"$watch"ing a service doesn't appear to update reliably - angularjs

Question

I am currently building a simple drag & drop directive so that I can move some SVG stuff around on the screen. At the moment I'm still in the early stages, but I have run into a strange issue with $watch that I'm hoping someone can help me with.
I have a service that maintains my mouse state. At the moment it's just the x and y coordinates of the cursor. I also have an attribute level directive that interacts with this service in order to bind to the mouse-move event and update the service whenever someone moves the mouse around. These two items work together like a champ. The directive keeps the service up to date with the mouse's position and since my service (Factory really) is a singleton, I can pull this data in to other directives/controllers to see what's going on with the mouse.
Here's the problem: I'm trying to allow a specific SVG element to be dragged around, so I created a super simple controller with two functions: a "trackDrag" function that begins tracking and moving a specific element, and a "releaseDrag" which stops tracking/moving the element (drops it where it is, basically).
Inside of my trackDrag function, I attempt to use $scope.$watch to watch the mouse service's current x and y coordinates. Since it's a factory, these values are returned in a function and my watch looks something like this:
$scope.$watch("mouseTrackingService.get()", function(){
// do some stuff here
});
This watch DOES fire off when I first start dragging an element but it doesn't fire as I continue dragging it across the screen. In my "releaseDrag" function, I deallocate the watcher and that seems to work correctly. I'm kind of stumped about why I don't see the watch fire off continuously, even though I can console write out inside of the service and I see that IT is updating correctly.
I've included a plnkr with some sample code below:
http://plnkr.co/edit/g3WEgiQWvd9oXCpFEByn?p=preview
If I just give in and use a $interval then this code works (updating the position every 10ms for example), but really I see that as a much less "angular" way of doing things vs binding.

Answer 1

Ugh, I'm just being dumb. I forgot that $scope.$watch can ONLY WATCH SCOPE VARIABLES.
I fixed this issue by adding the following wrapper around the service:
$scope.currentMouse = function(){
return mouseTrackingService.get();
};
I can then watch currentMouse:
$scope.$watch(currentMouse(), function(){
updateMousePosition(target);
console.log("noticed a change");
});
Of course that gives me that awful Digest error after more than like a half second of dragging:
Uncaught Error: [$rootScope:infdig] 10 $digest() iterations reached. Aborting!
But that's a different issue entirely.
Sorry folks, nothing to see here. Move along now :-p