javascript - Why is $timeout is needed for watch to trigger -

i have directive watches height of element. element updated controller uses factory method data. unless have $timeout in factory watch never gets updated. curious! can shed light onto why?

my controller:

$scope.update = function () {     apiservice.getlinks(function (response) {         $scope.links = response;         // if try $scope.$apply() here says in progress, you'd expect     }); }  quicklinksservices.factory('quicklinksapiservice', function ($http, $timeout) {      quicklinksapi.getquicklinks = function (success) {          //without watch in directive doesnt triggered         $timeout(function () { }, 100);           $http({             method: 'json',             url: '/devices/getquicklinkcounts'         }).success(function (response) {             quicklinksapi.quicklinks = response;             quicklinksapi.savequicklinks();              success(response);         });     } 

the directive i'm using here

basically angularjs provides $timeout service trigger function call after specified time, know use of $timeout not strictly needed, people have habit of writing mean need trigger watch after specified interval. in many cases $apply trick. thing need $apply(). reference please check this

also many times happens angulars $watch executes faster expected may send incomplete updates or response in such cases $timeout plays important role delaying $watch. can clear $timeout if $watch fast enough need, means $timeout way explicitly trigger $watch, $apply can itself. use of $timeout or $apply depends on how requirement is. hope clears you. luck.