angularjs - $watch a factory's function return -

i have factory manage standardized current time across app, has exposed function gettime().

i display on various pages; however, not updating minutes roll by. i'm trying below code in controller , expect once minute console log of current time, instead gets printed each time change page.

    $scope.$watch(function () { return timeservice.gettime() }, function (newval, oldval) {         if (typeof newval !== 'undefined') {             console.log("$watch", newval);             $scope.currenttime = newval;         }     }, true); 

how can watch return value of factory's function?

update

here timeservice code

    var _gettime = function () {         return _formattime(new date());     };      var _formattime = function (date) {         var hours = date.gethours();         var minutes = date.getminutes();         var ampm = hours >= 12 ? 'pm' : 'am';         hours = hours % 12;         hours = hours ? hours : 12; // hour '0' should '12'         minutes = minutes < 10 ? '0'+minutes : minutes;         var strtime = hours + ':' + minutes + ' ' + ampm;         return strtime;     }; 

there's no way angularjs can guess value of timeservice.gettime() changed. angular runs digest cycle responding things, user interaction, that's why works on page change. you'd need manually run $scope.$apply() fix this.

however, since need predictable change every 60 seconds, it'd better use $interval (https://docs.angularjs.org/api/ng/service/$interval) service:

$interval(function() {   $scope.currenttime = timeservice.gettime() }, 60 * 1000) // runs every 60 seconds, or 60,000 milliseconds