AngularJS : How to watch service variables?

I have a service, say:

factory('aService', ['$rootScope', '$resource', function ($rootScope, $resource) {
  var service = {
    foo: []
  };


  return service;
}]);

And I would like to use foo to control a list that is rendered in HTML:

  
{{ item }}

In order for the controller to detect when aService.foo is updated I have cobbled together this pattern where I add aService to the controller's $scope and then use $scope.$watch():

function FooCtrl($scope, aService) {                                                                                                                              
  $scope.aService = aService;
  $scope.foo = aService.foo;

  $scope.$watch('aService.foo', function (newVal, oldVal, scope) {
    if(newVal) { 

      scope.foo = newVal;
    }
  });
}

This feels long-handed, and I've been repeating it in every controller that uses the service's variables. Is there a better way to accomplish watching shared variables?