If the bookmark ID's are being changed in code handled by angular (such as in a controller, a directive, a service, an $http callback, etc.) then you don't need to do anything at all. Angular will automatically re-evaluate isBookmarked(event.id)
when it needs to in order to ensure that the UI is always up to date.
If, for some reason, the updates must be done outside of code that is handled by angular (such as in an external library), then you can manually force angular to update. However, it is highly recommended to just ensure that the code is handled by angular. You can usually do this, but it depends on the situation.
If you need to force it, then use $scope.$apply
as follows:
$scope.$apply(function() {
// Write code here to update bookmark ID's
});