comms

comms is a simple communication pattern abstraction on streams, created for communication between any classes. It allows Listeners to easily react to messages sent by Senders.

For use in Flutter projects, check out flutter_comms.

Installation

dart pub add comms

Basic usage

Imagine you had to connect a light bulb and its light switch. In real world you'd have to use a wire and connect them, in code we can create a Stream in the light bulb and pass its Sink to the light switch, but instead of writing all the boilerplate code you can use Listener and Sender mixins which do it all for you.

Creating a Listener

A Listener is a mixin which allows your class to receive messages from any Sender sharing the same message type.

/// Add a `Listener` mixin with type of messages to listen for.
class LightBulb with Listener<bool> {
  LightBulb() {
    /// Call `listen` to start listening.
    listen();
  }

  bool enabled = false;
  
  /// Override `onMessage` to specify how to react to messages.
  @override
  void onMessage(bool message) {
    enabled = message;
  }

  void dispose() {
    /// Call `cancel` to stop listening and clean up.
    cancel();
  }
}

Creating a Sender

/// Add a `Sender` mixin with type of messages to send.
class LightSwitch with Sender<bool> {
  void enable() {
    /// Call `send` to deliver the message to a Listener.
    send(true);
  }
}

Using Listener and Sender

void main() async {
  // Just create instances of both classes, comms will 
  // handle connection between them.
  final lightBulb = LightBulb();
  final lightSwitch = LightSwitch();

  print(lightBulb.enabled); // false

  lightSwitch.enable();
    
  // Because communication is asynchronous we have to wait until the
  // next event loop iteration for the message to reach the `lightBulb`.
  await Future<void>.delayed(Duration.zero);

  print(lightBulb.enabled); // true

  // Clean up resources once done.
  lightBulb.dispose();
}

Multiple Listeners and Senders

A Sender sends a message to all Listeners sharing the same message type, so whenever any Sender<A> sends a message every Listener<A> will get the it.

void main() async {
  final lightBulbA = LightBulb();
  final lightSwitchA = LightSwitch();
  
  final lightBulbB = LightBulb();
  final lightSwitchB = LightSwitch();

  print(lightBulbA.enabled); // false
  print(lightBulbB.enabled); // false

  lightSwitchB.enable();
  
  await Future<void>.delayed(Duration.zero);

  print(lightBulbA.enabled); // true
  print(lightBulbB.enabled); // true
}

Handling initial message

To handle the last message sent before creating an instance of Listener you can override onInitialMessage, which is called after your Listener calls listen passing the last sent message of specified message type as argument.

abstract class CounterMessage {}
class CounterIncremented extends CounterMessage {}
class CounterDecremented extends CounterMessage {}

class CounterController with Sender<CounterMessage> {
  void increment() => send(CounterIncremented());
  void decrement() => send(CounterDecremented());
}

class Counter with Listener<CounterMessage> {
  Counter() {
    listen();
  }

  int count = 0;

  @override
  void onMessage(CounterMessage message) {
    if (message is CounterIncremented) {
      count--;
    }
    if (message is CounterDecremented) {
      count++
    }
  }

  @override
  void onInitialMessage(CounterMessage message) => onMessage(message);
}

void main() {
  final counterController = CounterController();

  counterController.increment();

  final counter = Counter();
    
  print(counter.count); // 1
}

Listening for multiple message types

If you need to receive more than one message type in a single Listener class, you can use the MultiListener mixin.

class MyListener with MultiListener {
  MyListener() {
    listen();
  }

  @override
  List<ListenerDelegate> get listenerDelegates => [
        ListenerDelegate<CounterMessage>(),
        ListenerDelegate<AuthMessage>(),
      ];

  @override
  void onMessage(dynamic message) {
    if (message is CounterMessage) {...}
    if (message is AuthMessage) {...}
  }
}

Custom Senders

To create a custom Sender for example to send multiple message types, you can use getSend which returns send method for messages of passed type.

mixin CustomSender {
  final sendString = getSend<String>();
  final sendInt = getSend<int>();

  void sendMessages() {
    sendString('hello');
    sendInt(1);
  }
}

You can also use the getSend function to send messages from anywhere without even creating a Sender.

void main() {
  getSend<bool>()(true);
}

Communicating between blocs

To communicate between blocs you can just use Sender and Listener mixins, for more convenience there are added ListenerCubit or ListenerBloc classes and StateSender mixin.

Creating a ListenerCubit

A ListenerCubit works exactly like Listener but calls listen and cancel functions for you, enabling your Cubit to receive messages from any Sender sharing the same message type.

/// Use `ListenerCubit` instead of `Cubit`, second type parameter specifies
/// message type to listen for.
class LightBulbCubit with ListenerCubit<LightBulbState, LightSwitchState> {
  LightBulbCubit() : super(LightBulbState(false));
  
  /// Override `onMessage` to specify how to react to messages.
  @override
  void onMessage(LightSwitchState message) {
    if (message is LightSwitchEnabled) {
      emit(LightBulbState(true));
    } else if (message is LightSwitchDisabled) {
      emit(LightBulbState(false));
    }
  }
}

class LightBulbState {
  LightBulbState(this.enabled)
  final bool enabled;
}

Creating a StateSender

A StateSender mixin allows your bloc to send message with state every time a new state is emitted.

/// Add a `Sender` mixin with type of messages to send.
class LightSwitchBloc extends Bloc<bool, LightSwitchState> 
  with StateSender {
  LightSwitchBloc() : super(false) {
    on<bool>(
      (event, emit) {
        if (event) {
          emit(LightSwitchEnabled());
        } else {
          emit(LightSwitchDisabled());
        }
      }
    )
  };
}

abstract class LightSwitchState {}
class LightSwitchEnabled extends LightSwitchState {}
class LightSwitchDisabled extends LightSwitchState {}

Using ListenerCubit and StateSender

void main() async {
  // Just create instances of both classes, comms will 
  // handle connection between them.
  final lightBulbCubit = LightBulCubit();
  final lightSwitchBloc = LightSwitchBloc();

  print(lightBulbCubit.state.enabled); // false

  lightSwitchBloc.add(true);
    
  await Future<void>.delayed(Duration.zero);

  print(lightBulbCubit.state.enabled); // true

  // comms will automatically clean up resources on close
  lightBulbCubit.close();
  lightSwitchBloc.close();
}

Real life example

Most common bloc that each app has is AuthBloc or AuthCubit. It's important for all other blocs to know and react to whether user is authenticated or not. With comms, all you need to do is add a StateSender mixin to the auth bloc, which allows every other bloc to listen for AuthState changes.

class AuthCubit extends Cubit<AuthState> with StateSender {...}

For example, UserProfileCubit fetches the user's profile on sign-in and clears that data on sign-out.

class UserProfileCubit extends ListenerCubit<UserProfileState, AuthState> {
  UserProfileCubit({
    required UserRepository repository,
  }) : _repository = repository,
       super(const UserProfileState.initial());
  
  final UserRepository _repository;

  @override
  void onInitialMessage(AuthState message) => onMessage(message);

  @override
  void onMessage(AuthState message) {
    if (message is AuthStateAuthenticated) {
      fetchProfile();
    } else {
      emit(const UserProfileState.noUser())
    }
  }
  
  Future<void> fetchProfile() async {...}
}

Using conventional streams and sinks the same cubit would look like this:

class UserProfileCubit extends Cubit<UserProfileState> {
  UserProfileCubit({
    required UserRepository repository,
    required Stream<AuthState> authStateStream,
    required AuthState initialAuthState,
  })  : _repository = repository,
        _authStateStream = authStateStream,
        super(const UserProfileState.initial()) {
    _init(initialAuthState: initialAuthState);
  }
  
  final UserRepository _repository;

  final Stream<AuthState> _authStateStream;
  StreamSubscription<AuthState>? _authStateStreamSubscription;

  void _init({required AuthState message}) {
    onAuthState(initialAuthState);
    _authStateStreamSubscription = _authStateStream.listen(onAuthState)
  }

  void onAuthState(AuthState authState) {
    if (authState is AuthStateAuthenticated) {
      fetchProfile();
    } else {
      emit(const UserProfileState.noUser())
    }
  }
  
  Future<void> fetchProfile() async {...}

  @override
  Future<void> close() async {
    _authStateStreamSubscription?.cancel();
    super.close();
  }
}

This required 15 more lines of code, not to mention that you also need to pass the AuthCubit's stream and initial state in constructor which is not needed when using comms.