How to encapsulate single and temporal events in a service?

Question

I'm trying to encapsulate the events in a service in order to implement a mechanics to subscribe / unsubscribe the listeners when a controller's scope is destroyed. This because I have been using the rootScope.$on in the following way:

if(!$rootScope.$$listeners['event']) {
    $rootScope.$on('event', function(ev, data){
        // do some...
    });
}

or

$scope.$on('$destroy', function(ev, data){
    // unsubscribe the listener
});

So I just need one listener of this event, I need to delete the existing listener when the controller is no longer alive, because the function I registered earlier is still being triggered.

So I need to implement a $destroy event listener on my controller, to destroy the listener when the scope is destroyed, but I don't want to do that code each time I create an event. That's why I want to create a service in where I'm going to encapsulate the events.

angular.module('core').factory('event', [
    function() {
        var service = {};
        service.events = {};
        service.on = function(scope, eventId, callback) {
            scope.$on('$destroy', function(ev, other){
                //unsubscribe
            });
            service.events[eventId] = callback;
            // scope = null; I guess ?
        };
        service.emit = function(eventId, data){
            if (service.events[eventId])
                service.events[eventId](data);
            else
                return new Error('The event is not subscribed');
        };
        return service;
    }
]);

This could be done using $rootScope instead of my own methods but encapsulating the $on and $emit of $rootScope, but at the end I'll have the same issue here.

So these are my questions:

1. Is a good practice to pass the scope ref value to a service?
2. What is the meaning of $$destroyed? when this is true means that angularJS has no internal references to the instance?
3. Should I do a scope = null in my service to let GC delete the object or does angularJS handle an explicit delete?
4. Is there a better way to do what I want?

Answer 1

What you are trying to accomplish is basically an event bus.
You have also described very well what is wrong with the current implementation. A different way to approach the problem is to decorate the $rootScope with your bus (or any other event bus for that matter). Here is how:

app.config(function ($provide) {
$provide.decorator('$rootScope', ['$delegate', '$$bus', function ($delegate, $$bus) {
  Object.defineProperty($delegate.constructor.prototype, '$bus', {
    get: function () {
      var self = this;

      return {
        subscribe: function () {
          var sub = $$bus.subscribe.apply($$bus, arguments);

          self.$on('$destroy',
            function () {
              console.log("unsubscribe!");
              sub.unsubscribe();

            });
        },

        publish: $$bus.publish
      };
    },
    enumerable: false
  });

  return $delegate;
}]);
});

Considering the following $$bus implementation (kept basic for simplicity):

app.factory('$$bus', function () {
  var api = {};
  var events = {};

  api.subscribe = function (event) {
    if (!events.hasOwnProperty(event.name)) {
      events[event.name] = [event];
    } else {
      events[event.name].push(event);
    }
    return {
      unsubscribe: function () {
        api.unsubscribe(event);
      }
    }
  };

  api.publish = function (eventName, data) {
    if (events.hasOwnProperty(eventName)) {
      console.log(eventName);

      angular.forEach(events[eventName], function (subscriber) {
        subscriber.callback.call(this, data);
      });
    }
  };

  api.unsubscribe = function (event) {
    if (events.hasOwnProperty(event.name)) {
      events[event.name].splice(events[event.name].indexOf(event), 1);
      if (events[event.name].length == 0) {
        delete events[event.name];
      }
    }
  };

  return api;
});

Now all you have to do is subscribe or publish events. The unsubscribe will take place automatically (when the $scope is destroyed):

  $scope.$bus.subscribe({
    name: 'test', callback: function (data) {
      console.log(data);
    }
  });

And later on publish an event:

  $scope.$bus.publish('test', {name: "publishing event!"});

An important point to make is that the events themselves are subscribed to each individual $scope and not on the $rootScope. That is how you "know" which $scope to release.

I think it answers your question. With that in mind, you can obviously make this mechanism much sophisticated (such as controller event listener released when a view routed, unsubscribe automatically only to certain events, etc.). Good luck!

** This solution is taken form Here which uses a different bus framework (other then that it is the same).