Angularjs – What’s the most concise way to read query parameters in AngularJS

If you don't want to put CSS class names into Controller like I do, here is an old trick that I use since pre-v1 days. We can write an expression that evaluates directly to a class name selected, no custom directives are necessary:

ng:class="{true:'selected', false:''}[$index==selectedIndex]"

Please note the old syntax with colon.

There is also a new better way of applying classes conditionally, like:

ng-class="{selected: $index==selectedIndex}"

Angular now supports expressions that return an object. Each property (name) of this object is now considered as a class name and is applied depending on its value.

However these ways are not functionally equal. Here is an example:

ng-class="{admin:'enabled', moderator:'disabled', '':'hidden'}[user.role]"

We could therefore reuse existing CSS classes by basically mapping a model property to a class name and at the same time keep CSS classes out of Controller code.

Edit: The issue addressed in this answer have been resolved in angular.js version 1.2.7. $broadcast now avoids bubbling over unregistered scopes and runs just as fast as $emit.

So, now you can:

• use $broadcast from the $rootScope
• listen using $on from the local $scope that needs to know about the event

Original Answer Below

I highly advise not to use $rootScope.$broadcast + $scope.$on but rather $rootScope.$emit+ $rootScope.$on. The former can cause serious performance problems as raised by @numan. That is because the event will bubble down through all scopes.

However, the latter (using $rootScope.$emit + $rootScope.$on) does not suffer from this and can therefore be used as a fast communication channel!

From the angular documentation of $emit:

Dispatches an event name upwards through the scope hierarchy notifying the registered

Since there is no scope above $rootScope, there is no bubbling happening. It is totally safe to use $rootScope.$emit()/ $rootScope.$on() as an EventBus.

However, there is one gotcha when using it from within Controllers. If you directly bind to $rootScope.$on() from within a controller, you'll have to clean up the binding yourself when your local $scope gets destroyed. This is because controllers (in contrast to services) can get instantiated multiple times over the lifetime of an application which would result into bindings summing up eventually creating memory leaks all over the place :)

To unregister, just listen on your $scope's $destroy event and then call the function that was returned by $rootScope.$on.

angular
    .module('MyApp')
    .controller('MyController', ['$scope', '$rootScope', function MyController($scope, $rootScope) {

            var unbind = $rootScope.$on('someComponent.someCrazyEvent', function(){
                console.log('foo');
            });

            $scope.$on('$destroy', unbind);
        }
    ]);

I would say, that's not really an angular specific thing as it applies to other EventBus implementations as well, that you have to clean up resources.

However, you can make your life easier for those cases. For instance, you could monkey patch $rootScope and give it a $onRootScope that subscribes to events emitted on the $rootScope but also directly cleans up the handler when the local $scope gets destroyed.

The cleanest way to monkey patch the $rootScope to provide such $onRootScope method would be through a decorator (a run block will probably do it just fine as well but pssst, don't tell anybody)

To make sure the $onRootScope property doesn't show up unexpected when enumerating over $scope we use Object.defineProperty() and set enumerable to false. Keep in mind that you might need an ES5 shim.

angular
    .module('MyApp')
    .config(['$provide', function($provide){
        $provide.decorator('$rootScope', ['$delegate', function($delegate){

            Object.defineProperty($delegate.constructor.prototype, '$onRootScope', {
                value: function(name, listener){
                    var unsubscribe = $delegate.$on(name, listener);
                    this.$on('$destroy', unsubscribe);

                    return unsubscribe;
                },
                enumerable: false
            });


            return $delegate;
        }]);
    }]);

With this method in place the controller code from above can be simplified to:

angular
    .module('MyApp')
    .controller('MyController', ['$scope', function MyController($scope) {

            $scope.$onRootScope('someComponent.someCrazyEvent', function(){
                console.log('foo');
            });
        }
    ]);

So as a final outcome of all this I highly advise you to use $rootScope.$emit + $scope.$onRootScope.

Btw, I'm trying to convince the angular team to address the problem within angular core. There's a discussion going on here: https://github.com/angular/angular.js/issues/4574

Here is a jsperf that shows how much of a perf impact $broadcastbrings to the table in a decent scenario with just 100 $scope's.

http://jsperf.com/rootscope-emit-vs-rootscope-broadcast