STATIC.

void main() async {
  /// Initialize
  await initialize(isolatedFunc);
  ...
}

/// Global function is used to register blocs and called in Isolate
void isolatedFunc() {
  /// Register a bloc to be able to create it in main Isolate
  register(create: () => CounterBloc());
}

Create new Bloc instance

To create a new instance of bloc you can use Widget or function.

/// Create with Widget
IsolateBlocProvider<BlocA, BlocAState>(
    child: ChildA(),
)

/// Create multiple blocs with Widget
MultiIsolateBlocProvider(
  providers: [
    IsolateBlocProvider<BlocA, BlocAState>(),
    IsolateBlocProvider<BlocB, BlocBState>(),
    IsolateBlocProvider<BlocC, BlocCState>(),
  ],
  child: ChildA(),
)

/// Create with function
final blocA = createBloc<BlocA, BlocAState>();

Consume Bloc

IsolateBlocBuilder<CounterBloc, int>(
  buildWhen: (state, newState) {
    /// return true/false to determine whether or not
    /// to rebuild the widget with state
  builder: (context, state) {
    /// return widget here based on CounterBloc's state
  },
)

IsolateBlocListener<CounterBloc, int>(
  listenWhen: (state, newState) {
    /// return true/false to determine whether or not
    /// to listen for state
  },
  listener: (context, state) {
    /// listen for state
  },
  child: ChildWidget(),
)

IsolateBlocConsumer<CounterHistoryBloc, List<int>>(
  listenWhen: (state, newState) {
    /// return true/false to determine whether or not
    /// to listen for state
  },
  listener: (context, state) {
    /// listen for state
  },
  buildWhen: (state, newState) {
    /// return true/false to determine whether or not
    /// to rebuild the widget with state
  },
  builder: (context, state) {
    /// return widget here based on BlocA's state
  },
)

Blocs Observer

To observe single bloc or cubit you can override onError, onEvent, onChange and onTransition methods.

class CounterBloc extends IsolateBloc<CountEvent, int> {
  CounterBloc() : super(0);

  @override
  Stream<int> mapEventToState(CounterEvent event) {...}
  
  @override
  void onError(Object error, StackTrace stackTrace) {...}
  
  @override
  void onEvent(CounterEvent event) {...}
 
  @override
  void onTransition(Transition<CounterEvent, int> transition) {...}
 
}

Or you can use IsolateBlocObserver to observe all blocs or cubits.

void isolatedFunc() {
  IsolateBloc.observer = SimpleBlocObserver();
  register(create: () => CounterBloc());
}

class SimpleBlocObserver extends IsolateBlocObserver {
  void onCreate(IsolateBlocBase bloc) {
    super.onCreate(bloc);
    print('New instance of ${bloc.runtimeType} created');
  }

  void onEvent(IsolateBlocBase bloc, Object? event) {
    super.onEvent(bloc, event);
    print('${event.runtimeType} is added to ${bloc.runtimeType}');
  }

  void onChange(IsolateBlocBase bloc, Change change) {
    super.onChange(bloc, change);
    print('State is emitted in ${bloc.runtimeType}. New state is ${change.nextState}');
  }

  void onTransition(IsolateBloc bloc, Transition transition) {
    super.onTransition(bloc, transition);
    print("${bloc.runtimeType}'s state updated. "
          'New state is ${transition.nextState}, '
          'event is ${transition.event}');
  }

  void onError(IsolateBlocBase bloc, Object error, StackTrace stackTrace) {
    super.onError(bloc, error, stackTrace);
    print('Error thrown in ${bloc.runtimeType}. Error is $error');
  }

  void onClose(IsolateBlocBase bloc) {
    super.onClose(bloc);
    print('${bloc.runtimeType} is closed');
  }
}

Use Bloc in another Bloc

You can use Bloc in another Bloc. To do this you need to use getBloc<BlocA, BlocAState>() function which returns IsolateBlocWrapper<BlocAState> .

This function works this way:

  • waits for user's Initializer function
  • looks for created bloc with BlocA type
    • if it finds any, so returns this bloc's IsolateBlocWrapper
    • otherwise it creates a new bloc and adds to the pull of free blocs. So when UI will call create(), it won't create a new bloc but return free bloc from pull.
void isolatedFunc() {
  register(create: () => CounterBloc());
  register(create: () => CounterHistoryBloc(getBloc<CounterBloc, int>()));
}

class CounterBloc extends IsolateBloc<CountEvent, int> {
  CounterBloc() : super(0);

  @override
  void onEventReceived(CountEvent event) {
    emit(event == CountEvent.increment ? state + 1 : state - 1);
  }
}

class CounterHistoryBloc extends IsolateBloc<int, List<int>> {
  final IsolateBlocWrapper<int> counterBloc;
  final _history = <int>[];

  CounterHistoryBloc(this.counterBloc) : super([]) {
    counterBloc.listen(onEventReceived);
  }

  @override
  void onEventReceived(int event) {
    emit(_history..add(event));
  }
}

Use platform channels

You can just use platform (Method) channels in your blocs. About some limitations you can read here.

Examples

Articles

Helpers

Gratitude

Special thanks to Felix Angelov for the reference in the form of bloc package

Libraries

isolate_bloc