Watching changed in the service from the controller

Watching changed in the service from the controller

I am trying to listen to changes in my injected service (self-updating) in
the controller. In the below example you'll find two $watch cases - one
that works but I don't know exactly why and one that was obvious to me,
yet doesn't work. Is the second example the right way to do it? Isn't that
code duplication? What is the right way to do it?
Service:
app.factory("StatsService", [
'$timeout', 'MockDataService',
function ($timeout, MockDataService) {
var service, timeout;
timeout = 5000;
service = {
fetch: function () {
// Getting sample data, irrelevant, however this is what
updates the data
return this.data = MockDataService.shuffle();
},
grab: function () {
this.fetch();
return this.update();
},
update: function () {
var _this = this;
return $timeout(function () {
return _this.grab();
}, timeout);
}
};
service.grab();
return service;
}
]);
Controller:
app.controller("StatsController", [
'$scope', 'StatsService',
function ($scope, StatsService) {
var chart;
$scope.stats = StatsService;
$scope.test = function (newValue) {
if (arguments.length === 0) {
return StatsService.data;
}
return StatsService.data = newValue;
};
// This doesn't work
$scope.$watch('stats', function (stats) {
return console.log('meh');
});
// This works, don't know why
$scope.$watch('test()', function (stats) {
return console.log('changed');
});
}
]);