Passing current scope to an AngularJS Service

To let the controller know when something async happens, use Angular promises.

To provoke the $apply, you don't need the scope, you can call $rootScope.$apply, as there is no difference calling it in a specific scope or in the root.

Regarding the variable reading, it would be better if you received parameters. But you could also read it from a scope as an object parameter, but I would go with parameter, that would make your service interface much more clearer.

I would say if your functionality is specific to one controller only than you don't need a service.

The controllers tasks is to manipulate the specific model whereas a service should deal with global tasks. I would rather stick to this paradigm instead of mixing things up.

This is what the docs say

Service

Angular services are singletons that carry out specific tasks common to web apps

Controller

In Angular, a controller is a JavaScript function(type/class) that is used to augment instances of angular Scope, excluding the root scope.

PS: Apart from that if you need to digest you can also inject the $rootScope within your service.