AngularJS communication between directives

One way you can communicate between them using what is called eventing.

One directive can emit an event on the rootscope which can then be listened by anybody who wants to. You could use $rootScope.$emit or $rootScope.$broadcast to publish events with data and use $scope.$on to listen to the event. In your case you could just do $scope.$emit as well.

app.directive("firstDir", function(){
    return {
        restrict : 'E',
        controller : function($scope){        
            this.data="init value";

            this.set = function(value){
             //EMIT THE EVENT WITH DATA
              $scope.$emit('FIRST_DIR_UPDATED', value);
                this.data = value;
                // communication with second Directive ???
            }       
        },
        controllerAs : 'firstCtrl'
    };  
});

app.directive("secondDir", function(){
    return {
        restrict : 'E',
        controller : function($scope){    
          var _that = this;
          //LISTEN TO THE EVENT 
          $scope.$on('FIRST_DIR_UPDATED', function(e, data){
                 _that.data = data;
          });
          this.data="init value";   
        },
        controllerAs : 'secondCtrl'
    };  
});

Demo
Demo2

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Now speaking of which, it sometimes is really required to inject $rootScope just to have the eventing enabled to a different node in your application. You can instead have a pub/sub mechanism easily built in your app and make use of prototypical inheritance.

Here i am adding 2 methods publish and subscribe on $rootScope's prototype during app initialization. So any child scope or isolated scope will have these methods available and communication will be so easier without worrying about whether to use $emit, $broadcast, whether i need to inject a $rootscope for communication from isolated scoped directive etc.

app.service('PubSubService', function () {


   return {Initialize:Initialize};

     function Initialize (scope) {
        //Keep a dictionary to store the events and its subscriptions
        var publishEventMap = {};

         //Register publish events
          scope.constructor.prototype.publish =  scope.constructor.prototype.publish 
           || function () {
                var _thisScope = this,
                    handlers, 
                    args, 
                    evnt;
                //Get event and rest of the data
                args = [].slice.call(arguments);
                evnt = args.splice(0, 1);
                //Loop though each handlerMap and invoke the handler
                angular.forEach((publishEventMap[evnt] || []), function (handlerMap) {
                    handlerMap.handler.apply(_thisScope, args);
                })
            }

         //Register Subscribe events
         scope.constructor.prototype.subscribe = scope.constructor.prototype.subscribe 
            || function (evnt, handler) {
                var _thisScope = this,
                    handlers = (publishEventMap[evnt] = publishEventMap[evnt] || []);

                //Just keep the scopeid for reference later for cleanup
                handlers.push({ $id: _thisScope.$id, handler: handler });
              //When scope is destroy remove the handlers that it has subscribed.  
             _thisScope.$on('$destroy', function () {
                for(var i=0,l=handlers.length; i<l; i++){
                  if (handlers[i].$id === _thisScope.$id) {
                        handlers.splice(i, 1);
                        break;
                    }
                }
            });
        }

    }
}).run(function ($rootScope, PubSubService) {
    PubSubService.Initialize($rootScope);
});

and you could just have any place from your app publish an event without requiring a rootScope.

$scope.publish('eventName', data);

and listen anywhere on the application without worrying about using $rootScope or $emit or $broadcast:-

$scope.subscribe('eventName', function(data){
    //do somthing
});

Demo – PubSub