Alternatives to angularjs $watch to trigger ui change from service - angularjs

Question

I'm wondering whether there is a different approach to using $watch in order to achieve the following.
Setup:
ControllerA depends on ServiceA.
ControllerB depends on ServiceB.
Current browser view is showing both controllers.
Scenario:
ControllerA is initiating a function on ServiceA, which in turn changes the value of propery X in ServiceB which should be reflected in the UI of ControllerB.
http://jsfiddle.net/zexscvax/2/
html:
<div>
<div ng-controller="ControllerA"></div>
<div ng-controller="ControllerB">Progress: {{progress}}</div>
</div>
js:
var myApp = angular.module('myApp', []);
myApp.factory('serviceA', ['$q', '$interval', 'serviceB', function ($q, $interval, serviceB) {
var service = {};
service.start = function () {
var progress = 0;
var deferred = $q.defer();
deferred.promise.then(null,null, notifyServiceB);
function notifyServiceB() {
serviceB.update(progress);
}
$interval(function() {
if (progress == 0.99) {
deferred.resolve();
} else {
progress += 0.01;
deferred.notify(progress);
}
}, 50, 100);
};
return service;
}]);
myApp.factory('serviceB', ['$rootScope', function ($rootScope) {
var service = {};
service.update = function (progress) {
console.log('update', progress);
service.progress = progress;
//$rootScope.$apply(); // <+ ERROR: $digest already in progress
};
return service;
}]);
myApp.controller('ControllerA', ['$scope', 'serviceA',
function ($scope, serviceA) {
serviceA.start();
}]);
myApp.controller('ControllerB', ['$scope', 'serviceB',
function ($scope, serviceB) {
$scope.progress = serviceB.progress;
/* this works but I'm not sure whether this is performing
$scope.$watch(function () { return serviceB.progress; },
function (value) {
$scope.progress = serviceB.progress;
}
);
*/
}]);
Without the $watch in ControllerB for the property X in ServiceB, the UI would not get updated. I've also tried injecting $rootScope in ServiceB in order to run an apply() but that wouldn't work.
I'm not entirely sure whether there's a better way to setup this scenario or whether $watch is fine. I'm a bit worried about performance issues as the value of property X changes almost every 50 ms (it's basically a visual timer counting down).
Thanks for your input.

Answer 1

If you don't use $watch, you can use $rootScope to broadcast, and on controller B, you can $on this event and handle the view update.