AngularJS, sharing data between controllers

Question

So, this isn't the typical question of HOW to do it. I know it can be done with a service or a factory but I was wondering if someone could share what the advantages/disadvantages would be of just creating a basic service, injecting it into each controller, and then extending the service with functions from each controller. Something similar to the following example..

app.service('HelperService', function() {
    return {};
});

app.controller('Controller1', function($scope, HelperService) {
    $scope.somefunc = function() {
        //do stuff here
    };
    HelperService.somefunc = $scope.somefunc;
});

app.controller('Controller2', function($scope, HelperService) {
    HelperService.somefunc();
});

This works, and works well. I feel a bit stupid for asking this but it just seems like I'm missing something here as to why this isn't used or recommended?

Answer 1

It may work, but its a bad idea.

• Controller2 HelperService.somefunc() won't exist until Controller1 has been instantiated. So theres an implicit dependency of Controller2 on Controller1
• the code on HelperService isn't in one place where it can be understood together
• if you are doing some sort of data manipulation in that function, it really should be operating on data encapsulated by the HelperService.

Answer 2

The service is a singleton and it will be instantiated once calling new on the function itself -- the function you pass in is essentially a constructor. This will create the empty object you are returning for use everywhere, but if you want to return an object in such a way it makes more sense to use .factory, but this is not a big deal.

At any rate, you can consider your code to conceptually do this:

var HelperService = function () {}
var helperService = new HelperService;

function Controller1() {
    helperService.someFunc = function () {}
}
function Controller2() {
    helperService.someFunc();
}

I would consider this a dangerous thing to do for a couple of reasons:

1. Controller1 must be instantiated before Controller2 or else somefunc won't be available to Controller2. Ideally the controllers would have no knowledge of each other.
2. You are coupling Controller/ViewModel (since you're using scope) with service level logic, but these should be decoupled. HelperService shouldn't know about the controllers either. Instead, you should be injecting a service that has an API that the controllers expect to use. This doesn't always have to be HelperService, it just has to look like HelperService to the controllers and its API shouldn't change.

Without knowing specifics about what you're trying to do, it's hard to advise. In general you may rethink what you want to do, but you can extend functionality of services with other services. Consider services to be in their own layer.