Pubsub in Dart

gistfile1.dart

class Pubsub {
  Map channels;
  
  Pubsub() {
    channels = new Map();
  }
  
  subscribe(String channel, Function cb) {
    if (channels[channel] == null) {
      channels[channel] = new List<Function>();
    }
    channels[channel].add(cb);
  }
  
  publish(String channel, var message) {
    for (Function cb in channels[channel]) {
      try {
        cb(message);
      } catch (final Exception e) {
        print(e);
      }
    }
  }
  
  unsubscribe(String channel, Function fun) {
    if (channels[channel] != null) {
      int i = channels[channel].indexOf(fun);
      if (i > -1) {
        channels[channel].removeRange(i, 1);
      }
    }
  }
}

void main() {
  Pubsub pubsub = new Pubsub();
  
  void printMessage(var message) {
    print(message);
  }
  
  pubsub.subscribe('123', printMessage);      // subscribes on channel `123`
  pubsub.publish('123', 'a message');         // prints out `a message`
  pubsub.unsubscribe('123', printMessage);    // unsubscribes from channel `123`
  pubsub.unsubscribe('1234', printMessage);   // tries unsubscribing from channel `1234`
  pubsub.publish('123', 'a second message');  // prints out nothing
  pubsub.subscribe('123', printMessage);      // subscribes again on channel `123`
  pubsub.publish('123', 'a third message');   // prints out `a third message`
  pubsub.publish('123', ['one', 'two']);      // prints the array
}

Hi, I know this is a very old gist but I found it useful. I just updated the code to enable soundness. Here it is on DartPad, and I'm copypasting below.

class Pubsub<C, T> {
  final Map<C, List<_Callback<T>>> channels = {};

  void subscribe(C channel, _Callback<T> cb) {
    channels.putIfAbsent(channel, () => <_Callback<T>>[]).add(cb);
  }

  void publish(C channel, T message) {
    for (final _Callback<T> cb in channels[channel]) {
      try {
        cb(message);
      } catch (e) {
        print(e);
      }
    }
  }

  void unsubscribe(C channel, _Callback<T> fun) {
    if (channels[channel] != null) {
      channels[channel].remove(fun);
    }
  }
}

enum Channel {
  one, two
}

void main() {
  // A type-safe pubsub.
  final pubsub = new Pubsub<Channel, String>();

  void printMessage(String message) {
    print(message);
  }

  pubsub.subscribe(Channel.one, printMessage);      // subscribes on channel one
  pubsub.publish(Channel.one, 'a message');         // prints out `a message`
  pubsub.unsubscribe(Channel.one, printMessage);    // unsubscribes from channel one
  pubsub.unsubscribe(Channel.two, printMessage);    // tries unsubscribing from channel two
  pubsub.publish(Channel.one, 'a second message');  // prints out nothing
  pubsub.subscribe(Channel.one, printMessage);      // subscribes again on channel one
  pubsub.publish(Channel.one, 'a third message');   // prints out `a third message`
  
  // A dynamic pubsub.
  final dynamicPubsub = new Pubsub();
  
  dynamicPrint(var message) {
    print(message);
  }
  
  dynamicPubsub.subscribe("123", dynamicPrint);     // subscribes again on channel `123`
  dynamicPubsub.publish("123", ['one', 'two']);     // prints the array
}

typedef _Callback<T> = void Function(T);

Also worth mentioning that most modern Dart code would just use Stream for pubsub functionality (namely, broadcast streams).