如果想在某个属性发生变化的时候执行某些操作，那么scope.$watch是最佳选择

https://docs.angularjs.org/api/ng/type/$rootScope.Scope#$watch

    //如果是中文...

    $rootScope.$watch('session', function() {

        if ($rootScope.session) {

            var re = /[\u4e00-\u9fa5]/;

            if (re.test($rootScope.session.last_name + $rootScope.session.first_name)) {

                //has Chinese in name then firstname+lastname

                $rootScope.session.display_name = $rootScope.session.last_name + $rootScope.session.first_name;

            } else {

                $rootScope.session.display_name = $rootScope.session.last_name + ' ' + $rootScope.session.first_name;

            }

        }

    });