javascript - angularJs $watch not working with global variable -

im using facebook js sdk im trying hide/show div if user login via fb login im watching global variable this:

   $scope.$watch(function() { return $window.ustatus }, function(n,o){         console.log("changed ",n);     }); 

im monitoring variable changes when login/out watch function doesnt work.

if use $interval after $watch funtion realize variable changes. $watch function works if variable change in angular?

sory bad grammer.